Checking current

[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.