Lafayette PA conversion to a Gibson GA-20

[bluto]

Just got this done, still need to wire the mic channel and attend to some details but sounds great so far. Originally had old 6L6G's but decided to go with 6V6's as the rectifier is a 5Y3 and I don't need the extra power. The pots are the right values as is and the tube lineup was nearly the same so the GA-20 was a natural and has the sound I want...

[soundguruman]

That is a cool chassis.
However, GA 20 is kind of boring sound wise.
Make it into a quasi- Marshall 2204 preamp, I say.

Anyhow if you take the original circuit and parts, many times small tweaks will produce a decent guitar amp.
You don't always have to gut the inside and start over.

[bluto]

That is a cool chassis.
However, GA 20 is kind of boring sound wise.
Make it into a quasi- Marshall 2204 preamp, I say.

Anyhow if you take the original circuit and parts, many times small tweaks will produce a decent guitar amp.
You don't always have to gut the inside and start over.

Wow! that is really brilliant stuff...but I suppose no less can be expected from a self proclaimed "Guru". Amazingly I evidently don't have the same taste in tone or music as you do, but I reckon that is why there is a market for different guitars, amps, etc....I built a GA-20 because it has the tone I want. I have mostly been lurking here as I don't know much but it truly amazes me that you still post with abandon after being delivered a well deserved pimpslap dang near every day, but perhaps you are some sort of masochist.

[The Dude]

Wow! that is really brilliant stuff...but I suppose no less can be expected from a self proclaimed "Guru". Amazingly I evidently don't have the same taste in tone or music as you do, but I reckon that is why there is a market for different guitars, amps, etc....I built a GA-20 because it has the tone I want. I have mostly been lurking here as I don't know much but it truly amazes me that you still post with abandon after being delivered a well deserved pimpslap dang near every day, but perhaps you are some sort of masochist.

Only 15 posts in and you've already earned my utmost respect.

And, FWIW, I love the sound of the old Gibbys. Nice build!

[Leo_Gnardo]

However, GA 20 is kind of boring sound wise.

Yeah, right, sure*. GA20's I repair - get that Ry Cooder - Dave Lindley tone - how BORING. NOT!

*Credit quote to salvarsan.

I'm sure Bluto's Laffalot conversion will deliver plenty of fat juicy tone, no problem.

Most who own or get a good listen to a properly working GA20 say "wow that's a helluva lot of tone for a simple lightweight old amp." But there are some who believe they must have tons of bells and whistles or else their tone is naff - that's a psychological not an audio problem.

[km6xz]

I remember those when they were new. Only a few were cheaper at the time. Olsen copied it. No problem with the transformers, they were pretty good but the caps were a weak point so I suspect you changed more than the new filter.
You will have plenty of mic gain by just clipping the RIAA network out and using the phono input as mic in. That will give a 18db increase at the mid point and 35db at 12-15khz over it is now. Measure the gain at 50 hz now and that will be the gain flat across the spectrum after you clip the RIAA emphasis filter out. It would make a very decent harp amp without any other mods. The only thing to worry about it HF noise generated in the front end that is not heard with the RIAA filter in place. Replacing plate resistor of that first stage with metal film will help any carbon comp generated noise.
Looks like fun. I love the original Hammertone paint that was the rage in the 50s. The only thing better was the wrinkle black from a decade before. Sprayed it thick with multiple coats then put the chassis in the oven for drying and the results was a uniform shiny wrinkled texture that was tough and looks professional even if done in the kitchen.

[bluto]

Thanks very much to all for the encouragement! I did replace that ancient original 470K resistor with a modern 220K since the first post and that it does indeed sound better...also wired the second channel with a early Bassman design 6SC7 circuit but paralleled the twin triodes like a Matchless and that worked well. Still need to add a bucking xfmr to the primary and plan to wire switches and cathode resistors so the PI can be a 6SL7 or 6SN7 and the power tubes can be 6V6 or 6L6/5881 (with a 5V4 rectifier swapped in), that way depending on the setup the amp can be sort of a GA-20, 30, 40, or 50 AND have a sort of Bassman channel ;-) FWIW if the codes on the xfmrs are EIA they were made by Masco in 1949 and I think they might have made the whole thing, some Masco's are very similar...

[bluto]

FWIW I made some more changes and learned a valuable lesson for conversion projects. The B+ and filament voltages were running high and I was considering adding a bucking xfmr as I did not realize that the reason for this was the light load the 6V6's put on the PT, switched to 5881 power tubes and the voltages look much better. Also switched the 5Y3 out for a 5V4, both pull 2A @ 5V and Gibson used a 5V4 with several old 6L6 PP amps....and finally swapped the 6SL7 PI for a 6SN7 (this bumps the total filament load up only about 10%). The amp is now more or less a Gibson GA-50 and is wired pretty close to the schematic below (but no tremolo)

[Leo_Gnardo]

FWIW I made some more changes and learned a valuable lesson for conversion projects.

Good going Bluto . . . now how do it SOUND? Terrific, I'll bet.

[bluto]

Good going Bluto . . . now how do it SOUND? Terrific, I'll bet.

Thanks! it does sound great and nails the warm 50's jazz/blues tone I want....sadly I don't sound so good so it might be a year or two before I post any sound clips ;-)

[Leo_Gnardo]

Thanks! it does sound great and nails the warm 50's jazz/blues tone I want....sadly I don't sound so good so it might be a year or two before I post any sound clips ;-)

Good answer! You'll notice, and maybe you already have, that when you have good sounding gear it makes practicing enjoyable. For now you could ask a talented friend to play thru your amp to record some sound clips. All my customers can play circles around me but that's no worry. We all have our talents. I fix 'em. They play 'em. Best that way for the listening public. Put "tape over ears" smiley thing here.