"ghost" amplification...

[sbirkenstock]

Hi Everybody,
I was working on a Fender BF style amp today.
I was feeding a 1V peak to peak sine wave into the input.
When I turned the amps volume to 0, the sine was still well audible.
I experienced something like this before in other (Fender) amps, where some signal is audible despite the volume set to 0.

So I checked with my oszilloscope.
No signal at the mixing resistors, no signal at the input of the phase inverter.
But the signal was on both outputs of the phase inverter....

I also checked that the power supply was stable.

The amp works very fine though if you use it with a guitar!

Any hint?

Stephan

[sluckey]

1Vpp is too much. Turn it down to about 0.1Vpp or 0.2Vpp.

[Littlewyan]

Isn't this normally due to a poor grounding scheme? Old Marshall 1987 or 1959s do this on the High Treble channel

[xtian]

An unbypassed (or poorly bypassed), shared cathode resistor will pass signal to the other triode.