Help selecting Primary line

[angelodp]

On my way to completing a Vibroverbish ( encore ) build. I have this very beefy PT and did some tests on it. All readings done without tubes installed

It has 4 taps on the primary side. With the Bl - Bl/g ( A- B ) connection, inputing 117vac

I get slightly higher than called for voltages for the heaters with
7.1vac for the preamp/power tubes and 5.5vac for the rectifier tube. That also gives me 764vac across the two red leads for B+.

If i select the A-D connection for the 117v input, that brings the voltages down to 6.4vac and 5vac and 698vac.

Question is. If i use the higher voltage setup will I get back to the correct voltage with all the tubes installed…. a drop in voltage from the resistance of the tubes. I am not far along enough to test that proposition and i want to select the best primary connections in order to proceed.

Thanks Ange

[IMG:819:1024]http://i260.photobucket.com/albums/ii9/angelodp1/EncorePT-voltages_zpsef996ea0.jpg[/img]

[jjman]

The secondaries will drop under a load. Use the 117 connection.

[angelodp]

So the 117 taps should conform to the correct voltages once the circuit is in place. Can you detail the calculation on that.

Thanks

[tele_player]

There's no way to calculate it without knowing more details about the transformer's resistance and the actual load.

I'd build it using the 117 hookup, but test it after build before cutting any PT wires. If voltages are too high, maybe then rewire to use the 128 primary.

You mentioned it's an encore - is it an Allen transformer?

Robert

[martin manning]

I'm betting the the A-D primary hookup will work best in the end. Line voltage at 117 is a little low by today's standards and you could easily see 5% more than that. I would rather have the filaments on the low side. In any case, it's only one wire to change, so just wire it in and try it both ways.

[angelodp]

Its not an Allen transformer. It a doner PT from a Wurlitzer organ I believe ( all 4.5 lbs of it ). I typically see 117v at my place, but point taken. Its an easy swap to make.

Thanks guys.

[pdf64]

Expect a PT's regulation to be somewhere between 5 and 10%, ie the increase in secondary voltage from all secondaries at full load to all secondaries open circuit.