• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.


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Hi all,

I am in the process of constructing the EL84 Quad SE amp from the schematics posted in Bona's site. The url for the schematic is

http://www.infomaniak.ch/~bonavolt/el84_4.htm .

I need to fix a volume control in the input part of this amp. Could somebody in the forum please guide me to the location in the circuit where I can insert the volume control, and the recommended value for the potentiometer?

By the way, I will be using this amp with a pre-amp I have made from a schematic from the same site. The url of the preamp which I am using is http://www.infomaniak.ch/~bonavolt/12au7lin.htm

Any help in this regard will be highly appreciated.


Note: I do not know whether I did the right thing by linking to the schematics in Bona's site without prior permission.
No one will get upset if you post a URL to a public site. That's what they're there for.
The schematic, however...
Hmmm, I have to wonder if there's an error in the schematic. They're showing a 100k grid-stopper resistor which is pretty excessive--the highest I can remember seeing offhand is 10k, and *that* was pretty unusual. A more typical value might be on the order of, say, 100 to 1000 ohms.
More bothersome is that there's no resistor to ground from the grid of the input tube, so I'm concerned about the biasing of the tube, amongst other things.
I'd tend to think that the schematic should read 100k to ground and straight in on the grid. (Grid stoppers aren't really necessary. I generally don't use them, myself.) That would allow the tube to bias properly and set the input impedance, all in a perfectly normal way.
That said, I'd then make the 100k resistor a pot, with the wiper leading to the grid. 100k is a perfectly respectable value for the grid resistor in a tube circuit, even though it will sound high to those who think in terms of solid state equipment, where 10-47k is the norm.

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