Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tomof63 on June 17, 2020, 12:16:38 am
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Hi All:
Since I have the Epiphone EA-50T sorted out for now, I want to turn my attention to my '57 GA-5T. This amp is a Blonde Tolex version which had been serviced for caps, three wire with ground cord and a Voice of Rock (VOR) 8" Alnico replacement speaker. I have made another baffle and fitted a 10" Gibson Ultrasonic from a '70 G10 solid-state amp. The Gibson 10" speaker was an improvement, as the VOR was quite dark sounding. The VOR was new, so it may break in over time. I'll just hang onto it for the time being. What I'm looking for is a way to brighten up the amp circuit and maybe it get to break up earlier. I do have the schematic and the my version of the amp (I know...Gibson is all over the place with different versions) has a 12AX7 for V1, 6BM8 for V2 and a 5Y3GT rectifier.
I can post pics of the chassis for clarification and identification of components.
Thanks in advance!
Tomof63
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Here's the Schematic.
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga_5t_61_skylark.pdf
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Breakup
* I'm not seeing a grid leak R on V1a. 220K - 1M would be good.
* B+ is very low. This darkens tone in the preamp. Reduce value of B+ dropping R's. Shoot for 180 - 220VDC on the preamp plates.
* Preamp plate R's are very hi value. This darkens tone. Use 220K or 100K. 220K is more gain. Keep existing Cathode R's; or maybe change to 3.3K with a 220K plate R. Pay attention to resulting plate voltage.
* Add cathode bypass cap to Stage 2. This requires splitting the cathode resistance if you keep NFB. See VibroChamp schematic.
Brighter tone
* Some of the prior mods address this
* Decrease preamp's cathode bypass cap value to 1 - 5uF. This brightens tone.
Do such mods in stages; not all at once.
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I know...Gibson is all over the place with different versions
That's for sure, and that version is likely the rarest. To get it to breakup earlier I might try reducing the negative feedback by increasing the NF resistor.
I agree with jjasilli in doing one mod at a time. And I would try the other mods before raising the B+. And if you do raise it, I would be cautious about raising it on the power tube by keeping 1K dropping resistor as is.
You might also try reducing the value of the first or both .022 signal caps in addition to the cathode changes.
I have 6BM8s in a coupe of PP amps; one a Gibson and one a Geezer/Tubenit type circuit. Love the sound of that tube and will be interested to see what you come up with.