VVR3 help

[andresound]

Hi guys,
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???

[strelok]

When you swapped the MOSFET's did you change the protection diodes as well?

Edit: just saw that you said you tested them. You might try changing it anyways. Something is causing the voltages to drop off suddenly and you're correct is saying it sounds starved, because it is Might also check/replace the pot. Its a linear pot so at 3 oclock it should read about 3/4 of its resistance. (or 1/4 depending on what side you're measuring from.)

[andresound]

Thank you strelock. I will change the diodes. There are two pots. Do I change both, or can I measure them in circuit.?

[strelok]

It should be one pot, dual ganged. I would take it out of the circuit to measure it just be sure. Use some clip on leads and adjust the pot over its range to see if there is a sudden change in resistance. Do this for both sections.

[andresound]

Sorted!! One of the pots was faulty. Have ordered new pot.
Thank you strelok

[strelok]

No problem. Glad I could help!