Checking current

[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/homebrewtweak..._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/homebrewtweak..._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