class-A PP amplifier with 2x 845 question

[hesener]

hi, currently looking into building a class-A PP amplifier using 845 at a B+ of 900V, 100mA anode current, and I am looking for recommendations on the output transformer to use.

what primary impedance would you recommend? from the datasheet, various values between 3500 and 6000 ohms seem possible.

Would prefer something off the shelf, e.g. plitron or lundahl, but if anybody can point me to a good source I would be very grateful!

thanks a lot

[jane]

A output transformer in the range 3k5 to 6k will make it a class AB amplifier. Max class A power output is limited of the reflected load of the OPT and the standing current.
P_max_class_A = { 2 x (Iq/1.414)^2 * Raa/2 } = Iq^2 x Raa/2, where Iq is the standing current and Raa is the plate to plate impedance of the transformer.

6k and 100mA limits max class A power to 30W, 3k5 to 17.5W

A better choice for true class A is probably Raa = ~10k

Jan E Veiset

[7N7]

I am sure that Jane is correct, amthough without looking at the curves from memory i would think that you might get away with 8k and of course with a lower primary impedance there is better chance of good HF performance. I am not saying it's impossible with 10k, but I should say it's tricky.

Majestic Transformers, Poole, Dorset England, could make you an excellent transformer; they are superb.

7N7

[hesener]

thanks a lot for your inputs!

[Brett]

Lundahl 1620, 1679, 1688 or 9202. I used the 1620 for my PP 813 (triode) amps.