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Transformer identification

Posted: Sun 09/19/21 5:30 pm
by Ronron
I am planning to build an 18 watt amp. First problem is trying to find out if the power tranny I already have will be sufficient. All I know about it is, it works, when it's fed 117v it has the following secondary voltages under no load. Ht is 325-0-325. with 6.3 and 5v for filament and rectifier. It is quite hefty, 7.6 lb. Also the filament leads are solid copper and much thicker than all the other leads which are stranded. I am really not too savy about substituting components or electronic theory, I can basically follow layouts and schematics but get over my head in a hurry when trying to deviate from the plans. Any help would be much appreciated in determining if this tranny will suffice.

Re: Transformer identification

Posted: Sun 09/19/21 11:44 pm
by TriodeLuvr
That's a huge transformer, probably enough to power a 25W/ch stereo amp (or more). That much weight suggests a 5V winding at 3A and a 6.3V winding at 5A or 6A. Difficult to know for sure about the HV winding, but it should be way more than you need for 18W.

These are guesstimates, of course. Don't blame me if it's something weird rated for 6.3V/40A and 650VCT at 1 mA. :lol:


Re: Transformer identification

Posted: Tue 09/21/21 6:57 am
by colossal
325-0-325VAC secondary at 117VAC input is way too hot for a traditional 18W amp. The target plate voltages are on the order of 340-345VDC. If your mains input is actually 120-123VAC or even more (don't know if you were testing at 117VAC on a variac), the rectified voltage is going to be even higher. You might get into the ballpark using a 5Y3 rectifier.

Using zener diodes to drop voltages or other means is something you could also consider to shoehorn that transformer into your plans, however, IME/IMO getting the right transformer from the start is the easiest approach.

Re: Transformer identification

Posted: Wed 10/06/21 5:17 pm
by katopan
Sounds like a suitable PT for a nice 30-35W output amp, valve recto and either EL34s or 6L6-GCs. Too big for an 18W.

A JTM45 replacement transformer is listed a bit above the 7lb mark, to give you a comparison.