Gibson GA-40 OT?

[luthierwnc]

Hi Guys,

I'm DIYing a version of a Gibson GA-40 Les Paul amp (6sq7 oscillator). I can't remember where or why but I thought I read Gibson used a heavier output trannie on this than they usually did on their other 6V6 amps. Does anyone know the approximate heft of the OT on this unit?

Thanks, Skip

[Diablo]

I don't have an answer for you, but another question on the GA40 OT. Anyone have specs on the primary resistance?
Is it a 6.6K ohm primary or did Gibson use something else?
The schematic doesn't list any specs.

[Two Tone]

my ´58 GA40 measures 10K on the primary,
just like my ´55 GA40

[Two Tone]

uh,sorry that´s a ´55 GA20

[Diablo]

my ´58 GA40 measures 10K on the primary,
just like my ´55 GA40

Thanks Two Tone for the information. How did you measure the primary impedance of the OT? Did you use a method like this?

Output Transformer Impedance

[Two Tone]

yes,exactly
in fact,it was more well over 10K,
1oK5 I believe

[Two Tone]

OK,I looked it up,
it was 10,2K for the GA40

[Diablo]

Thanks Two Tone for the info, I'l make a note of it for when I will build my GA40 clone. Might you have any notes on your 6V6 plate voltage? The schematic says around 310 volts.

[luthierwnc]

Thanks TwoTone. You see 10k and north on a lot of 6BQ5/6V6 amps of that era. I think the one in there now is around that. On your amp, how big is the OT physically?

sh

[Two Tone]

yes, those voltages are spot on,
I´m using a 5AR4 with
a 270 ohm cathode resistor,
adds up to 370v and a little more
headroom.
Regarding the size of the OT
I have to check this next week

[Diablo]

Thanks Two Tone.....great info...

[luthierwnc]

pics

I've gotten a couple PM's on my build so here it is. This was a Hammond AO-29 organ amp (great platform donors, btw). It is ready to spark test. I'll run it through an EVM12L Thiele as a reference and make some educated guesses on the permanent speaker from there. I'm leaning towards a Weber 12A150A.

sh

[luthierwnc]

Speakers for GA-40

Now that this thing is done, I keep looking at how long that chassis is and that it could easily be a 2X10 combo. I don't have one of those right now.

Whadda you think this circuit would sound like with a couple of 10's?

Skip