• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Using isolation transformer as a PSU transformer???

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I just read threads by Sy and Tubelab that using isolation transformer such as Triad N-68X is appropriate for PSU.

They have a 115 / 230 volt dual primary and a single 115 volt secondary. I have been known to hook them up backwards making a 115 to 230 volt transformer.

Could anyone explain to me how to hook this up backwards? I think I understand. But, high voltage always scares me. :eek: And I am a newbie! :)

Thank you.
The dual primary means it has two 115V windings on primary side and one 115V winding on secondary side.

To hook it up backwards, hook up the secondary winding to the mains, and connect the two primary windings in series and hook this up to your power supply.

Make sure to connect the two primary windings in-phase. Your transformer should say which end is 0V and which end is 115V on each winding. This sounds like it's producing DC, but it's actually indicating phase direction. So connect the two primary windings in this order: 0V-115V^0V-115V.

Hope this helps,

Thanks James for the reply.

I have already hooked up 115v to 115v before. So, I assume that the transformer is already in phase? I am a scary cat. Reading through the high voltage warning page got me more scared, having already been zapped! :hot:

Thanks again.

Maybe I'm a worry wart. It would not surprise me to find that the single secondary has a few more turns than the dual primaries have. That would compensate for regulation effects.

A 100% solution to the B+ problem is to use a "full wave" doubler in combination with the single secondary. Low noise UF5408 diodes will surely stand up to the conditions. A pair of low ESR 250 WVDC/470 muF. 'lytics in the doubler stack ensure a "stiff" rail is the outcome. Follow the doubler stack with a low DCR choke of approx. 1 H. Finish the supply off with a final reservoir capacitor.

100 mA. of DC at slightly more than 300 V. is easily available from the 50 VA N-68X.
Thanks Eli

Thanks for the reply. I am thinking of using 600v @3 rated Freds as fullwave since I have them in my hands. Duncan's PSUD seems to indicate that that will be acceptable.

Eli, I read one of your post at "preamp for my dad" thread. I've been collecting PSU parts for the plan. And I decided to go with resistor load instead of choke.

Thanks again.

If the way you hooked up worked (voltage measured correctly and transformer did not heat up quickly, and no magic smoke escaped :) ) then it sure is in phase.

It's a good practice to beware of high voltage but don't let that turn your enjoyment of DIYing into a stress. The whole point of DIYing is to enjoy building things by yourself. Things can't go wrong as long as you follow basic safety advices given by members here.

Kind regards,
No Smoke!

Thanks James

No smoke for sure! Wow! That will save a lot of trouble for me. I now have enough of B+.

If I may ask another question. For heaters, I have 20v @2A transformer with a center tap which gives me 12-0-12. Is it ok to use two differnt rectifiers? In other words, I am going to regulate one 12-0 for 6.3v heaters and the other 12-0 for my remote controlled volume?

Thanks in advance.

It sounds like it was because of reaching maximum allowable input-output voltage difference but the data sheet for LM350 states that maximum allowable difference is 35V.

Getting another transformer of right voltage for this job sounds like the least troublesome option. I recommend getting a 15V transformer and use 6V and 12V regulators, to account for diode forward voltage drop and loss from regulator reference voltage for 12VDC. 12V transformer could work if you use loss loss diodes and regulators.

Hope this helps,
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