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Repair and disassembly guides for guitar amplifiers.

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Old Kay amp needs attention

This is a very old guitar amp probably from the late 1940's. I believe it is single ended in that it has an 80 rectifier tube, a 6C5, a 6N7 and a 6L6. The socket for the 6L6 says 6N6. The amp works but I can see on my variac amp meter the current draw goes down instead of up when I strum the guitar. This seems odd to me as all other amps I've tested this way increase the current draw. It also seems to have a large current draw at idle, (no input signal) at .9 amps. The amp has had some work done, perhaps a lot as much of the circuit has been rewired and new capacitors. What if anything is out of the norm here? In other words shouldn't the amp meter rise with a strong input signal? If so what might be the reason it's decreasing?

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@bigdrums I'd start by replacing the 6N6 with a 6L6 or a proper substitute for that.

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That was already done when I got it. I am thinking that whoever rewired it got the plate, the control grid, and the screen grid wires mixed up. Without the 6L6 in place I measure 472v on the plate, pin 3, 474v on the control grid, pin 5, and 476v on pin 6 which is supposed to be unused. From my notes on previous projects using 6L6's pin 6 sometimes has the highest voltage for what I don't understand. then the plate then the screen in descending order.

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Hi @bigdrums ,

Does the amp have a model number on it?

If so then this link may be of some help. (The 'donate' option is up to you)

For what it is worth it seems as though your amp's circuit has been modified to accommodate the 6L6 valve in lieu of the 6N6.

If it has you may have to "trace all the wiring and draw it out" to get a circuit diagram of the amp that you can work from.

According to this link:

Some known problematic tubes to replace: 6N6 (chinese), 6N6P (russian). This tube has NO known direct replacement: the best replacement is another 6N6 – 6N6P. However if you are experienced, and with the proper tools, it could be replaced (given socket rewiring and maybe some circuit tweaks) with 5687, 12BH7, ECC99 or E182CC. This last tube seems to be the closest equivalent.

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It appears to have been rewired quite a bit. There is the original schematic taped to the bottom of the chassis and it's nothing close. Trying to establish some absolutes like shouldn't there be a bias negative voltage present on pin 5 control grid regardless of the class amp ?

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Hi @bigdrums ,

Been awhile for me with valve circuits but my recollection is that the control grid was negative bias.

Here is the data sheet for the valve which may be of some help. Apologies if you've already been there done that.

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Jim Mcsharry will be eternally grateful.
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