Checking current

[The Captain]

Ive noticed my PT getting very hot, so tried a current check with a MM but the amp made a horrid loud ERRR noise before standby flipped (closed), as if it coincided with caps charging up.. I had my MM set to 200m for Amps, red probe on standby switch, black probe on 6v6 pin 3, all tubes and speaker in too (is this right?). I want to check this in case its wrong, if anyone can help- thanks, Captain.

[stokes]

With the speaker connected and the meter on your plate,you will get noise.Not sure where you have the standby wired,I usually put it on my PT's CT to ground.Just to be clear,the red lead goes to your OT's CT or first filter cap or pin 8 on the rectifier and the black lead to the plate (pin 3,6v6).If you have a shorting jack on the output jack to the speaker,just pull the speaker jack out to avoid any noise when checking the current,if not you will have to short the output.I have to think that if it were the power tubes current causing the PT to heat up,the tubes would be red plating.After you check the current on the 6V6's and are sure they are within safe limits,pull all your tubes and check the volts at each filter cap,with no tubes the volts at the rectifier output (pin 8) and all the filter caps should be the same,with no load (tubes) there will be no voltage drop across any of the dropping resistors in the B+ rail.If you see a voltage drop anywhere in the rail with no tubes,you have a problem with a leaky cap.If you remember when we last did the test with the current limiter,I wasnt 100% convinced there was no problem,the way you described the lights brightness levels.A leaky cap wont always cause the fuse to blow,but the increase in current will cause the PT to slowly overheat and can eventually burn it out,which was what I thought with your first PT problem.Check the volts as I described at each of the 3 filter caps and pin 8 of the rectifier and post your findings,we'll get to the bottom of this.

[The Captain]

Great help as always Mr. Stokes- for some reason Im nervous about this current check, so I have to double check..

I've a non-shorting spkr output (2 lug) so Ive pulled spkr jack out & wire-linked output lugs for the check, so thats now shorted? I've tubes in and flipping standby(closed) after AC switch on. Im looking to get 20-28mA with MM set to 200m on its A section: Once Im sure this is right I can post up voltages of all caps with and without tubes in. cheers Capt.

[stokes]

Its always best to use a "shorting" jack on the speaker out,in case you forget to plug the speaker in.20-28ma's is a safe range for the 6V6's.No need to check the volts on the B+ rail with the tubes in.What we are looking for with this test is to see if there is any current being drawn in your power supply.With no tubes in the amp there should be no current being drawn,so you shouldnt see more than a couple of volts dropped from one cap to the next,the volts at each cap and the rectifiers output (pin 8) should all be the same,or within 2-3 volts at most.So if you see,lets say,a 20v drop at one of the caps that would indicate that the cap is leaking current and causing the PT to overheat.

[The Captain]

Assuming Im checking current with all tubes in-? after AC switched on, and waiting for a bit I read 88.5. I shut it down before flipping standby(closed) as I guess summat is not as it should be/ I have a prob? (Black probe was on 6v6 pin 3/ red probe pin 8 5y3gt, reading with MM set to A, spkr unplugged, 2 lugs joined of spkr output -configed as a shorting socket would be ie tip to switch to gnd all linked).

[stokes]

If I am reading this right,you are seeing 88.5ma's with the amp in standby?If that is the case then I have to assume you have the switch wired so you get dc to the first filter before you turn the standby switch to the play mode.If this is the case,you must check the current with the amp in play mode.If the current is still that high in play mode,then we have to adjust the cathode resistor to lower it.Dont be concerned about flipping that standby switch to play,as long as you have no signal going thru the amp,it'll be okay.Yes,you have to have the tubes in the amp to check the current.

[The Captain]

I get 88.5 with just AC switch on- I didn't dare flip standby to play as I thought there was a major prob. So thats AC switched on, and standby switch...

I really hate this standby terminology: Standby On means effectively 'off', or 'not flipped'? All I can do is hope my 'Open' and 'Closed' (as in switch contacts) is understood, being the clearest way I can describe its position; I can never be sure of anyone's On or Off, or 'Amp in Standby' either. 'Play' I am taking as my closed/ ie a singing, ringing 5e3. If not I am considering opiates. Play is clear- thanks Mr. Stokes.

So I will check current again, this time with standby to play/ closed, and report back.
-Am I keeping tubes in for this current check?

[Tom Phillips]

Assuming Im checking current with all tubes in-? after AC switched on, and waiting for a bit I read 88.5. I shut it down before flipping standby(closed) as I guess summat is not as it should be/ I have a prob? (Black probe was on 6v6 pin 3/ red probe pin 8 5y3gt, reading with MM set to A, spkr unplugged, 2 lugs joined of spkr output -configed as a shorting socket would be ie tip to switch to gnd all linked).

Captain,
Do you realize that with your current meter connected as described above, you are providing a path for current to flow around the standby switch even if the switch is in “Standby mode” (Open contacts) Then the current can flow from pin 3 of one 6V6 to the output transformer secondary. From there it can flow through the OT secondary and out the other side and out the center tap to effectively try to power the whole amp. Think of the current meter as a low resistance wire, draw it that way on the schematic and I think you will see what I mean.
This can be a cause for confusing results

Regards,
Tom

[stokes]

When I say "amp in standby"I mean you get no sound,you say open.Standby on means you get sound,or as you say,closed.The tubes must be in to check the current,you are checking the current the tube is drawing.Is my assumption of your standby being wired so you get dc voltage at the first cap when the switch is "open"?

[The Captain]

So Standby On is closed/ play? well Im a dutchman... not to worry, I'll use my terms just to be as clear as poss. Cheers Tom, now Ive redone checks your post makes sense.

Anyway, current checks redone: at 1st 6v6 (nxt to recto) pin 3= 40.3 mA. And at other 6v6= 43.3 mA. Both switches flipped/ play.

[The Captain]

I can't understand this..

V at pin 8 recto (no tubes now)= 0.26V.
And at 1st filter cap (25uf/ 250r 5w)= 0V.
And at adjacent 16uf sprague= 0.11V.

I checked each 3 times. Both switches to play/ closed. No spkr in as before(?). MM set to DC, all connections sound- WTF?!

[stokes]

It sounds like you have no rectifier tube in there.The rectifier should be in the amp or you will get no dc volts.I guess i should have made that clear when I said "with no tubes".You have to leave the rectifier tube in.

