Please share your opinions and expertise on how to size and choose audio
output and power transformers.

I want to build a tube amp, (possibly 2 channel) and beleive it all starts
with choosing the transformers and output tubes.

I've chosen to use two EL-34 in push-pull configuration.

I'm looking at these transformers from www.tubesandmore.com
P-T1650R TRANSFORMER, OUTPUT, PUSH-PULL, HAMMOND, 100 WATT ~$70.00 ea
P-T282X TRANSFORMER, POWER, HAMMOND, 500-0-500 V, 200 mA ~$77.00

abdulla abdulla wrote:
Please share your opinions and expertise on how to size and choose audio
output and power transformers.

I want to build a tube amp, (possibly 2 channel) and beleive it all starts
with choosing the transformers and output tubes.

I've chosen to use two EL-34 in push-pull configuration.

I'm looking at these transformers from www.tubesandmore.com
P-T1650R TRANSFORMER, OUTPUT, PUSH-PULL, HAMMOND, 100 WATT ~$70.00 ea
P-T282X TRANSFORMER, POWER, HAMMOND, 500-0-500 V, 200 mA ~$77.00


Are you copying a particular design? Or, are you just experimenting and
want some place to start? The design that you working with may help you
choose the right transformers.

I'll make a couple of assumptions: You'll be building a class A-B amp with
the EL34 screens attached to an "ultralinear" primary tap.

The P-T1650R primary impedance is in the right range at 5000 ohms, but
at 100 Watts rating it's probably bigger and more expensive than you
need. The 1645 (5K 30W) or 1650H (6.6K 40W) would do. I think that the
Dyanco ST-70 output transformers were 6800 ohms at a 35W rating. All of
these transformers from Hammond have the ultralinear priamry taps for the
screens.

The P-T282X would be alright with a choke input power supply filter
(about 400-450v), but otherwise with a more common cap-input filter the
voltage will be too high (500v+) for most EL-34's. The 278X
(400-0-400 20ma) or 274BX (375-0-375 175ma) or 273BX (350-0-350 175ma)
would likely be a better match for a cap-input design.

I hope this is helpful. Good luck with your project!

Paul D. Spiegel wrote:

The P-T1650R primary impedance is in the right range at 5000 ohms, but
at 100 Watts rating it's probably bigger and more expensive than you
need. The 1645 (5K 30W) or 1650H (6.6K 40W) would do. I think that the
Dyanco ST-70 output transformers were 6800 ohms at a 35W rating. All of
these transformers from Hammond have the ultralinear priamry taps for the
screens.

A follow-up to myself.

I'm told that the ST-70 A470 output transformer was actually a 4300 ohm
primary. So, that may be a better value to shoot for.