stereo valve amp question

9 volt · Post by **9 volt** » 17 Jul 2011, 10:21

Hello, I have an old single ended stereo valve amp (basic amp with 6wg8 valves). I noticed that the balance control is a pot that connects the cathodes (triodes) to ground. Cathode to 150ohm on each side of the pot and a 20uf on the middle lug going to ground. Anyone ever heard of this? I thought the balance would normally send the signal to ground......Thanks

JiM · Post by **JiM** » 24 Jul 2011, 18:53

9 volt wrote:I noticed that the balance control is a pot that connects the cathodes [...] to 150ohm on each side of the pot and a 20uf on the middle lug going to ground.

This looks like a variable cathode bypass cap.
There should be proper cathode resistor(s) to ground, and the pots brings a variable amount of decoupling cap, which increases gain (above a certain frequency, set by the cap value) without changing the DC bias. This should indeed be pretty effective as a balance control.
More info here : http://www.freewebs.com/valvewizard1/gainstage.html

9 volt · Post by **9 volt** » 30 Aug 2011, 01:23

Thanks Jim, this makes sense. Also thanks for the link, much appreciated!