VVR Pot Scratchiness

[MarshallPlexi]

I just built an amp with the VVR on the Screen and Plates but for some reason the volume control on the amp and the guitar are still scratchy. I thought this would only happen is you scaled the whole amp. I'm not scaling the preamp at all.

Any ideas?

I can of course install the 0.1uF caps but this is just odd to me and I wasn't expecting it. What is causing it?

[ChrisM]

Did you install isolating diodes?

[Deric]

What kind of amp? What's the plate voltage on the input stage?

[MarshallPlexi]

The only diode is found at the entrance of the VVR after the rectifier. Is that it? It's an AC30 type build and the plate voltage for the 12AX7's is around 275V.

[Colossal]

The only diode is found at the entrance of the VVR after the rectifier. Is that it? It's an AC30 type build and the plate voltage for the 12AX7's is around 275V.

If you are scaling the power section (plates and screens) only, it is typical to isolate each rail with a pair of diodes at the fork in the B+ road, so to speak. So you'll still have that diode in front of the VVR (and after too if you want to be thorough), but you'd add another diode going to the other rail which is supplying the rest of the amp's B+.

[ChrisM]

Follow this VVR configuration. You need those isolating diodes.

[img822]http://rh-tech.org/public/18-Watt/18_Wa ... PI_141.gif[/img]

[MarshallPlexi]

Great! I'll add that diode and see what I get. I'll report back here. Thanks!

[MarshallPlexi]

Okay I added a 1N4007 before the 22k dropping resistor for the preamp supply and the noise was still there when I moved the Volume control on the amp or the guitar. I added a 0.1uF from the Volume control to the grid of V2 and it solved the volume control problem. I added the same cap from the input jack to the grid of V1 and it solved the problem with the guitars volume. Strange....

[Richie]

You might look to see if you have a leaky cap. if the preamp voltage is not dropped, you should not have to use the added caps or resistor. Those are usually just used when lowering voltage on the whole amp,or preamp section.

If you are using mica caps, look at measuring those to see if any are leaky.
Many people have quit using the newer mica type caps because so many are bad or leaky or show up a few days later. So i would check and see if there was another problem. Could be a tube also,so swap those and check.

Kind of like taking pain meds for a toothache, it might help the pain, but not the problem causing it.

[MarshallPlexi]

You might look to see if you have a leaky cap. if the preamp voltage is not dropped, you should not have to use the added caps or resistor. Those are usually just used when lowering voltage on the whole amp,or preamp section.

If you are using mica caps, look at measuring those to see if any are leaky.
Many people have quit using the newer mica type caps because so many are bad or leaky or show up a few days later. So i would check and see if there was another problem. Could be a tube also,so swap those and check.

Kind of like taking pain meds for a toothache, it might help the pain, but not the problem causing it.

I will check this. You are dead right about the Silver Mica's! I've had a LOT of them go bad recently.