small tube output transformer

[ddameron]

I decided to try to build a small SE output transformer. Because I was interested in the low frequency response, did not interleave the windings, just the secondary over the primary. The lamination core was 0.75 inch (1.9cm) square. The primary was 1600 turns #33 awg thick build, the secondary (8 ohms) was 85 turns #22 (0.69mm diam). This is a turns ratio of 18.8:1. These windings barely fit in the window. The first tube I tried was a 50L6 with about 120v plate voltage with a 150 ohm cathode resistor, 45 mA plate current. The low frequency 3dB frequency I measured was:
Butt joint of EI laminations 70Hz,
Tracing paper gap (0.045mm) 100Hz,
Bond paper gap (0.095mm) 110 Hz.
So I see a larger core is required for lower frequency response. A 6BQ5, which is my goal, will need more wire and a larger primary impedance.
Thanks.

 

[TonyTecson]

give it more turns with a #35 and you can get lower Hz...

 

[ddameron]

Pix of transformer

Thanks.
I wired this output transformer into a 6BQ5 amplifier, the chassis on the right. It is the amplifier on the left side. The right side is using a smaller vintage 6BQ5 output transformer. The power supply chassis is on the left. The amp is driving a Dayton B652 speaker and its input is a Toshiba "walkman" style radio.