PY 81 as power rectifiers?

[Kalee20]

Carry on the good work... in the meantime I will have a look as I am sure I have a few small output transformers that I removed from dumped TV's years ago, which would be 'near enough' for the circuit... and would be interesting to compare performance against the mains transformer.

I wanted to do the same for comparison, but haven't found a suitable victim!

Thanks,
Amie.

Hi Amie,

I found the small single-ended output transformer I had in mind - salvaged from an old TV years ago, and I've never used it. It's a neat little transformer, just 2 terminals primary, 2 terminals secondary. If you want it, PM me your address and I'll stick in the post.

Here are vital statistics:

1) Rpri = 348Ω so will drop only 5V at 15mA anode current. Not bad.

2) Connecting to a Variac and applying 50Hz AC, Vsec = 1.95V at Vpri = 100V. So turns ratio = 51:1 thus a 3Ω loudspeaker will give 7.8kΩ load to the output valve. Should be fairly good for the ECL80.

3) Again with the Variac, Ipri = 39.2mA @ Vpri = 100V. So reactance = 2.55kΩ @ 50Hz thus primary inductance = 8.1H. (I also measured at 1V primary, got 0.552mA implying 5.8H. This is pretty typical for a small transformer with an air gap. If the laminations had been interleaved, inductance would be much higher, but far less constant. I also tried to measure at 200V but the current started rising disproportionally, indicating core saturation was kicking in at about 60mA).

Now 8H and 7.8kΩ load gives a low-frequency 3db cut-off of 155Hz so if you like deep thumpy bass, you'll be a bit disappointed. On the other hand, I'd bet it will do better than the mains transformer you currently have. And it was good enough for TV viewers in the 1960's and 1970's.

It should do you fine.