[Tom Phillips]

So Standby On is closed/ play? well Im a dutchman... not to worry, I'll use my terms just to be as clear as poss. Cheers Tom, now Ive redone checks your post makes sense.
Standby terminology is a little confusing the way people use the terms differently. I prefer to just say the amp is in “standby” mode or “operate mode.” Anyway…on to the other issues.

Anyway, current checks redone: at 1st 6v6 (nxt to recto) pin 3= 40.3 mA. And at other 6v6= 43.3 mA. Both switches flipped/ play.
OK. That’s making more sense now. So the amp seems to be under biased therefore drawing too much bias current. You have not posted your plate and cathode voltage so we need to sort that out in order to calculate the plate dissipation. The current is higher than you will want but should not hurt anything during the time it’s on for temporary tests.

I can't understand this.. V at pin 8 recto (no tubes now)= 0.26V. And at 1st filter cap (25uf/ 250r 5w)= 0V. And at adjacent 16uf sprague= 0.11V.I checked each 3 times. Both switches to play/ closed. No spkr in as before(?). MM set to DC, all connections sound- WTF?!
You wouldn’t have measured the above currents if the voltage was not present.
Either something just failed like the rectifier tube as Stokes mentioned or it’s a measurement problem.
Checkthese things:
1) Are your meter leads still plugged in the the current inputs? If so, you need to plug back into the voltage measuring inputs. Note: If you try to measure the DV voltage with the meter configured for current measurement you could blow a fuse in the meter. This would make the current measurement section stop working.
2) Check where your meter ground lead is connected when you make the voltage measurements. It should be on chassis ground.
3) Check another known DC voltage (like in another working amp) to verify that the meter is working OK.

Regards,
Tom

[The Captain]

Ah Ok. So with just recto tube in, and spkr plugged back in (?)..

1st 16uf (layout left of three)= 523V
2nd 16uf= 518V
3rd 16uf...... the 1A amp fuse blew/ pilot light lit briefly-? this on AC switch flipped, standby switch not applied yet (open).. wtf.

[stokes]

If you are seeing 523v at the main and 518v at the screen with the standby open then we have a problem.The idea of the standby switch is to keep dc from going beyond the rectifier till the tubes have a chance to warm up.Are you saying the fuse blew before you had a chance to measure the voltage at the 3rd cap?Or are you shutting down after each measurement?Where exactly is the standby switch wired into the circuit.When I install a standby switch,I connect it between the PT CT and ground,this keeps any dc volts out of the circuit altogether.Where is your standby switch?

[Tom Phillips]

Captain,

I have a few quick comments then need to get on the road for our Easter weekend.
1) Voltages seem high even with no load.
2) Voltage drop between 1st & 2nd is higher than I'd expect with the tubes out. This indicates that something is drawing extra current.
3) Fuse blowing is unexpected. Maybe an intermittent short that was jiggled when you were probing.
4) What amp model is this? Is the history discussed in another thread?

Regards,
Tom

[The Captain]

I got both 523/518v at caps once I flipped standby to play- zero v just with AC switch waiting for recto tube to warm up.
Fuse blew as soon as just AC flipped for third 16uf measurement, so MM read zero.
I always shut down and restart one switch at a time between measurements- there's no way Im taking off/ reconnecting a probe to ~500v.

My standby switch is wired as per this (easier with a diagram)- http://www.weberspeakers.com/store/5e3_layout.jpg

It'd be good to establish if this is correct, or not, before I try anything else: I have one tube in, the rectifier- all other tubes pulled out -& I have the spkr plugged in. cheers Capt.

[Tom Phillips]

Captain,
Couple of questions.
Do you have a link to the schematic diagram of the version you built?
Are you using a normal blow or a Slow Blow fuse?
Tom

[The Captain]

Hi there Tom-

yes this schematic I believe is identical to the weber layout I used: http://www.kilback.net/homebrewtweaks/amps/5e3/deluxe_5e3_schem.gif

I have a 1A slow-blo (in UK). Im 100% sure probe didn't shift when fuse blew (Id never touch a probe with 500v around, so I secure probe/ turn on, check/ turn off & reset probe for next check/ cap).
Its a scratch-build, and yes it has history (!). Ive had a PT replaced by weber as it kept blowing fuses- now this new one has blown one.. I think Ive finally lost the cause.

[Tom Phillips]

Hi there Tom-

yes this schematic I believe is identical to the weber layout I used: http://www.kilback.net/homebrewtweaks/amps/5e3/deluxe_5e3_schem.gif

I have a 1A slow-blo (in UK). Im 100% sure probe didn't shift when fuse blew (Id never touch a probe with 500v around, so I secure probe/ turn on, check/ turn off & reset probe for next check/ cap).
Its a scratch-build, and yes it has history (!). Ive had a PT replaced by weber as it kept blowing fuses- now this new one has blown one.. I think Ive finally lost the cause.

OK. The 1A Slo-blo fuse is fine for your 220V line voltage. However, with the standy switch before the first filter cap, there is quite a bit of stress on the fuse when the standby switch is moved to the operate position. You can blow the fuse if yu switch the standby too many time within a short period. The original 5E3 had no standby switch at all and I don't believe that one is needed. If it were mine I would move the first filter cap to the hot side of the standby switch.

I still believe, as I previously stated, that your B+ is too high. It will be lower under load but your readings seem much too high even with all the tubes removed. What is the AC voltage reading of your PT secondary? This could be your main problem. With all the tubes installed and the amp properly set up, I would expecdt the B+ to be less than 400V.

Tom

[The Captain]

Too much switching causing the fuse to blow- Im hoping to heck it could be that and not a 2nd fried PT in under a week!! I do have an inexpensive TAD 5y3gt which may be cause of higher than ideal V iirc.

1) When folk refer to the '1st' filter cap which are they refering to? to move it, Id have to lift out the board/ pots/ inputs/ rewire etc.. I just can't face doing it again! is this a must-do?
2) When you say AC reading of PT secondary -my innitial check procedures now 1/2 forgot you see- can you remind me; do I MM (set to Ac) probe on one red HV point on the rectifier and black probe to chassis, with recto only tube in?
3) Please can anyone help here-- should I have spkr plugged in or out for such general V checkings?

