Gibson GA-75 OT impedance?

[katopan]

Does anyone happen to know the primary impedance of the OT in a GA-75? I've found out they used EI-125 laminations but am having trouble finding any mention of the impedance.

Schematic says the PT is 700V centre tapped, which is easy enough to find. Solid state rectified with B+ of 470V shown. It doesn't mention the choke size either but as a full current pi filter with 20uF downstream, something around 5H would be more than adequate filtering.

*Edit* - Forgot to mention I'm looking for the 60's version rather than 50's. Don't know if they were different in OTs, but they were in numerous other ways.

[jazbo8]

Really rough estimation, R = V^2/P = 470^2/25 = 8.8k.

[martin manning]

I think 4k based on anode and screen both at 470V, 30W Pa max for 6L6GC.

[jazbo8]

I think 4k based on anode and screen both at 470V, 30W Pa max for 6L6GC.

In which case, the output power would be much higher than the "25W" rating. Anyway, I'm guessing the real impedance is probably between 4k-8k...

[katopan]

Thanks guys. This one has me going.

Published output power of the amp is 25W. My rule of thumb calc which is generally close for clean output power is P = (V x k)^2 x 2 / Z where k is 0.7-0.8 depending on the valve voltage drop left at clipping. With 25W clean that puts Z at 8-11k depending on what factor you use. And if the 25W is actually a bit conservative then Z could be as low as 6k6. All viable options.

If it turns out to be 8k them JTM45 iron could be suitable for a build.

[martin manning]

Is this the schematic you are referring to? 2x 6L6GC with 470V on plates and screens and 25W output power does not make much sense.

[katopan]

Yeah that's it Martin. Specs are quoted as 25W output in the link below. Can't find it now, but I've also seen a scan of the catalogue page for this line of amps and it has the same output power listed. Lower power than typical 6L6s at that voltage must mean higher than normal OT impedance.

http://www.0rigami.com/gg/amps2.html#GA ... _Crestline