Vox AC15VR Oscillation

[telecaster]

Please disregard the pictures in my prior post - time to retire the old B+K, at least for higher frequency measurements. This picture is from a 60MHz Tektronix - picture taken at the Speaker output (probe on Ext Speaker (+) and grounded to chassis). Amplitude 5.5 div X 2V/div = 11V peak-to-peak. Frequency 5.2 div X 0.5 microsec / div = 385 KHz if my math is correct.

[Helmholtz]

Do you see oscillation on any of the supply voltages?

Do you still see oscillation when you lift one end of C49 or R31?

[Helmholtz]

BTW, this is a rather tricky (clever) circuit.

The 12AX7 is actually operated like a little PP power amp, driven by an LTPI (Q10/Q8).
The opposite phase output signals of the 2 triodes are combined by U5A just like an OT would do.

Still thinking about the multiple NFB arrangement.
Looks as if they're varying plate load(s) via U6A/U6B to emulate the poweramp-speaker impedance interaction of a real tube amp.

How do these amps sound? Close to a tube amp?

[telecaster]

Helmholtz - there are oscillations on the Supply Voltages, which are ok DC-wise at +/- 27V and +/- 15V. Measurements of oscillation amplitude at frequency below:
U4 pin 1 = 80mV at 385KHz
U5 pin 7 = 8V at 15.625KHz
U4 pin 3 and pin 5 = 0.7V at 385KHz
U5 pin 1 and pin 8 = 0.48V at 385KHz

I will lift R31 and get back to you with the result.

[Helmholtz]

Check all power supply filter caps, especially C12.

U5 pin 7 = 8V at 15.625KHz

Strange to have 2 different oscillation frequencies at the same time.

[The Dude]

.......U5 pin 7 = 8V at 15.625KHz......

That is very near horizontal scan frequency of many CRT units (15.750KHz). Do you have a TV or bench monitor nearby that could be radiating into the amp?

[telecaster]

Helmholtz: C12 looks bad - 1.5K ohms in circuit, compared to over 400K ohms for C14. I was doing a Mouser order anyway so I ordered replacements for C8, C9, C12, and C14.

The Dude: I did have the B+K on the bench too, but it was off. I will recheck that measurement after I replace the caps