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In Reply to: RE: Since we're talking ramps.. posted by dirtyvinyl on March 29, 2017 at 14:36:56
On the 68k-ohm grid loading resistors or the 1meg-ohm grid to ground resistor? Jumper wire from 68k-ohm to signal grid?
Most DR had the 68k-ohm resistors on the jacks. Some had them on the board.
That's what I'd look at first.
Thanks for the enlightenment.
I'm usually plugged into the reverb channel, playing clean. Now and then I get my crunch on through the dry channel via a tone bone(tube)
That amp looks like a possible R/&R screamer for recording. El84 powered? What's that power polarity for?
Did you go through it? It's so clean inside.
I believe the polarity switch is similar to ones seen in earlier blackface Fender amps. Changes polarity of the AC line onto the primary side of the PT. Sort of a hum control.
Will post photos, once the amp arrives!
Those dual electrolytics that CMI used were notorious to fail. Luckily, on that chassis, there is plenty of room for four individual caps.
I don't know of a source for the dual caps.
Like the 20/20/20/20 at 475VDC, that Fender used on the Princeton Reverb. The new CE product (rated at 525VDC) is pretty nice.
Yeah, prolly go with 4 individual 20/500 caps.
The brown Spragues in these are almost always bad.
I favor using discrete capacitors like Panasonic EE radials. Mount them on terminal strips. 450V should be high enough unless this amp is a lot higher voltage than some others I've worked on.
Think I'll re-cap my GA20, thanks for the motivation. I suspect
they share many of the same parts.
I'll try to post re-do photos, once I get the amp & start digging into it.
You've probably seen the service video's on u-tube. Very
well done. One thing is, the schematic can be off as Gibson
used some different value parts and mods as they saw fit,
not shown in the diagram. It looked liked the OEM cap was
a multi-section, 50/50/50 or something, in a paper can, with
leads out one end. It's on circuitry side of chassis.
It's unusual to find a Gibson amp that actually matches the schematic exactly.
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