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Dirty-South Blues Harp forum: wail on! >
56 Gibson GA20 rebuild.
56 Gibson GA20 rebuild.
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Barley Nectar
250 posts
Jan 25, 2014
7:54 AM
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About the only things worth keeping on the chassis are the pots, jacks, sockets. I have a new Magnetic Components PT which is more correct, voltage wise. The OT and original Jensen are in real nice shape. This amp has 4 inputs, each going to a dedicated triode. This gives me some options for preamp configurations. I'm thinking grid leak as Gibson used on the mic input . I thought paralleling V1a & b for this and also changing the 1 Meg grid leak R to 3M. For the second triode I was thinking Fender-ish cathode bias with each triode set up differently, gain wise, and signal cap value. Heck, anything goes here. Have any sugestions? Here's the schmo 
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jimr
61 posts
Jan 25, 2014
4:22 PM
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You seem to be in a solderin' mood, so consider
V1a- preserve grid leak bias
V1b- cathode bias (forget paralleled triodes) using cap/resistor values from a Gibson GA6
V2- redo with 9 pin minature input pentode like a 5879, as per a Gibson Maestro. The result will be very similar to a Maestro but for the tone circuit. I just got a Maestro and it is a nice amp.
One thing is weird to me about the schem- a single plate resistor feeds three first gain stage plates. I'd probably alter that during a rebuild.
Should be a killer amp, modded or not.
Good luck
Jim
Last Edited by jimr on Jan 25, 2014 4:23 PM
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Barley Nectar
251 posts
Jan 26, 2014
1:09 PM
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Yes Jim , you are correct about the plate R. The mic channel is much preferred tonally, partially due to that and the low grid leak R value of the three instrument channel inputs. I have never played a 5879 amp but I do have a couple here with a 6SJ7 input. They are very nice indeed! I'm looking into the 5879 as we speak.
I take it you have tried parallel triodes, I have not. What is your opinion on parallel triodes?
Thanks Jim, I know you know your stuff...BN AKA Goose
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