Thanks alot, Captain.

[Tom Phillips]

Too much switching causing the fuse to blow- Im hoping to heck it could be that and not a 2nd fried PT in under a week!! I do have an inexpensive TAD 5y3gt which may be cause of higher than ideal V iirc. .
The filter caps draw a surge of current each time the standby switch is moved to the operate position. This stresses the fuse. That is usually not a problem unless it is done too quickly. This may not be your problem but your voltages seem too high so there may be more stress.

1) When folk refer to the '1st' filter cap which are they refering to? to move it, Id have to lift out the board/ pots/ inputs/ rewire etc.. I just can't face doing it again! is this a must-do? . This is not a big deal. For now, until the main cause of the problem is found, I’d suggest that you just leave the standby switch closed as if the switch were not installed. This is like the stock 5E3 which has not standby switch at all.

2) When you say AC reading of PT secondary -my innitial check procedures now 1/2 forgot you see- can you remind me; do I MM (set to Ac) probe on one red HV point on the rectifier and black probe to chassis, with recto only tube in? .
Yes. Measure from the red wire to ground. Each red wire should measure about the same. Use the AC meter position.

3) Please can anyone help here-- should I have spkr plugged in or out for such general V checkings? .
It doesn’t really matter at this time. I would just leave the speaker plugged in.

Also measure the heater voltage for reference.
Did your transformer come with multiple primary taps? If so, what are they and which one are you currently using?

Tom

[The Captain]

Thanks there Tom, multiple PT primaries yes- 110, 230 and (240v for here), colour-coded, the PT has wiring chart corresponding to weber layout too. Im 110% sure I have that correct. All unused PT wires heatshrink/ sealed ends.

Generally speaking just to inform yourself Im not a right bodger- I have checked all wiring around circuit more than 4/5 times, continuity checks 4 times, cap polarity 5 times, liffted out board etc etc.. basically all conceivable bases covered; all is neat neat neat, and Im almost anal when it comes to care/ safety with checks and preparation.

Im very concerned with that fuse blowing- as I said it happened as soon as AC switch applied/ (standby was still idle/ open) pilot lights briefly, just as my previous PT (now dead) continously blew fuses.

[stokes]

I dont think the repeated switching would cause any additional stress.The caps get charged up when the switch is first turned on and the caps will hold the charge to some degree causing less 'stress" the second and third time the switch is flipped.Cap'n,I would use the pin type probe instead of the clip on type and check the volts one after another on the 3 caps,just keep your non probing hand behind your back when checking the volts,no need to fear the 500volts.Keep the clip lead on the ground or black lead of course.That 5 volt drop from 523 to 518 is just what I have been talking about forever here and in the last thread.I suspect you will see more volts drop when you do check the volts on that third cap.

[Tom Phillips]

Captain & Stokes,

Here are my thoughts and ideas.

I am not really concerned with the 5V drop between the first and second filter nodes. That is a 5V drop across a 4.7kΩ resistor which means that it is only 1ma of current flow. That will not cause transformer overheating or fuse blowing.

I think it is a good idea to concentrate on the PT and hopefully rule it out as a problem. To do this, we should isolate the PT and test it before moving on. This can be done without unsoldering any wires. Remove ALL the tubes and the pilot light bulb. Make sure the Standby switch is in the open position. Now, unless there is a short in the wiring, a tube socket or the pilot light holder (all very unlikely) there is nothing connected to any of the PT secondary windings that would draw current.

Now, here are the tests I would do:
1) Turn the power on. The fuse should, of course, not blow.


2) Measure the AC voltage at each secondary winding. Remember that without a load, the voltage readings will be higher than the final working voltage. I am just doing a sanity check and looking for evidence that may indicate a problem. Note: for these tests, you do not need the speaker connected.
a) 5V winding (measure between the yellow wires)
b) 6.3V winding (measure between the green wires)
c) High V winding. (measure each half separately)
Measure between the red wire connected to pin 4 of the rectifier tube socket and chassis ground.
Also measure between the red wire connected to pin 6 of the rectifier tube socket and chassis ground.

3) Leave the PT connected for an hour or two in this no load condition. It will get a little warm but it should not get hot to the touch. It starts getting HOT then I would turn the power off.

4) You can also measure the no load primary current draw. This can indicate the health and quality of the transformer. The easiest way to do this is:
a) Unplug from the power line
b) Remove the fuse
c) Clip you current meter between the fuse holder terminals and set the meter to measure AC current. Start with the meter set to the highest scale if you do not have an auto ranging meter. You are now ready to measure the current the the transformer draws with nothing connected to the secondary windings. This is the magnetization current.
d) Apply power. Remember that the standby switch should still be in the open position, no tubes installed and the pilot bulb removed.
I’m away from the shop so I don’t have access to my notes so I can look up my historical measurements. However, I believe that the magnetization current should be less than about 50mA.

Let me know what you find out and we will proceed from there. This sounds like a lot of work but it really can be done quickly except for waiting for the PT temperature to stabilize.

Regards,
Tom

[The Captain]

Thanks for detailed thoughts chaps, much appreciate your help.
Right then I have my work cut-out again it seems.. it'll take me a week to do those checks I reckon. I just had a worrying thought- as I read 523v/ 518v on 2 of the sprague 16ufs (iirc it was 390v ish with the previous PT when I got that running innitially-?) I forgot these caps are rated 475v! Im therefore surely unwise to risk any more checks around 520v as they should have blown up? I cant believe Im back at sq1 again- Im really hating this fiasco of an amp now.

[Tom Phillips]

You can get to the bottom of this. Just keep at it one step at a time.
The last checks I suggested will not stress the filter caps because they will not be connected to any voltage at all.
I suggest that we verify that the PT is OK first and then move on from there.
Cheers,
Tom

[stokes]

Tom,this is a brand new PT.We went thru about 120 posts with the last one.Look for the Capn's last thread.I forget the title,but we left the last thread hoping it was just a bad PT to begin with,but since he is having trouble with the new PT getting too hot,I am assuming something is drawing too much current.Although you say not to be concerned with the 5v drop across the 4.7k resistor,I have to disagree,with no tubes he should be drawing no current at all.In any healthy power supply I have never seen more than 1 or 2 volts drop,even across a 10 or 20k resistor,with no tubes to draw current.I would really like to see the voltage at that 3rd filter cap,before discounting the power supply as the cause.If there isnt a larger drop across that second resistor (22k),then I'll agree the 5v across the 4.7k is not the main cause,but 5v is still more than should be there and I suspect the 3rd cap could be the problem.

