Converting Dynaco ST70 Power Transformer for 12.6V

[eeyore]

Hi, is there any way of connecting the two 6.3VCT heater windings on the ST70 power transformer for 12.6V operation? I was wanting to convert one for use with the GU50 tubes?

Thanks!

[Tom Bavis]

Yes just connect them in series. If you get 0.0V instead of 12.6V, reverse one.

[eeyore]

But what about the CT on each of the 6.3VCT windings? Do I just leave them unconnected? Then do I make the series connection point the CT of the new 12.6VCT?

[eeyore]

Is this what you mean? Option 1 or 2?

[llwhtt]

Both of those options are fuse testers or xfmr fryers. Option 2 is close but no cigar, skip both CTs and use the junction of the 2 windings as the CT. Another possibility is Option 2 again, skip both CTs and use the junction of the 2 6DJ8s as the CT. Both will provide the correct voltages but one may be more less noisy than the other.

Craig

[eeyore]

Craig, do you mean not to ground the two CT, just leave them unconnected, and then ground the connection point/junction. But in theory, is it just better to get a purpose 12.6V transformer?

[JesseG]

Just disregard the two CTs for the 6.3V windings - in fact, make sure that they are well insulated so they DONT make a connection. Then use the connection point of the two 6.3 windings as your CT.

It probably won't matter which end of the 6.3 windings you connect together, but it might. If the 6.3V windings are stacked together, they might try to interfere with each other. You might want to try it one way and then the other. The easy way to test this is to apply primary power to the trafo with no load and listen up close to the transformer. If it is silent, then you have the 2 windings connected the best way. If you can hear a slight hum, try connecting them the other way 'round. With no load, the trafo should be completely silent.
BE CAREFUL of the HT winding wires while doing this - they can BITE!

Jess

[eeyore]

So is this what I should do? I need to make sure that the phase or polarity is of the two 6.3V windings are correct and I need to make sure the CT of the 6.3V windings are not connected and well terminated so no possible contact can occur.

If done as per the advice, will this give a safe operating environment?

[llwhtt]

I was thinking use the junction of the 2 windings as the CT "OR" the junction of the 2 6DJ8s as the CT. But your last diagram should work also. In all cases DO NOT use the CTs of either winding, tape or heatshrink them so they don't short out on anything.

Craig

[JesseG]

Originally Posted by eeyore

If done as per the advice, will this give a safe operating environment?

You got it