Choosing the proper PT

[craigtone]

Tom,

I like your P/S from a price standpoint but am curious why you used the 2nd 120/12 transformer? Couldn't you just pull the filaments out from between the first 120/12 and second 12/240? Obviously pick one with enough current to handle the mains and filament like a 4A 120/12 and a 2A 12/240. Just trying to keep cost down! Yellow_Light_Colorz_PDT_09

If that is correct, I need 2A at 6V and 200mA at 240V for my design. My thoughts are as follows:

Line -> 3A Fuse -> 120/12 3A -> filament tap (run them in series) -> 12/240 4A -> Rectification -> B+

I'm assuming on the first transformer:
2A for filament
1A for Main (only need 200mA)
=3A

Second transformer:
240/12 = 20:1 turns ratio, so 200mA * 20 = 4A

I need 6V filaments, but can run both tubes in series and use 12V.

The idea is to keep this build as cheap as possible! I just want to keep it at my desk here at work and run uncompressed CD's via iTunes into it for a headphone amp. I'll have to buy some decent ear muffs now though! I'm even toying around with getting a bare HagUSB board and only populating the USB, DAC and NOT SPDIF and wiring an input selector switch to switch between RCA in and the headphone out of the HagUSB so the amp would have RCA and USB inputs!

[TomMcNally]

Well, I have a bunch of the NOS Triad 12 v 2 amp transformers. At first, I used one for the filaments and one for the back to back, feeding the other transformer which is tappable 6/12 and 120/240 (see them at mpja.com) Anyway - they ran kind of hot, so paralleling them cooled them down. Ideally, maybe a 12 v 4 amp transformer would be better. I have a feeling the reverse transformer isn't all that efficient.

So yeah, I built it with all junkbox parts except for the new Hammond OPT's ... I've since upgraded to new electrolytics, the 70's vintage junkbox caps may have been part of the heating problem - excessive leakage, because it runs cooler now.

I sent your 4-400C tube - you should see it Monday. I shoulda waited to read this and I coulda sent you some filament transformers. I have plenty.