[stokes]

Just looked,the last thread is titled "Odd normal channel volume".Take a look at that,Tom,it'll give you a better idea of what is going on with this amp.

[The Captain]

Mr. Stokes- Im interested to know the 3rd cap V too, but as previous 2 wereover 500v is it safe to continue forgetting as I did these Sprague blue 16ufs are 475vdc rated? also interested to know how they coped with this v without exploding too :eek:

[stokes]

Spragues can take some overvoltage usually.With no tubes in the amp you should be drawing no current,so the overvoltage will be less devastating.You shouldnt have a problem in the short time it takes to check the volts.

[The Captain]

Thank goodness fuse didn't blow (placcy probe shaft was resting against spkr jack tip testing that 2nd 16uf, maybe that was it-?) anyway BIG relief! so then, with just recto tube in:

1st Sprague 16uf (LHS next to 25uf/250r) was 523v
2nd was 518v
*3rd just checked= 510v.

Maybe now Im getting somewhere..

[stokes]

Although it doesnt seem like a lot,there are too many volts dropping in that B+ rail.I have a dozen amps in my basement and I can open any one of them and I wont see more than 1 or 2 volts drop down the whole line.Unless you have something wired wrong and there is something touching ground and drawing current somewhere (but I doubt thats the case),you have some leaky caps.Even if the PT was the problem and the caps were okay you wouldnt see that many volts dropping further downstream of a faulty PT.I strongly suspect the leaky caps are the source of all the problems you have been experiencing from the start,including the crappy tone you first described.It isnt drawing enough to blow a fuse,but the extra heat slowly burnt up your first PT,and I suspect when you put the tubes in and start putting a signal thru it,the caps likely get worse under the stress of the full load along with the signal.

[The Captain]

Ok, I gathered that those Vs are too high, and shouldn't be lower from one cap to next. The 3 Spragues are still pretty new- which is odd no? they recently replaced 3 TAD 16ufs - a week or so before 1st PT fried Ive just thought.. (*one thing I did do, perhaps 'lazily' was just snip the TADs leads so I could just solder Sprague caps to these rather than lift board out/ solder new leads direct into eyelets. I checked later all continuity & wiring under board etc, so wasn't concerned.. but-?).

Is there any resistor, or wire guage or anything in area I should look at 1st, or just replace all 3 Spragues do you think Mr. Stokes? I wasn't entirely happy with those new PT limiter checks not dimming at power tube stage, as I think you'd concur also. Could you remind me what Vs I should be looking twds instead of my ~523-510v with only recto in.. Cheers alot Capt.

[stokes]

Its very possible the Spragues are just bad,it happens.I am a big fan of Sprague,use them almost exclusively,but like everything else these days,quality control isnt always the best.I dont think the way you snipped the old leads is a problem,I've done many re-cap jobs the same way.I am pretty sure you didnt bring the voltage up slowly with a variac,and if those caps sat on somebodies shelf too long it is possible the electrolyte started to dry and could cause premature failure of the caps.I know you dont have a variac,but using a current limiter,which I know you have now,is an acceptable way to "form" the new caps.I dont think it is as good as using a variac,but I know some people use it and say it is fine.I have a variac so that is what I use.All it involves is,after you put new caps in,just plug into your limiter as you did before,turn the amp on and after a couple of minutes the caps will be "formed".It is definately better than not doing it at all and hitting those new caps with full voltage and current all at once.As for the 523 volts with no tubes in,it seems to be quite high,but lets see what happens when you put the new caps in and put the tubes in.With all tubes in the volts should come down to under 400 volts.I think a true 5E3 should be in the area of 380 volts on the plate.Its up to you,but if you are ordering new caps I like 40uf on the main and screen,tightens up the bass a bit.Leave the preamp filter at 16uf.

[The Captain]

Ive had a word with supplier, saying I definitively nailed a prob to leaky caps (only weeks old) who tho adamant he's never had a prob with all his Spragues will post some more 16ufs once mine are returned- pretty good that (Hotrox UK). The C12N jensen replacement nailed the wallowy bass, so 16ufs will be ok I think. I'll report back once new caps in to just recheck forming with limiter procedure, then (hopefully) I'll be done! great help Mr.Stokes.

[stokes]

Thats great that he will replace them,you got lucky twice,first the PT replacement and now the caps.Forming the caps with the limiter is as simple as just plugging the amp into the limiter and turning it on.Should see the bulb glow for a second or so when the standby is "closed" and the dc hits the caps,then dim when the caps charge up (2-3 seconds) and that is it.I learned to do it with a variac,but then read about using the limiter,I still prefer the variac,but I am told the limiter will work if it is all that is available.Another precaution to take when soldering the caps in is to use a heat sink,an alligator clip works fine,just clip it between the solder joint and the cap itself to keep the heat from getting to the cap.Hopefully this will solve the problem,I cant see anything else at this time.Looking forward to hearing soon that the amp is up and running okay.

[Tom Phillips]

Captain,
Good luck with this. However, I encourage you to keep an open mind because I don't see how replacing the filter caps again will solve your reported problems of fuse blowing and overheating power transformer.
Regards,
Tom

[Gee]

Ive had a word with supplier, saying I definitively nailed a prob to leaky caps.

Captain, I don't really think you have got to the point where the caps are the problem.

From reading through this thread it sounds to me like you have a wiring problem somewhere downwind of the 1st cap.

Here's something you could try. With only the rectifier installed, unsolder the 1st, 2nd and 3rd caps (+ ends), the 5K 1st dropping resistor and the second 22K dropping resistor. Just unsolder one end for each and pull the lead up and away.

Then measure the resistance to ground at each of the points you just unsoldered. All should read open circuit.

Resolder the 1st cap and check resistance to ground. Then turn on amp and standby, i.e. so that you get the full DC voltage on the first cap. Then measure the DC voltage to ground for the connection points on the board for the 3 cap (+) connections. Then resolder the next component down the line (obviously with amp off). I.e. 1st cap, 5K, 2nd cap, 22K, 3rd cap. And check the resistance to ground and then the voltages. Progress your way along the board from the 1st cap and note the voltages.

