• 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.

Gray Research AM-50

Hello Everyone,
I picked up a pair of these and I am in the process of rebuilding them. I was wondering if anybody could give me a hand at what bias voltage these should be set at. They have the Dynaco A-431 opts( 4300 ohm primary). They are running 400v UL on EL34 ouput tubes. As it stands right now I can only get about -8.5v with the bias adjust pot wide open. Is that enough bias at that plate voltage? Should I replace that pot so I have more adjustment ? I could see that would be needed to run 6l6gc in the amp if I wanted to. Also if anybody can recommend any mods to help with stabilty or tube longevity or anything. I sure would listen. Thanks
From my skill, I know, bias for EL34 /or 6L6/ is more -20 v. -8.5 v are doesn't enough. You can use mixed bias /fixed - 8.5 plus automatic bias about -20 v, using common cathode resistor /for PP/, shunted with electrolitic cap.
Thanks for the reply. I did notice that there is a 10k resistor attatched to the bias pot going to ground could I change that to get a different bias voltage range?
You need something like -25V to -35V on the grid, unless it uses mixed grid/cathode bias. Show us the circuit for the output stage.
I cant find ANY info on these. from what I gather its a dynaco mk 2 with out 460v on the plates of the outputs. there is also a 1k resistor going from the selenium rectifier to the wire coming from the bias pot. I removed it and fired the amp back up and it smoked the 100v bias capacitor. So that has to go back in or something like it .


2008-08-23 7:28 pm
First: I think, maybe something is wrong with -8.5v, this is not suitable for something in Your amplifier. You can check bias rectifier. You can disconnect AC wires from it /that go from transformer and check how is AC. Than connect to rectifier and check in it output DC/when output is disconnected from caps/. .....Or check rectifier to short /but first disconnect all it wires/. If there is short, You can change it.
Second: About Cathode bias, /maybe there is/
You can see, according data sheet, and found where are the cathodes of pair EL34. They must be connected and from common point the wire go or to ground, or to common resistor. If there is resistor, it must be about 100-200 ohm and check how is U on it.
More, after give us more info.
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Hi Jason ,

You got bad coupling caps , a change value resistor or a bad selenium rectifier . It looks like a copy of a Dyna MKII does it have a 7199 in t to drive the EL34's .The bias should be about -36 volts

Its got a a 6an8 doing the driving. before the 1k resistor it has -27 volts after the 1 k its about -8. If I lower the 1k to say 500 ohms what will that do?