Gibson GA-20 circuit?

[Phil_S]

Yes, the old Gibson GA-20 with a 6SJ7 input and cathode biased 6V6's...anyone have any idea about plate voltages and what sort of PT? Thinking about building one just for the fun of it. Only one input channel, so I won't be dividing the B+node for the 6SJ7's.

For the OT, would this be 6600, 8000 or some other primary?

I think I've got enough stuff on the bone pile for one of these.

Thanks.
--Phil

[PaisleyTube]

Don't know about the plate- and B+ node voltages but I hope to get some info about that from a tech-buddy.

Some GA20s did have 220k plateresistors for the 6SJ7s and those are nicer sounding according to my buddy.

Over here (Netherlands, Europe) they're very rare, I once did have one at my shop for a quick fix but I didn't have the time to check/measure the amp more in depth....

MercuryMagnetics does sell excellent transformers, also the OT and PT for GA20s and it's possible to get them with non usa-primary-voltages.

[Andy Le Blanc]

Go with what you know as far a power side, tranny, power tube, plate volts,
rectifier, etc.....
With a simple circuit the mistake is to be overly concerned with the details of
a very generalized design. The PS has 2 resistors and 3 caps.
RC-18 has a resistance coupled chart on the tube type, the design max plate voltage is 300v,
but if your going to try dressing it as a triode the max is 250v as is the "typical operating conditions" data.

Most V1 Ep figures I've seen on Gibson were pretty low, 110v to 150v,
even B+ figures on some after rectification as low as 300v.

[Phil_S]

Andy,
If you must know, this is your fault . After that exchange you and I had for the 6SJ7's, I found another in the bone pile and began to think there must be a use for it.

What I know: it uses a 5Y3GT, which has a max of 350-0-350 on the input side, and that would yield around 385VDC. I agree with you that Gibson would not have pushed to this limit. Let's say those 6V6's are in the upper 200's; let's call Va=~275, maybe less, not more. I guess I just let the rest fall into place from there?

I've got a 230-0-230 and a 240-240 (no CT). I suppose those are reasonable choices?

I've also got a 114-114 no CT that I could run through a doubler.

This being a "low voltage" amp, you think it matters much if I go with a N4007 diode rectifier?

--Phil

[flood]

i have a ton of 6SJ7s, and thanks to my current build (see: https://tubeamparchive.com/viewtopic.ph ... light=6sj7) i've been looking for other pentode based amps. specifically 6SJ7s, cause i happen to have a LOT of them...

since i have parts and transformer sets (275-0-275 PT, 8k CT OT) for 4 more 18-25 watters, i think the gibson GA20 all-octal version is going to be part of my growing mania.... this one particularly because i have a couple of 6SC7s and some top-notch 5Y3s among other rectiiers ready to go.

[Andy Le Blanc]

Whats the old rule of thumb, use components that are rated twice the working voltage.

There is something very sweet about a tube rectifier. You might have a
chance to use a lower volt/ma rated tube, 6X4, 6C4A, 5Y3.
Maybe even an old four pin 80, your very close to some of the old BR's
with that scheme, you could use very old tubes with it and get a chance to
hear some interesting tube type's.