Help please, amp passing audio with volume turned down

[randalp3000]

Just finished a new amp based on the 18watt super light but with a top boost tone stack. Getting quite a bit of signal with volume turned all the way down. I replaced the volume pot and even tried shorting pot center lug directly to ground. I can't figure out where the signal is bleeding through.

any ideas would be greatly appreciated.

thanks
randy

[xtian]

Bad/shorted bright cap on volume control?

Try shorting grid of V1b to ground. Still getting audio thru?

[randalp3000]

thanks for the ideas.

I already removed the bright cap with no luck. Shorting grid of the second gain stage to ground still has audio passing but with less volume.

[Smokebreak]

Change v1 tube? Maybe one triode's bits are shorting to anothers'?

[JazzGuitarGimp]

This in probably nothing, but I noticed in the schematic that the first two gain stages use the same half of the tube: pins 1, 2 & 3.

[Stevem]

X2, bad tube!

[randalp3000]

Lou, yep I used the wrong template for V1b. oops

thanks for the tube suggestion, makes sense to me also. I have had 2 tubes in v1 with the same results but I'll try another.

[Malcolm Irving]

If the 20uF cap which feeds the first two stages is not doing its job, then I think you would get the symptoms you describe. It would have to be effectively an open circuit - either due to a faulty cap or a bad connection.

[randalp3000]

If the 20uF cap which feeds the first two stages is not doing its job, then I think you would get the symptoms you describe. It would have to be effectively an open circuit - either due to a faulty cap or a bad connection.

I will give that a try also.

thanks

[jaysg]

Without really looking, when there are high gain stages and wires are in parallel, you can get a transmitter/receiver pair. In other words, it could simply be lead dress. Sometimes a plate feed needs to be shaped like a big S.

[gui_tarzan]

Would you please teach everyone else that takes photos of their projects how to take large, clear photos? I LOVE your pic, when it's expanded it's crystal-clear and very easy to look at.

[Structo]

I've read reports about faulty silver mica caps in recent years, the dog bone type.

Not sure what they manifest for symptoms but I thought I would throw that in.

[Andy Le Blanc]

look at the input jack... its grounded to the chassis...

and... you have it grounded to an additional point behind the tone stack

aluminum chassis? that's a loop, right in front of you... its your grounding through out the build.

yes a good picture is very helpful

I don't see that you cleaned the chassis metal under the nylon stop nuts

or used toothed washers under then either.

[Andy Le Blanc]

yup.... revise your grounding... looks like the green wire bottom right in the pict might make a good star point... install a bus or buses instead of grounding to the chassis... nylon jack input grounded back to the star

[Littlewyan]

It must be the grounding like Andy said. All old Marshalls did this and I believe it was the grounding in those amps. Its not a massive issue though is it? Its only a very tiny bit of signal normally.