Tube Squeal and Hum, 1st Stage Pre-Amp Grounding

[@LarsonGuitarPlayer]

Will save this and watch again when needed. It's a next level knowledge for me. Gonna let this sink in. Thanks! ❤

[@cascadian3679]

Excellent! Thank you very much sir. I just resolved a very annoying and rather loud 120 Hz hum from the 1st stage 12AX7 on a 1964 Univox U-42 (schematic sometimes labelled U-45). One must separate these grounds from a terminal with several other component grounds and wires to other grounds eventually landing on chassis at the speaker output and instead route them to the input ground to chassis. :)

[@jetblakink]

This is absolute GOLD!… Thank you so much.

[@triclone123]

I love it. Thank you!

[@voxpathfinder15r]

If I lift the ground on the amp to test for squeal to go away and nothing changes. Is it safe to assume a 7 pF capacitor from plate to grid on the first stage isn’t the fix?

[@deepblueharp]

The 7pF will not fix the squeal.

[@voxpathfinder15r]

@@deepblueharp I tried the smallest value in my capacitor switch box with a 100 picofarads didn’t work, I assumed the value was too large. Then another source I read talked about going between plate and cathode of 100-350 picofarads. I had those values at my disposal, didn’t work. In both cases it introduced hum as well. And the internet claimed these were fixed introduced by fender & Marshall.

[@johanhansson4574]

@@voxpathfinder15r There are other causes of squeal than bad ground. In Marshalls case aka the 2203 it's signal wires going back and forth.The bandage of 100pF on the input tube stopped the oscillation. One should think carefully about the layout. When building higher gain amps doing the old turret board type with tubes on one side pots on the other wires going back and forth isn't a good layout. Modern amps have the tubes placed in the middle of the circuit with the components around them with short pcb traces. U can do the same with a turret board so U get really short wires to the tube socket.