Monkeymatic Tamalpais #3 (5F6A/JTM45)

[xtian]

I sold this amp to a local buyer, and he found an odd behavior when my amp is attached to his Two Notes Torpedo load box or OX load box. When the signal is fully attenuated, my amp goes into low-frequency oscillation--I can see blue light in the power tubes pulsing at about 4 Hz. Looks a lot like the bias-vary tremolo in a Princeton. This effect is audible in the output. As soon as you hit a power chord, or ease up on the attenuation, the oscillation stops.

Any idea how I can solve this issue?

I'm going to try tacking in another 47uF in parallel with my reservoir cap.

[Guy77]

Hi xtian. This maybe more for my curiosity then solving the issue but I am curious if you can reproduce the issue by turning your master volume all the way down and then crank your volume all the way up to maximum and then finally start to raise your master volume slowing until you start to just barely get some sound. Then listen to hear how it sounds.

Cheers

Guy

[xtian]

Tacking in extra filtering made no change. I also turned all the pots up and down, together and individually, and nothing made a difference in the visible pulsing of the power tubes.

There is no master volume. Preamp volume for normal and bright channels.

The issue is repeatable with the Two Notes Torpedo Live (no speaker out, line out only) and the UA OX, but no issue with my Weber MASS reactive load.

[xtian]

I solved the oscillation by lifting the gNFB. I added an SPDT to make it switchable.

[Guy77]

Glad to hear its solved!

[Roe]

problems like these could be caused by lead dress. try to move the gNFB away from other wires

[pdf64]

Mention was made in the early posts of a frondelli master vol, pic shows it between power switch and lamp? Has it been removed or is it just left up full and forgotten about?
Bear in mind that the 470ohm g2 current limiting resistors are grid stoppers too, best mounted on the socket terminals. Assume that the g1 grid stoppers are on the socket terminals? Upping them to 10k may help.
The value of the 47pF HF roll off between the LTP plates shouldn’t be seen as fixed in stone, it’s there to compensate for parasitic effects, which will be different for every build; try smaller, none, bigger.
Did you check that the g2 HT node ecap wasn’t a bit wonky?