Metropoulos Forum

Since you mentioned earlier that the noise disappears when bypassing the presence pot, try disconnecting the nfb wire from the impedance selector and see if the noise goes away. The amp will be a bit looser and overdrive a bit easier with the nfb disconnected but lets see if that makes a difference. If that helps you can try hooking it back up and then swap the plate leads on pin 3 of the 6v6's.

Another thought that I had was that the diodes may be damaged from the soldering process and causing this noise in the power supply. Did you use individual diodes for the HT voltage or a bridge rectifier?

Have you posted pics.

I'm afraid that a single scope trace does not help me (maybe someone else can help?)

I feel your pain. I had a strange issue building a clone Supro 1688T. I found that I had not fully tightened the volume and tone pots of the normal channel causing some low level hash on occasion; it was an intermittent issue. Took me forever to find it. The pot is not isolated from the chassis in this design, and slight vibrations compromised the pot shielding. Once tightened the problem went away.

At this point you have narrowed the cause to be somewhere in the NFB loop, right? Have you by chance tried swapping out the presence pot, or ensured it is properly shielded?

Good luck resolving this issue. It's enough to drive you mad, right?

Sorry, I've been away for a minute... However, today I had someone suggest in a Facebook group that i use a 0.001uF capacitor across the plate load resistors. I found this to solve the issue when placed across the 100k PI plate load resistor...
Now my question... Is this the solution for the issue or just a patch? Is there a better way or do i solder this baby in and call it a day?

Sorry, I've been away for a minute... However, today I had someone suggest in a Facebook group that i use a 0.001uF capacitor across the plate load resistors. I found this to solve the issue when placed across the 100k PI plate load resistor...
Now my question... Is this the solution for the issue or just a patch? Is there a better way or do i solder this baby in and call it a day?

Often use that to take unwanted high end off, particularly in extra stagers with gain +

Most of the early Marshall amps used a 47pf-100pf ceramic cap between the two PI plate resistors. Is this where you soldered your caps in or did you place individual caps across each plate resistor? As Neikeel mentioned, they are often used to remove some of the higher frequencies in high gain circuits but if it's working for you I would roll with it.

bhyatt1 wrote: ↑

Thu Nov 26, 2020 9:28 pm

Sorry, I've been away for a minute... However, today I had someone suggest in a Facebook group that i use a 0.001uF capacitor across the plate load resistors. I found this to solve the issue when placed across the 100k PI plate load resistor...
Now my question... Is this the solution for the issue or just a patch? Is there a better way or do i solder this baby in and call it a day?

Did you put one across both or just the one 100K Pi Plate Load Resistor...

Just the one

So... The hair is still there on the phase inverter. I'm one step away of disassembling the whole thing and starting from the get go.

I can't remember if you posted any pics of the amps wiring. It may be helpful to put up a few pics of the board, pots and socket wiring.