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Bought a 1966 Gibson Explorer amp. Seems in pretty good shape. Interesting guitar amp. Has interstage transformer.
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I rebuilt a very similar vintage Gibson Skylark for a friend. 12AU7 front end into the interstage transformer driving a pair of EL84s.
The thing that plagues some Gibson amps of this era is their use of T-filters (part of me thinks that's the wrong name) for tone shaping. The theory I've heard is that with the era of the jangly, bright guitar, Gibson wanted to make amps that made their darker-sounding guitars sound sparkly. The result is that many of these mid-60s Gibsons can sound very odd, unless you delete or modify these filters.
What do ppl do to the tone stack to fix tone shaping issues? Mod them or replace with another type of stack?
Thanks!
"What do ppl do to the tone stack to fix tone shaping issues? Mod them or replace with another type of stack?"
People do either, depending on the particular amp and/or the final sound they want.
On the Skylark, for example, I added a resistor between the ground of the filter and the circuit ground to reduce its effect.
In other words, these mid-60s Gibson amps have good cabinets, transformers, and decent speakers, and the weird tone-shaping can be remedied fairly easily.
It's been low on my list of projects but is now rising up a bit. I first heard this amp in 1964 and really loved the sound (and the girl of course which may bias my judgment here). I have one major non- musical project to finish and then gonna tackle the Baldwin. Here's an image (not mine).
https://www.gearslutz.com/board/attachments/so-many-guitars-so-little-time/340001d1365778810-baldwin-c1-custom-professional-amplifier-supersound-baldwin1.jpg
Sim
I repaired a couple of them when he came through Charlotte a few years ago. His road manager, Pooty (now deceased) brought them in. Willie likes the way they interact with his nylon string guitars. He owns four of them.
Hopefully I won't need to replace the transistors.
Sim
Hopefully not. Some of them are Germanium, and difficult to source. Usually, the problems come from old electrolytic capacitors that they used for interstage coupling.
Those multi-color drawbar type switches can be problematic, too, but can be cleaned.
And I now know who to consult as I move forward. Fortunately I have the schematic diagram.
Sim
Reminds of something you might see in the "Jetsons." Very cool!
Good find, very similar to my GA20RVT. Has great tone!.
The dry channel on my deluxe reverb is acting up.
Intermttent signal loss at the input jack. After repeatedly plugging/unplugging the cord, the signal comes back . BOTH hi/low inputs.
Any idea what this could be?
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!
8^)
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.
8^)
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.
Beautiful amp. Are you going to keep it or sell it?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Add to my low-midrange vintage guitar amp collection. Fender Princeton reverb, Ampeg Jet, Deluxe Reverb.I like this wattage range (10-22 watts). Good tone, can be driven hard without blasting every one out of the house.
8^)
Edits: 03/29/17
Steve
You have some of the most beautiful and exotic guitar amps I've ever seen.I would love a couple but I can't play a guitar and make it sound like music.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Lovely.
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