Gibson Scout GA-17RVT tonal improvent

[Randall]

A Chicago blues harp guy brings me a 1964 Gibson Scout 1x10 combo that he complained has no bass response. Indeed, it did sound too bright with my Strat shop guitar. Someone had already replaced a bunch of caps with orange drops, probably an attempt at fixing the issue. I have heard Gibson voiced their amps for their humbucker guitars, as did Fender for their single coils, but this was unusable with a Fender or a harp mic. I found a video (Guitarologist) with the exact same amp with the exact same complaint. He pointed out what he refers to as the Sprague Tone Suck Network, which is a three legged module consisting of C3, C4 and R6 and R7. He says whenever he sees those in like era Gibsons, he always removes them. He also pointed out that the .001uF C2 just before it is way to small, so he replaced all of that with one .047uF cap. He then changed the .1uF cathode cap on V2b to a more reasonable 10uF.

I followed his lead and did the same, and sure enough the amp now is louder, breaks up nicely, and has more bottom end. I think my blues playing customer will be happy. Gotta wonder what was Gibson thinking with these values and that Sprague module? There is a significant improvement overall in this amp making those three simple changes. The reverb is thicker and stronger now as well, it was pretty lackluster before.

http://www.ampwares.com/schematics/gibson/ga17rvt.pdf

[Leo_Gnardo]

Well done Randall! Some of those 60's Gibsons were absurdly bright - no wonder we don't see many around. Something to remember, Gibson didn't build amps, they had them made by job shops. Sometimes I think Gibson also didn't design the circuitry either - they had some idea what they wanted the amps to sound (so they make our woofy-tone guitars sound good!) & look like & left the rest to the job shops, resulting in some wide variations in builds, also amps that don't match any known schematic. There's another "suck filter" I see in some of them, consisting of a cascaded series of caps & resistors.

I'm sure your customer will be positively impressed.

[Jazz P Bass]

Brad the (Guitarologist).
He has some good videos of repairing equipment.
I like the one where he had a terrorist related popup.
It was a spoof.
And then the FBI showed up at his door!
https://www.youtube.com/watch?v=sETDPFyHQsM

[Randall]

Yes his videos are good, if not usually way too long. I could say what he says with 80% less words.

[Helmholtz]

It's a bridged-T notch filter. These can be used to counteract acoustic resonances. So the purpose might have been killing the acoustic feedback with Gibson's hollowbody Jazz guitars.

See here: http://www.muzique.com/lab/notch.htm

The other RC chain in the schematic is the phase shift circuit of the Tremolo LFO.

[nhbassguitar]

Yes his videos are good, if not usually way too long. I could say what he says with 80% less words.

Yep. And that's why I had to quit watching his stuff. Seems like an ok guy, but it was really getting annoying. Best of luck to him, though. I think he recently separated from his wife. That's never good.