Gibson GA 40 with no tremolo and tone switch

[mr.tom]

Hi everbody,

I would like to build something like a Gibson GA40 amp. I don't need the tremolo, but i would keep the second channel (without the tremolo) to thicken the tone. I would hardwire the two channel inputs so I don't need a jumper cable and only two jack inputs would be needed.

I would like to build something like a tone control in this too. Something like the matchless 6 position switch. I would keep the cut control, i think its verry useful.

I added a little bit of the schematic. I circled something i don't understand in the schematic.

Is this an interesting project? I think so because of the combination of the pentodes (5879) with the 6V6's.

Can someone help me figure out the schematic and maybe a layout?

Thanks dudes,

Ps. Please excuse me if my english is bad.

[Phil_S]

That is the tremolo. Here is your picture with the tremolo erased!

[mr.tom]

Hi,

I attached drawings of what i think my schematic would probably look like, and the orignal:

I redrawn it without the trem part, thanks Phil S! Is this correct, i mean the two channels in this configuration?

I added a tone switch. Would this work? I mean in the DC30 schematic this tone circuit is in front of the 1Meg volume pot.

[overtone]

You might want to take a look at what a guy in Germany has done building a stripped down GA-40 without the tremolo:
http://www.tube-town.de/ttforum/index.php?topic=8873.0
I have not checked the schematics posted there and good luck with the language!

[David Root]

The circled red components are just a fixed steep bass cut on the microphone input. You will notice the gain on the mic input 5879 is way higher than on the guitar input 5879, 510K plate resistor vs 100K plate resistor. This is because non-crystal mikes in those days were very low output. Gibson first did this with the EH-150 fifteen years before the '57 GA-40. Putting a humbucker in that mic input makes it sound like a browned out Marshall, very tasty!

The tremolo is the other section you cut out in your next posting.

[mr.tom]

So the erase of the tremolo is correct in my second post? What about the tone control? Can this be put here, between the 1meg pots and the phase inverter?

[David Root]

Yes that should work.

[mr.tom]

Thanks, I will redraw the schematic and a nice layout and post it over here.

Is there a reason why matchless puts the 6 position tone switch before the volume pot in het ef86 channel?

Thanks

[KindaFuzzy]

I did one of these for a friend, with each 5879 voiced differently, and the 6sq7 was a gain stage before a proper Fender type tonestack, it was great.

[mr.tom]

@Kindafuzzy:

Do you some schematics lying around of that amp of your friend? or maybe a layout

greets

[KindaFuzzy]

I think this is the schematic of what I did, I might have ditched the two volumes, and just hardwired a single resistor, but you get the point.

[andrew]

Hi, I am using this PI with 4 EL84s but the 1M pot at the 220k - .02 -.005 - to ground, has no effect at all. I have some KT66s if the EL84s are a poor choice. Thanks for any help.

[Andy Le Blanc]

Thats a para phase inverter, thats where the gain is, choose a low gain
tube type with the hi gain pentode pre.

That line of cap and resistors to ground you questioned can be used as a
contor control if you add a 1m pot between it and ground.
the pentode pre is going to want to see a hi impedance tone control
which will limit the choices.