Fender Twin Amp Vol Control Noise in 25W Mode

[93DMTX]

Hey Guys,

I've got something going on with a 2005 Fender Twin Amp (Pro Tube).

This one...

http://support.fender.com/schematics..._schematic.pdf


This may have always been there (I just never noticed) and may be normal.

ONLY when the the amp is in 25W mode, the clean channel volume and the gain channel gain controls act/sound like they are dirty or scratchy. When the amp is in full power mode they are dead quiet. My first thought was "could there be DC on the pots"? The wipers of those two pots are connected to the grids of V1B and V2A. I haven't pulled the chassis yet but at the input a guitar cord tip goes to about -58mV when in 25W mode and is about 0.3mV in full power mode.

My knowledge of theory gets a little weak here. So I ask...the 25W power mode drops plate supply voltage (X+) for V1 & 2 from 242VDC to 125VDC (per the schematic not measured yet). Could that account for the neg voltage measured on the V1A grid (and I suspect I'll find the same on V1B and V2)?

If yes, then maybe this is normal operation. There is no problem when playing, only when changing the levels.

Thanks Much,
Doug

[93DMTX]

So I went ahead and coupled the Ch 1 Vol pot wiper to the V1B grid with a 1uF NP cap. No more noise so it seems like it was/is a DC on the pot issue. I still just don't know if I'm covering up a problem or a design 'feature'.

TIA if anyone has any thoughts...

Doug

[Chuck H]

If a capacitor between V1b and the volume control removed DC from the pot I would say that V1 is a bad tube. The only way DC gets blocked in your scenario is if it's comming from V1b grid (happens all the time). Try replacing that tube and removing the newly installed cap.

[Enzo]

Tubes can accumulate a negative charge on the grid. In fact grid leak bias circuits count on it happening.

I can see no reason for the change really, but try this theory on for size. B+ drops from 242v to 125v in low power mode. That means the plate must be down in the 60-70 volt range. perhaps that extra low plate voltage has a harder time attracting electrons, so they find it easier to accumulate on the grid as they pass by. SO the extra -50mv occurs. I can't think of anything else. All low power does is reduce the B+ voltages throughout the amp. (Drops bias voltage some too) No circuit changes are made in the stages.

You stuck a cap between grid V1b and the rest of the world? So there is now no resistor from that grid to ground? You need to add one then. 1 meg would be fine. You can't leave a grid floating, it will cause various issues.

Perhaps different 12AX7s will respond to the low voltage differently.

[93DMTX]

I've tried several different makes of CP tubes in V1. A JJ, Shuguang, Tung-Sol and a Sovtek. Also and older Toshiba. I may try as a final shot a Sylania 5751 (I'll just jump the cap). I'll probably just end up adding the resistor on the grid and calling it good.

Thanks much for the help.

[93DMTX]

Ok, added a 1M from the grid to ground. No pot noise and the amp sounds great. I'm calling it done.

Thanks again for the help.

[Chuck H]

Ahhh... Right. As often happens, I'm on the right track and Enzo refines my theory. Good catch on the V1b 0V bias reference Enzo. And nice trouble shooting 93DMTX.