Stromberg Carlson AU-42 Restore and Loud Squeal

[misterc57]

I am trying to resurrect this AU-42.

I have replaced every cap with the exception of two domino style caps, C5 and C8.

Also installed a 68K grid stopper at V1.

V1 is a 6SF5
V2 calls for a 6J5 which I do not have, I am using a 6N7 and have jumped the grid pins 4 and 5 to make the 6N7 work.

After completing all the work I am hearing a very high pitch and loud squeal with all pots at zero.

I have scoped the output.

Suggestions greatly appreciated.

Technical manual in pdf below.

Thank you, MarkO

radiomuseum-stromberg-carlson-co-amplifier-au-42-2482481.pdf

[misterc57]

Just recalled that I should check for OT primary wires reversed.

[Leo_Gnardo]

Just recalled that I should check for OT primary wires reversed.

Yep! Certainly worth a try, doesn't take long.

However, if OT wiring wasn't changed can't see why it would start squeeking due to incorrect polarity at that point. Sometimes old tubes set up a squeal, also insufficient power supply filtering.

Domino caps? Sometimes one has been at the root of some problems I've encountered. There was one x-pensive boo-teek tweed a customer brought, the builder used all new, good parts except for one domino which he reckoned was vital for the "vintage tone." Amp sounded like a hurricane with whooshing wind and crackling lightning. It was the domino. Swap in a competent cap and now the $3000+ boo-tiki sounded proper.

[misterc57]

I had a used pair of Made in China Coke bottle 6L6GC that I had never tried before and was suspicions of. Swapped output tubes and the squeal is gone.

I am left with a -57 db 120 Hz hum. Do I need more filtering?

Specs call for 40 uF and 30 uF. I installed 47 uF and 30 uF.

Can 120 Hz hum be caused by mis-matched output tubes? Though these same output tubes sound fine in my AU-34 amp.

[misterc57]

Treble pot does not seem to do much to my ears. There is a 1M pot with a domino 500 mmF cap. Any suggestions to make this tone control more effective?