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Loud hum, and high AC at idle on one output tube.
Loud hum, and high AC at idle on one output tube.
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Old 15th November 2019, 06:53 PM   #21
JMFahey is offline JMFahey  Argentina
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Donīt overthink it, go for the simplest hypothesis: that stage is oscillating like crazy, all symptoms match that.

1) as suggested by the bedouin, pull V3, the phase inverter and check: do the tubes still oscillate/hum/redplate?

Will add more questions later but start by answering this one.

Throwing 1000 hypothesis on the table and trying to solve all at the same time only muds the field.

2) A couple suggestions about your image posting problems:

a) you linked a TDPRI image which can *only* be seenby currently logged in TDPRI members and nobody else.
You see it because you are so, most others are not, and DIY Audio Forum even less so it can not grab that image to display it here.

b) you are trying to post a PDF as if it were a graphical image, but it is not, so it will not display.
Solution is to open the "advanced" window when you re posting, and **attach** the PDF file as an attachment.
It still wonīt display inside the answer but clicking the attachment it will open in Adobe Acrobat or maybe some browser PDF extension.

Hey, look at this, I attached a Fender Blues Jr .GIF image, straight from my own hard drive (so no external hosting involved), and in a format which does not require Adobe Acrobat, can simply be opened in another browser window, and to boot, can be edited (meaning add voltages, whatever)
Even better, you can see a thumbnail inside this answer.
Attached Images
File Type: jpg Fender-blues_jr.jpg (503.8 KB, 36 views)
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Last edited by JMFahey; 15th November 2019 at 06:57 PM.
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Old 15th November 2019, 10:50 PM   #22
bendyha is offline bendyha  Sark
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Quote:
Originally Posted by JMFahey View Post
................. A couple suggestions about your image posting problems..........
Thanks, I think I get it now.


And thank you for the other suggestions everybody.....something to be getting on with, I will get back with answers when more is ascertained.
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Old 15th November 2019, 11:07 PM   #23
bendyha is offline bendyha  Sark
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Quote:
Originally Posted by baudouin0 View Post
I would then suggest removing the driver valve v38 and seeing if the output stage is the problem.
The problem is still ther with the PI removed.
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Old 15th November 2019, 11:15 PM   #24
bendyha is offline bendyha  Sark
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Quote:
Originally Posted by baudouin0 View Post
... This could produce a shorted turn causing all sorts of HF strange behaviour. .................
You could check DCR on each transformer primary, but you may have to measure inductance. You could put the transformer across a signal gen I guess.
The DCR measurments of both sides of the primary seem OK, but checking the inductance, not done that yet.
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Old 19th November 2019, 03:26 AM   #25
ChrisABC is offline ChrisABC  Canada
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check here:
page 65(41)
https://americanradiohistory.com/Arc...cs-1953-05.pdf

page 55
https://americanradiohistory.com/Arc...-1950-11-R.pdf
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Old 21st November 2019, 08:58 PM   #26
bendyha is offline bendyha  Sark
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Originally Posted by ChrisABC View Post

Thanks, I'll read through them.
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Old 21st November 2019, 09:02 PM   #27
baudouin0 is offline baudouin0
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If your really stuck you could add say 47pf caps (high voltage) from both plates to ground since you say your meter lead stopped the oscillation and the hum stopped. Then you may be able to find the underlying issue.
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