VVR adjustment started to introduce some noise

[Gee]

I have an amp which is a bit of a test bed that is currently an 18 Watt LiteIIb (SS recto and PA & PI scaled). The VVR installation is my own layout and follows Dana's circuit and component specs.

The VVR circuit has worked flawlessly for about 2 or maybe 3 years and I put it together not long after Dana first make public the VVR circuit.

Recently I have noticed that if I adjust the VVR pot, that just after the point at which the rotation of the pot stops, that I get a few seconds of what sounds to me like white noise. After a few seconds the white noise goes away and the amp is as quiet as normal. My first thoughts were that maybe the VVR pot needed cleaning so I sprayed in some pot cleaner and wanged the pot about a bit. The problem remains however.

Also I have used this amp with the VVR on the same setting for most of the time and I was wondering if there might be some damage inside the pot or arcing (just thinking out loud here).

Anyone else noticed a problem like this?

[goodtone4u]

It may be the VVR Mosfet oscillating at high frequencies causing the audible white noise.
Most of the VVR circuits do not bypass the pot wiper or power connections with .01 up to 0.1uf 600V caps to ground.
Your filter caps may have aged some and may be contributing to this symptom.

Ask the VVR designer for hints how to solve this.

I do not use commercially built VVR's because of Mosfet location has to be on a big heatsink. (100 watt amps with 500V)
But I have seen oscillations at the output of the Mosfet drain connection (Output) going to the output trans CT.
Have stopped the oscillation by adding a .047uf 600V at the wiper to ground.

Good luck!

[goodtone4u]

Small typo in my last comment the output of the Mosfet is the SOURCE connection going to the output transformer and screens.

I need more coffee!

good luck!

[Gee]

It may be the VVR Mosfet oscillating at high frequencies causing the audible white noise.
Most of the VVR circuits do not bypass the pot wiper or power connections with .01 up to 0.1uf 600V caps to ground.
Your filter caps may have aged some and may be contributing to this symptom.

So are you saying that it could be the MOSFET showing signs of aging or staring to fail that could be causing this?

I would be interested to hear any others experience like this. Those MOSFET's are hard to find in the uk, expensive and a pain to replace in the circuit. So I would want to be confident that it is the MOSFET before I change it.

[wokkel]

I thought that the noise was due to re-biasing of the Mosfet, the Mosfet clicks on and off and on due to changes in the gate-source voltage.

I read about the crackle-OK pot in a SHO pedal and I always thought this was similar to what happens when you adjust the VVR.

http://www.freestompboxes.org/viewtopic.php?p=975#p975

(I may be completely off though)

But if this is true, then you should have experienced this from the start.

Harald

[goodtone4u]

If you are getting voltage control, no need to replace Mosfet.

After seeing your reply I checked my amp VVR with a scope on the output of the Mosfet feeding the output transfomer.
(Carefully measure using a 0.1uF 600V cap in series with scope probe)
Very clean, no noise except when at maximum a little power supply ripple. (Night Train clone as test bed )

Check your amp stages at near no voltage condition for the source of white noise. Do you have the PI powered off variable or fixed voltage.

Hard finding Mosfets?
I bet UK has junk computers and CRT monitors that have SMPS for power.
Some Mosfets I found were 800V @ 12 Amp fully insulated which is perfect.

The Mosfet is only a voltage buffer that tracks the pot wiper voltage.
The wiper voltage typically about 3.5 volts higher than Source connection of Mosfet. (or output of VVR board)

Good Luck!