OT impedance for P-P 6GT5?

[bob p]

I'm looking at an old PA amp that used a pair of 6GT5 in a push-pull output stage. Does anyone have any idea what sort of primary impedance the OT would have for this kind of beast?

As you know, this is a horizontal deflection tube, and data on these things in audio applications seems sparse.

any help would be appreciated (short of telling me to go measure voltages and do the calcs).

thanks.

[madkatb]

A quick google search of "6gt5 circuit" came up with this:
Wade's Audio And Tube Page
About halfway down the page, look for "Frazier F106c power amp". There's an attached schematic if you follow the link as well as a circuit description. It's four output tubes but they tell you the best bias current so just chop it in half. 12AU7 inverter.

[bob p]

Thanks. I had seen that schematic, but I didn't find the answer I was looking for. Am I missing something?

[tubeswell]

Based on Old Tele Man's rule of thumb for Pr Z on 1/6 to 1/8 of the plate resistance for pentode and tetrode output tubes, and these here pages:

NJ7P Tube Database Search

http://scottbecker.net/tube/sheets/049/6/6GT5.pdf

http://scottbecker.net/tube/sheets/049/6/6GT5A.pdf

I'd say 1k8 to 2k5 at a rough guess

[bob p]

Hmm. That's lower than I was hoping. Now I'm thinking the OT iron won't be useful for much else.

Thanks.

[tubeswell]

Well I'm only rough guessing about it. You could always half the load on a 4k Pr OT (if that's what you have)