If you then post back your results, a bad cap or short should be evident from the readings.

[The Captain]

Captain,
Good luck with this. However, I encourage you to keep an open mind because I don't see how replacing the filter caps again will solve your reported problems of fuse blowing and overheating power transformer.
Regards,
Tom

These filter caps weren't put in to solve a PT prob, but just for vanity issues (the TAD cheapies may have saved a PT if Id left them), but thanks anyway for input to this Q.

[Bruce / Mission Amps]

For what it's worth.... keep in mind I might go through 200 to 300 power supply caps a year but I have had at least a dozen or more bad Sprague 16uF and 20uF filter caps over the last year.
They were all internally shorting and pulling the B+ down to fuse blowing levels.
This has been a big enough problem for me that with the rediculous price Sprague is asking for the caps, I started phasing them out and using the less expensive, RUBY 22uF/500v and F&T 20uF/500v caps.
So far I have had ZERO failures with these.

[The Captain]

Thanks chaps- just did Gee's tests:
(with recto in only, standby switch closed so AC switch turns all on, +cap legs lifted and one leg of each R all continuity as should be) so..

1) 1st (LHS) 16uf + leg back in: 537v
2) with 4.7k back in: 530v
3) 2nd cap back in: 531v
4) with 22k back in, @ 1st cap: fuse blew (no pilot light & fuse really blackened)- ?
5) after a wait.. refused & 3rd cap in (all bits back in now), @ 2nd cap: 529v, @ 3rd cap 516v.

Fuse is odd, maybe it just had enough already what with all the onning and offing.. the wait was ~20mins before retrying. If results help any diagnosis it'd be good, as Im not clear. Can anyone say what I should be looking twds (I know 380v ish once tubed) as in above scenario, ie should they all be equal and much lower than ~520v?

[Gee]

1) 1st (LHS) 16uf + leg back in: 537v
2) with 4.7k back in: 530v
3) 2nd cap back in: 531v
4) with 22k back in, @ 1st cap: fuse blew (no pilot light & fuse really blackened)- ?
5) after a wait.. refused & 3rd cap in (all bits back in now), @ 2nd cap: 529v, @ 3rd cap 516v.

That fuse blowing is significant. Did you measure the resistance to ground at all the points above? I would look for a V3 or V4 socket lugs shorting out or stray wires shorting to adjacent lugs OR a short after that 22K resistor on the board.

Something is not right with your circuit. From my understanding of the 5E3 circuit, with no pre or power tubes in place, there should be nothing to draw current through the circuit. You have about 0.5uA going through something.

Reconnecting the 3rd cap may have fixed the short - just a thought.

Also the 2nd cap voltage is 1V higher. Let's put that down to the DVM.

Are you sure that your DVM leads are in the right sockets for measuring voltage as opposed to the socket configuration for measuring current?

[stokes]

Captain,
Good luck with this. However, I encourage you to keep an open mind because I don't see how replacing the filter caps again will solve your reported problems of fuse blowing and overheating power transformer.
Regards,
Tom

Tom, the idea is that the leaky caps are drawing more current than the PT is rated for,causing overheating and eventual PT failure.His first PT eventually failed,my theory is that the cap is not shorted,as this would blow the fuse,but leaky enough to slowly overheat the PT and cause failure.If you read the prior thread I pointed you to,you will see we did some testing with a current limiter,that took quite a bit of explaining to complete.When he got the new PT he and Steve thought the test showed all was okay,I had my doubts,he went a head and used the amp for a while and he is now experiencing PT overheating again,so we are back to tracing the short,or as I believe,leaky caps.If you got something better,by all means please let us know,as this is getting quite frustrating on both sides of the ocean.

[Bruce / Mission Amps]

You have to have a dead short to ground after that 22K resistor.
Pull all those power supply caps off their dropping resistors and measure the DC reistance to ground from the connection point of the to 12AY7 100k plate load resistors, then at the 4k7/5K1 to 22K junction.

[The Captain]

There was no resistance to gnd at any stage or any place of checks. MM set for DC of course. All valve pins/ continuity checked numerous times around amp, as is wiring etc etc..

I can't be sure now these caps are defo shagged..? Im no closer knowing wtf is wrong with it.. unless either a 2w resistor is a duffer (checked again- read 4.7k/ 22k) or a crap 0.1 uf (RS) or bad 5Y3GT (TAD) could be the cause.

What voltage is normal at the filter caps with just recto in; am I 100v too high? should these caps all read the same?

[stokes]

Cap'n,with just the rectifier in the voltage at each cap should be the same as there is no current being drawn without tubes in the amp.Whether or not the 500v is too high is not relevent to the problem.As I have said before,any healthy power supply I have ever seen shows no more than 1-2 volt drop down the whole line.The fact that you are seeing a 5volt and 8 volt drop indicates there is current being drawn,the only thing in the circuit to draw current is the caps,and they would not draw current if they were not leaking.If the 500+ volts you are getting out of the PT is too high,it is because the PT is not the correct one.Lets get to the bottom of the overheating PT,then we can put the tubes in and see how high your volts are under load.

[The Captain]

Ok Bruce, so Im pulling the 3 16uf caps + sides up again, powering up both switches closed and measuring V at that 100k junction to ground & at junction of 4.7k and 22k? forgive my ignorance but when you say 'DC resistance'.. Im muddled- resistance in ohms, DC as a voltage..

[Tom Phillips]

If you got something better,by all means please let us know,as this is getting quite frustrating on both sides of the ocean.
OK. Below are my current thoughts. (I stand by my previous posts too)

If you read the prior thread I pointed you to…
Yep, I actually followed that thread as it played out and I see why this has been a frustrating experience for the Captain and those who are trying to help. The Captain is just learning electronics, troubleshooting techniques and the proper use of test equipment. Not an easy thing to do.

… fuse really blackened)- ? Fuse is odd, maybe it just had enough already what with all the onning and offing…
The black in the fuse indicates that it blew because of a heavy current surge as opposed to a long minor overload. It appears that you have an intermittent short and when it does happen it draws lots of current.

