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Old 10th April 2012, 05:24 AM   #1
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Default Determining the reason for mystery? cap in schematic.

In a parallel thread I started off about the problems I encountered wiring in a volume pot, a question put to me during the course of the thread was: the purpose of a cap (circled in green) on the attached schematic that came with this little SET amp I am attempting to troubleshoot. I checked and it`s not a mistake as they are definitely wired in this way on both my EL84s

If I can`t even wire in a simple pot then you can imagine how far over my head this question is.

So any ideas on the purpose of this .001 cap between Pin 7 of the EL84 and B+..?

And of course, if anything jumps out at anyone regarding this particular schematic as being particularly problematic or improperly executed I`d be interested in your observations and suggestions.

Thks in advance. Leon
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Last edited by devilsindetails; 10th April 2012 at 07:52 AM. Reason: spelling
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Old 10th April 2012, 07:19 AM   #2
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Hi devilsindetails
It's a bit hard to make out in that scan but it appears to be conected between anode of output tube and B+ supply.
My best guess would be to stop some high frequency oscillation.
It could have been connected from the anode to ground but would require a higher voltage cap. Maybe done for cost reasons.
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Old 10th April 2012, 07:20 AM   #3
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Correct! It prevents the EL from oscillating.
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Old 10th April 2012, 07:43 AM   #4
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Quote:
Originally Posted by Stefkorn View Post
Correct! It prevents the EL from oscillating.
Thanks to you both, now I can return to regular programming and continue troubleshooting my SE EL84...as well as answer the question with proper attribution to both of you, of course.

Just for my own curiosity...if this cap (which presently is one of those small clay-orange ceramic discs) were off spec...ie: bad...any idea of what the sonic signature (symptom) would be?) High pitched squeal?

As I am replacing the caps in this amp I imagine I shouldn`t be sloppy and ensure I replace these as a matter of course as well?
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Old 10th April 2012, 07:44 AM   #5
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Quote:
Originally Posted by Bibliophile View Post
Hi devilsindetails
It's a bit hard to make out in that scan but it appears to be conected between anode of output tube and B+ supply.
My best guess would be to stop some high frequency oscillation.
It could have been connected from the anode to ground but would require a higher voltage cap. Maybe done for cost reasons.
Thanks Biblio...
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Old 10th April 2012, 07:53 AM   #6
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Oscillation probably outside of audible range.
Possible effects could be overheating of the tube and maybe some audible distortion.
Maybe even interference with AM radio reception if frequency high enough.
Ceramic caps are pretty reliable but replace them if you feel more comfortable.

Last edited by Bibliophile; 10th April 2012 at 07:57 AM.
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Old 10th April 2012, 07:56 AM   #7
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Quote:
Originally Posted by Bibliophile View Post
Oscillation probably outside of audible range.
Possible effects could be overheating of the tube and maybe some audible distortion. Ceramic caps are pretty reliable but replace them if you feel more comfortable.
Thanks...your answer may have helped me identify why one of the 84s was running hotter than the other after I replaced all the caps (the exception being these little buggers)
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Old 10th April 2012, 08:04 AM   #8
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Could be a tube problem.
Try swapping it with the other channel and see if the problem moves.
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Old 10th April 2012, 08:12 AM   #9
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Quote:
Originally Posted by Bibliophile View Post
Could be a tube problem.
Try swapping it with the other channel and see if the problem moves.
Yes...I was thinking ahead of myself...I was running the tubes earlier in the same amp prior to the larger capacitor renewal I undertook and both tubes were behaving equally...after my cap job one was running hotter...so I though perhaps it was likely because I hadn`t done ALL the caps. But yours is the best first course in troubleshooting. THKS
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Old 10th April 2012, 08:27 AM   #10
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If you have a multimeter, measure between control grid an ground.
Normally you will see 0 Volts. If it is even a little positive, you could have a leaky coupling cap from the driver. What sort of caps did you use ?
Control grid is pin 2.

Last edited by Bibliophile; 10th April 2012 at 08:40 AM.
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