VVR3 help (strange problem)

[andresound]

Hi guys, I have posted this in the TW section, but thought I would get a wider audience here.
I have had a VVR3 in my express build for nearly 2 years. I have been extremely happy with it and it has proved to be reliable. However, something went wrong!! My bad. Let me explain.

I received a bunch of NOS MULLARD and Phillips 12AX7/ECC83's (10 of each). So I decided to test them for noise and microphonics. V1 in an express is the best tube tester in town!!! Being impatient, I "hot swopped" some and repeatedly switched the amp on and off while doing others. I believe this is when the problem started.

Symptoms are:
With the VVR not engaged (10), sound is great. The VVR at 9.5 and 8 and 7.5, do attenuate in the normal fashion.

However, at a little before 3 o'clock (7 or 25% attenuation) the amp starts to sound terrible. The initial note comes through and then it dies out very quickly and is "jagged" sounding. Almost like the amp suddenly gets starved. Sorry this is the best description I can come up with.

Checked the VVR. Both mosfet's (changed both), resistors, and diodes test fine. It seem the VVR is functioning as it should? The voltages with tubes in are;
10 (not engaged) = B+ In 398v, B+ out 393v and bias -33.7v
7.5 = B+ 321v and bias -36.7v
5 = B+ 73v and bias -17.6v
2.5 = B+ 38v and bias -3.7v
0 (fully engaged) = B+ 34.4v and bias -3.1v

I hope this all makes sense.

Any ideas???

[sliberty]

The best way to know for sure is to take the VVR out of the circuit. See if the amp is doing anything odd without it. Raise and lower the volume, etc. f all is well, then I'd say that the MOSFETs and diode need to be replaced, even though they may have tested good. It's a cheap enough circuit that brute force is your best bet.

[andresound]

Took VVR out of amp, and sounds great. Will replace the diodes as well, reinstall and report

[andresound]

Double post

[andresound]

Sorted!! One of the 1M pots was faulty.