Tom, the idea is that the leaky caps are drawing more current than the PT is rated for,causing overheating and eventual PT failure… my theory is that the cap is not shorted,as this would blow the fuse,but leaky enough to slowly overheat the PT and cause failure
We don’t agree on this point. I calculated and reported previously that the “leaking” current is around 1ma. IMHO that is not the cause of the transformer overheating or the fuse blowing. There is a low resistance short somewhere. Hence the blackened fuse glass that Captain reported. It appears that the short is intermittent. That can be difficult to find. Furthermore, if we are to trust the plate current readings that Captain reported, the power tubes are drawing way too much current. This could be because the plate voltage is too high or the cathode bias resistor is off. The high bias current is much more significant than the extra 1ma of “leaking” current. I still suspect that the PT secondary voltage is too high. That’s why I asked for the AC voltage readings of the unloaded secondary windings a while back. I’ve been burned before when I assumed that a part was OK just because it was recently replaced.

Cheers everybody,
Tom

[Enzo]

DC resistance means we are not talking about impedance - which would be "AC resistance." It just means your ohms scale on the meter. We get used to using terms like DC resistance because so many times we have to correct people who wonder why their 8 ohm speaker (impedance) measures only 6 ohms on their meter (resistance), as an example.

I wonder why that 4.7k resistor drops the voltage 7 volts BEFORE the second cap is reconnected. It is only a milliamp, but where does it come from. Or is the wiring from these nodes still attached?

A dead short to ground on the low side of the 22k would be like 537 volts across 26.7k (22k+4.7k). That is roughly 20ma. I don't see that blowing fuses.

What meter are you using?

[The Captain]

Im using a Mastech M-830B DMM perfectly fine it is, not ££s though.

Im now wondering if the replacement PT has correct 2ndary windings! I checked the PT model no. was correct of course; there surely tho must be a simple solution to this prob. Can someone remind me how to check 2ndaries V again.. is it: reading MM for AC, pin 4 of 5y3GT to ground, and rectifier tube in only?

If not this and or caps, I'll have to take whole thing apart, bin power tube Rs caps etc, replace and start again I guess.. I don't think I can face just for another prob to crop up.

[Gee]

Im using a Mastech M-830B DMM perfectly fine it is, not ££s though.

Im now wondering if the replacement PT has correct 2ndary windings! I checked the PT model no. was correct of course; there surely tho must be a simple solution to this prob. Can someone remind me how to check 2ndaries V again.. is it: reading MM for AC, pin 4 of 5y3GT to ground, and rectifier tube in only?

If not this and or caps, I'll have to take whole thing apart, bin power tube Rs caps etc, replace and start again I guess.. I don't think I can face just for another prob to crop up.

Take things one at a time. Whether the PT secondaries are wired correctly is not the immediate issue. You have something shorting out or a failing component that is causing current to flow when there are no tubes.

One way to find out the problem would be to remove all components from the board and add them back in one at a time. Then each time turn on, measure voltages and resistance to ground and you should find the problem when you either add back in the problem component or disturb or fix the short.

[Tom Phillips]

... Can someone remind me how to check 2ndaries V again..
See my first post of 4/8/2007 in this thread.

[Bruce / Mission Amps]

OK... I've been very ill over the last three weeks and haven't been real good about reading all the threads...
I realise now you previously said the main 1a fuse blows with a black film in it... that does mean very heavy and instant current flow.

At +500vdc the 22K, preamp dropping node resistor would limit the current to around 23ma. The 30VA power tranny and primary fuse could suppply this current for hundreds of hours with no serious problems, but the 1/2 watt 22K resistor would burn up.
Using simple Ohm's law...
23ma x 23ma x 22Kohm = 11 Watts!!

What causes a fuse to blow like that is something like a dead shorted filament supply or a miswired socket where the raw high voltage AC (if the rectifier) or the B+ from the rectifier, standby, + ends of the power supply caps, vacuum tube's plates or screens is shorted to ground... maybe even through the filament supply ground reference... instanly or after a few moments when the tubes warm up.. etc.
Another cause is when using a multi tapped primary power tranny.
As used for the combination of North American and Euro consumers, where there is the possibilty of the primary windings being miswired or accidentaly wired out of phase if it is a split primary wind.
If yours is an auto transformer, (where there is not seperate windings) using taps pulled to create the proper turns ratio for 100v, 120v, 220v and 240v mains, then this is probably not the case.
Unless... the unused leads have not been dressed properly and are shorting somewhere together or to the chassis or mistakinly wired wrong.
Because you are a novice builder and the fact that you have had two transformers go bad or over heat and blow fuses suggest to me that, there is a miswired lead or socket somewhere that has simply eluded you right now and it is after the power tranny.
The lead that can cause this has to be one where excessive current can flow and that would be close to the B+ rail circuitry, filament supply or tube socket.
This is where years of trouble shooting experience is so handy.
Many of us, who if having this amp on our benchs in real time, could probably sort it out in a matter of minutes, but being thousands of miles away and having to read text makes it very hard and frustrating for all.
I know this is a pain in the rear end but you've come this far....can you get a camera out and take good, focused pictures of each tube socket, the standby switch (if used), the main filter caps and the wires from the PT... did you say it was a second or replacement WeberVST Deluxe power transformer?
I did much of the BETA testing for Weber and their transformer line a couple years ago and I did real destructive testing on them.
Although inexpensive, Chinese made and not the best made iron in the world,
if the transformers are not defective and the circuit is correctly assembled, their Deluxe power tranny is more then capapble of running a 5E3 amp for years.

[Bruce / Mission Amps]

No, what I'm talking about doing is measuring resistance in ohms to ground from the B+ nodes with the postive end of the filter caps lifted.
Other possible problem areas need to be checked and the amp has to be off and unplugged from the mains with all the B+ drained off.

With your black negative lead of the DVMM grounded, use your red positive probe from the rectifier lug 8, the standby switch lugs, the centertap of the OT, lugs 3 of the power tubes, both sides of the 5K1 resistor, both sides of the 22K resistor and the filament wires on all sockets with the center tap of the filament supply lifted if grounded.
If it using the 100 ohm balancing resistors, then that's OK, you'll see 50 ohms or so unless shorted to ground.

