Dynaco A470 and 4P1L Options

[gnnett]

I have a pair of Dynaco A470 output transformers that I have not got around to sourcing the rest of the parts for an ST70 build. But I do have a handful of 4P1L that I picked up when I had a Bottlehead Smash. The Smash is gone but wondering if this parts bin combo is a possibility?

I don't think the 4.3K center tapped primary impedance would work with the 4P1L, which I have seen recommended impedance of between 2.5K to 3.5K.

Would paralleling tubes, or using 4, or 16 ohm taps, change this?

Speakers are again parts bin Goodmans Axiette II 8in, which I would run open baffle down to 80-100Hz, where 5 to 10W would be more than enough.

Cheers

Grant

 

[6A3sUMMER]

Accurately measure the DCR of your Goodmans Axiette II 8in.
Suppose you measure 12 Ohms. That is the Minimum impedance of the Axiette.

A470 4300 Ohms and the 16 Ohm tap:
Load the 16 Ohm tap with 12 Ohms.
4300 x 12/16 = 3225 Ohms at the '4300' Ohm primary.
Seems OK to me.

Or,
A470 4300 Ohms and the 8 Ohm tap:
Load the 8 Ohm tap with 12 Ohms.
4300 x 12/8 = 6450 Ohms at the '4300' Ohm primary.
This seems Ok to me, and . . .
That 8 Ohm tap will give a higher damping factor, and a lower transformer insertion loss than using the 16 Ohm tap.

Try using the 16 Ohm tap
Try using the 8 Ohm tap
See which one you like better.

Be sure to use the A470 as a push pull output transformer. You need equal current from one end to the center tap, and the other end to the center tap.

About 28 Years ago, I carefully pulled an A470 apart, and re-stacked the laminations, all E's on one side, Kapton tape for the Air Gap, and all the I's on the other side.
Then I used the modified air gapped A470 to build an SE amplifier.
I used a single EL34 triode wired, and with the cathode on the 16 Ohm tap for 3 dB of cathode negative feedback
It went all the way out to 60kHz, (better than my tweeters, and better than my ears, ha ha)

Have fun designing, building, and listening.