5F6-A circuit never goes to '0"

[Roberto Lasco]

First msg here. Congrats for the new house !

Have a 5F6-a circuit on the bench that never cut output, even with volumes at zero. It does on both channels. I don't think it's potentiometers fault (2 different brands on bright/normal channels); when zeroed, CF's input has about 60mVAC (which gives an output power of 1.3W; it takes about 110mV to give full undistorted output). Funny is that before the mix resistors, AC voltage is a bit smaller (oscillates between 40-50mV), so it seems to be induced from anywhere after the mix resistors.

Already changed first coupling caps. Not sure if raising the 270K mix resistor to Marshall values would do anything.

Any ideas ?
thanks, roberto

[Carl / Zwengel Amps]

Ok, are you getting an oscillation at audio frequencies or is it just that the guitar signal is bleeding through just a little bit? If you're seeing an oscillation at that high a level with NO SIGNAL applied, that's the first thing you've got to get fixed before you can even think about troubleshooting anything else in the amp. If its just the guitar signal, I'd be willing to bet that you're getting coupling through the power supply capacitors. Increasing the values to something around 47uF should reduce this effect substantially.

That's my 2 cents

-Carl

[Roberto Lasco]

Thanks for the response Carl.

No oscillation at all, just a bit of signal bleed. This guy is pretty well filtered, 50mF/choke/50mF/25mF/25mF, but I added a 47mF on the preamp node, no change.
Subbed the 270K mix resistors for 470K, still no help.

Something on the layout maybe ?

[LooseChange]

Don't worry about it. You may never solve it. I have seen many amps that do this.

[Enzo]

Crosstalk might be what you hear, or the pots simpply don't go all the way to zero ohms at the end of travel. Connect a clip lead to ground and ground the wiper of the volume pot. If it kills the signal, then the pot wasn't going to zero. if the signal remains it is crosstalk.

However, when do you ever play an amp when it is turned to zero. Is all the way down still too loud? In other words, why do you care that the sound never entirely goes zilent?

[R.G.]

I've run into situations where too-big stopper resistors on grids let the grids pick up capacitive cross talk. If you isolate a grid behind a big resistor - 50K and up - then the grid cannot be pulled to ground by a volume pot on the other side of the grid stopper, at least not completely. Then the grid picks up capacitive signal at low level from almost anywhere nearby. The composite effect is that the volume pot won't turn it all the way down.

In cases like this, lowering the grid resistor quiets the amp.

[Roberto Lasco]

It's really crosstalk, grounding the pots do nothing.
Well, not sure if it's a big deal to the owner (amp didn't came in due to this), just trying to make it neatier as I can...

On the normal channel it's a pretty wimpy bleeding, but on the bright one is almost Champ volume like (vol on 4 ?); since owner will play guitar AND harp (although I voiced just the normal channel for harp), not cutting fully could eventually cause problems with a mike...

[Roberto Lasco]

RG, if I understood correctly, the mix resistors act like grid stoppers. How low can I get in 5F6-A circuit ? I tried 20K and indeed reduced the problem, but didn't kill it...

[dai h.]

hi Roberto,

normally there isn't such a problem with the 5F6-A cct., so how is the amp different from usual (layout, etc.). Is this a homebrew or a production amp with the same circuitry?

[Roberto Lasco]

Hi Dai, yes it's a homebrew, Ptp (not quite eyelet or turret), but with a board with more and less the usual layout, aluminum chassis (maybe narrower than a Bassman one, 10cm width); besides this "feature" is well done, very quiet noisewise and sounds good...