[The Captain]

Much help there Bruce: Yes thought a std continuity test must be what you meant (a few terms for similar things now and then tricky for the learner): so all from pin 8 5y3gt along B+ rail nodes read no resistance to ground.

I do get resistance to ground at all filament nodes/ tube heater pins. I am right in assuming this is correct.. right? cheers capt

[MWJB]

Yes, a little resistance (probably ohms rather than several hundreds of ohms or Kohms) to ground from the filament connections will be normal as there will be relatively little resistance from either leg of the heaters to the grounded centre tap, or if you have used 2x100ohm resistors these of course connect directly from each leg to ground...then there's the pilot bulb bridging the 2 legs to consider...removing this may affect your reading?

[The Captain]

Sh*tbags- I thought I had it solved there!! rereading hoffman excellent heater info I saw those 100ks would go to gnd therfore being resistance there anyway.. one thing I did read there was caps leaking/ noise resulting etc: I did notice some noise when I played it last, like a 'ghosting' distortion from the spkr when strings played increacing on hitting strings harder, even at low vols/ clean sound: an only slightly irritating distortion in background. Just seems worth mentioning this now..

Im waiting on cap replacement so will reprt back rechecks once formed/ all back in etc. Cheers folks, Captain.

[stokes]

Sh*tbags- I thought I had it solved there!! rereading hoffman excellent heater info I saw those 100ks would go to gnd therfore being resistance there anyway.. one thing I did read there was caps leaking/ noise resulting etc: I did notice some noise when I played it last, like a 'ghosting' distortion from the spkr when strings played increacing on hitting strings harder, even at low vols/ clean sound: an only slightly irritating distortion in background. Just seems worth mentioning this now..

Im waiting on cap replacement so will reprt back rechecks once formed/ all back in etc. Cheers folks, Captain.

I thought we covered the resistance to ground earlier,and found no resistance to ground on the B+ rail,no?If you read one of my earlier posts I said the caps are likely the problem if there is no other resistance to ground on the rail.That is why I arrived at the caps being the problem.The ghosting you describe is a symptom of bad filter caps.If you are using 2x 100ohm resistors for a CT on the heater you will see 50 ohms on each leg to ground,if you have a CT on the heater winding you will see close to zero.In this latest thread the fuse blowing was intermittent,I was assuming this was something you were doing wrong.I hate to sound like a broken record here,but if you see no resistance to ground in the B+ rail with no tubes in the amp and there is a voltage drop,the only thing that could be drawing current is the cap.This is of course assuming you wired everything right.Some time ago,Capn,I was going to suggest you ship the amp to me and I would look at it for you,and as somebody pointed out this is a problem that any one of us could find in 5 minutes if we had the amp in front of us.I was really hoping somebody would come up with something you and I arent seeing here Capn,but I still keep coming up with leaky caps.As someone pointed out the amount of volts being dropped with no tubes does seem to be small,my theory is that when the tubes are in the amp and the caps are stressed this seemingly insignificant current draw becomes worse,not to the point of popping the fuse,but enough to slowly burn the PT.You have to remember before the first PT failed you had a 3amp fuse where a 1 amp belonged.It is possible that the problem would have blown a 1amp fuse,but not a 3amp.Hey,I could be wrong here,as all we have to go on is what you are telling us,so keep the ideas coming guys,I'm sure we all want to see this amp working,after all the effort and typing we have been putting into this one.I dont know what the cost of shipping this amp would be,but I would be willing to have a look at it if all else fails.

[The Captain]

Yes I was re-rechecking reisitance as Bruce and others have helped out with ideas/ let them know etc, and as that fuse went around the 22w/ 2nd cap back in.. I take in all your input Mr. Stokes! & thanks so much for the offer of taking a look at it- some kind folk here for sure. Now then, this may put a spanner in the works: I found my old caps (TADs- thought I binned them) so put them in & fired it up= ghosting as before (as above slight but there), hot as before too (ie after 15 mins all warm, after 1.5 hours playing power tubes were fiercely hot/ chassis pretty hot/ PT hot). I'll do those V checks no tubes etc tmrw. I wasn't expecting these findings though. Im eying the .1uf coupling caps now (given to me, perhaps old..)- just a vague idea as mine always are. Cheers Capt.

[stokes]

While you have the tubes in you might want to check the current draw on the power tubes,I think,if I remember correctly,things are getting hazy with all the info we've been throwing around,you were drawing in the area of 88ma's.With the meter set to measure ma's put the red lead on your OT CT and the black lead on pin 3 of one power tube and see what we got.If you are reading over 30ma's we have to address that.

[MWJB]

You are UNDOUBTEDLY going to get a reading of over 30mA's with a stock 5E3, running a 250 or 270ohm cathode resistor and 370v or more B+. Run plate voltage, plate current and cathode voltage by us before you take any action on this front.

I would expect your current to be around 35-40mA per tube (normal current range for a 5E3), if it's higher than 40mA try a 330ohm cathode resistor. This will push up B+ voltages but should reduce overall plate dissipation slightly.

[The Captain]

Im waiting on those replacement spragues- Id post up current current, but temp caps are 450v so my knees would be knocking if 520v are flying about.

So another course of opinion it seems, notably the idea maybe 40-43mA is not quite so alarming, caps poss ok so try increacing the bias R to 330r- to my original course/ Mr. Stokes' idea that caps most probably cause for a defo alarming current drop/ amp heat shinanigans; a little tricky to know for sure where to go.. such is nature of forums I guess.

[stokes]

I was basing that 30ma's on you saying you have 500+v's with no tubes,and estimating that with the tubes in you are still going to have 400+'s.In a normal 5E3 35+ma's would be okay,but that would be if you had under 400v's on those plates,with over 400v's you would not want to see to much more than 30ma's.Class A with a pair of 6V6's at 400v's would be about 30ma's.

[Bruce / Mission Amps]

With correct primary voltages applied, most 5E3 plate voltages (with respect to ground) are around 350vdc to 365vdc and the power tubes idling mildly hot at around 36ma-40ma.

[The Captain]

Having a word with Weber giving Vs and current, they just said.. 'use the red/whites instead'.. these secondaries are 540v/? as oppsed to 640v/150mA between the 2 reds used as per normal on layout.

Are these usually used for a different amp? meaning I'll get significantly lower B+ ? Im not sure if thats good, bad or what.. Im still no closer to getting the amp as is solved whatsoever!

[MWJB]

640VAC (red wires) should give you spot on 5E3 rectified B+ voltages.

540VAC (red/white wires) will give towards the low side of 5E3 voltages even if you use a GZ34 instead of a 5Y3.

Sounds like you either have the wrong PT or have connected the wrong primaries?

[Bruce / Mission Amps]

The Chinese made Weber W25130 power transformer is a loose version of my old American made TMI 20 watt PT.
My TMI PT is a loose version of a silver face Deluxe Reverb PT.
However, my TMI PT had two different high voltage taps.
The 275vac secondary for my EL84 Crusader amps or higher 340vac secondaries for my 6V6 Crusader amp and a low power 6L6 amp I was building.
I don't use TMI anymore but my new USA made MA20/18 power transformer is very similar to my old TMI PT.
The 6V6 secondary is now voltage tweaked for the 5E3 kits when using a real NOS 5Y3GT rectifier but works great with a GZ30 or GZ34 in the 6V6 Aurora amp.
Side note:
When helping Weber with their transformers a few years ago I suggested he have his Chinese winder copy the specs of my Chicago made TMI power transformer.
I dont know if the more recent W25130 PT is differrent now, but at the time, the Chinese Winder (or maybe it was Weber) called it a 150ma PT (I doubt it) while mine was rated at +120ma.
However, I beta tested both the Chinese Weber PT with my TMI PT in a test jig I invented, which included loading the filament supply leads down to their current rating. I took these trannys right to the point of near meltdown.
I ran the high voltage secondaries at 80ma for 24 hours, 120ma at 12 hours, and for 2 solid hours at 150ma!
The Chinese one got scarey hot... smelly hot, feel it on your face hot....hotter then the hubs of hell!
My TMI got very warm too but not like the Chinese one.
Regardless, neither PT gave up the ghost in what was an extreme two hour, near total destruction test.
What I found was, at the same secondary loads my TMI PT could handle the same secondary currents but with less heat for some reason.... maybe TMI was more conservative and mine is +150ma too... based on some "Chinese duty cycle" rating. Ha ha...

[The Captain]

I have the reds connected ok, always did.. just Weber succinctly(!) just said.. 'use the 540v red/whites instead'. Even more confused now, I think I'll just have to live with a hot PT until it dies, then just shelve this amp.

[MWJB]

OK, so long as your heater voltages are 7VAC or under and over 6.2VAC (important, confirm this) and your B+ is under say 385v with a 270ohm cathode resistor, or under 420v with a 330ohm cathode resistor, WITH EITHER pair of B+ secondaries you will be OK.

If, with the 540VAC secondaries you only get 300-320vdc recitified (with 270ohm cathode resistor), that's not going to really be a problem (it would be nice to have over 30mA on the 6V6s), you can always use a stiffer rectifier to bump up the B+ *if you want*, OR try a smaller cathode resistor like 165-200ohms (don't do both). If the amp is just for home use, the lower B+ might even be in your favour, even try some 6K6s to drop power slightly?

PT heat - You have told Ted Weber what B+ you have with the replacement PT but I couldn't find a note in the previous posts (apologies if it's in there) so it's not easy to determine whether your high PT temp is solely due to the PT itself (wrong part/primaries) or an indirect symptom like very high plate currents due to raised B+. In any case some PTs do run "burn yourself after 20mins" hot under normal operation, as Bruce intimated. What you consider "hot" and what your PT considers a regular day in the office might be the same thing. On the other hand you could be right and the PT might be a problem...so just keep playing it and IF it dies you were right. It only takes an hour tops to chuck in a new one and you are now well versed in how to do it :-)

Building amps (especially 5E3s) isn't as easy as "put tab A into slot B", a lot guys get lucky...a good proportion don't. That's why kits like Bruce's Mission kits sell well (proven parts, construction methods, instructions and support) and cost more than other bare bones kits. You have jumped in at the deep end and you're probably swimming better than you think, but with limited experience it's difficult to determine with which components your problems lie, if there is a component issue at all - things like that only come with experience.

Now stop that silly "sheve it" defeatist talk, connect up the 540VAC taps (be aware that this is a nominal voltage, once you allow for manufacturing tolerances and 240v operation the reality might be quite different) and tell us what you've got. You're in danger of snatching defeat from the jaws of victory (I might have said that before? Well, everyone has to have a catchphrase.)

Good luck & chin up.

[The Captain]

Thanks for that info- I will try the other wires instead as a last resort. Its started blowing fuses again so Im defeated now Im afraid.

Thanks to all for help though- especially Mr. Stokes!
Captain

[stokes]

As we have all said,stick with it Capn,you got a lot of good people behind you,we'll have none of that "shelving" talk.I have been away for a few days,so I have to do some reading to catch up on where we are at here.

[The Captain]

I really am at my wits end now Mr. Stokes.. I need therapy again.. board's out now but the bits seem to be just laughing at me. Wiring checked THOUGHRALLY for the *th time- ALL good.

Q- if I had old or dried out 2x 0.1uf caps, could they lead to that mild distorty ghosty sound, or even this hot PT/ fuse popping piffle? Capt

[MWJB]

Re. the .1uf caps. Desolder the ends farthest from the plates resistors (or cathode @ V2,8), connect your red meter lead to the free end of the cap (making sure the free end can't short to ground), the black lead to the chassis. Measure the DC voltage leaking through the cap, replace if over 0.1v.

Having said that, did you get these noises with these caps and the old PT?

I wouldn't go changing anything else until you have the lower voltage secondaries wired up and confirm voltages/currents.

[The Captain]

No didn't get thse noises with old PT (same caps).

I hooked up the other red/white secondaries ('540v').. no sound at all, just background murmurs. Desoldered, resoldered, checked pins 4 & 6 recto.. same. Im nervous about even turning the amp on now.

[MWJB]

What voltages did you get (AC @ pins 4 & 6 of recto, dc @ pins 3, 4 & 8 of 6V6s)?

The worst thing that can happen is that the amp will smoke/smell/blow a fuse (as long as you have your hands in your pockets) and you'll have to turn it off & replace the PT...which you suspect anyway...and you've had practise doing.

Get back on the horse.

[stokes]

Use the current limiter to fire it up for the first time if you are unsure of anything.