by EWBrown » Fri Dec 07, 2007 7:20 am
I have one of the Trafomatic "ST35" toroidal power trannies and the OPT pair, obtained from Boris Sasic at Vista Audio Engineering (and Shannon has the other set). Yellow_Light_Colorz_PDT_03 .
I finally got around to setting up and testing it last night on an older DIYtube Rev B board.
The power trannie has a lot more capability than the stock PA774s, after two hours run time, it barely got warm.
The "gotcha" (there is always an unexpected "gotcha", it seems) is that it delivers excessively high B+ (well over 400VDC) with my local AC power which runs around 123VAC. EL84s won't last very long with that B+ level :o
I had to turn down the Variac to 113VAC output, in order to get a more reasonable 380VDC B+ at the OPT CTs. At that setting the filament voltage was 5.95VAC, but everything ran fine.
Since this is the G*S*G topic, I figure that this trannie might be just about perfect for a SS rectified 300B GSG design, as the 300B likes a higher B+ voltage, and 420VDC isn't excessive for that tube.
The "fun" part was figuring out the definitely non-standard color codes of the trannie leads, blue are teh two "hot" secondary HV leads, red is the CT, one filament pair is white, the other gray, and all four primary leads are orange. There are attached numbers on small plastic clips, but one still has to figure out the windings' phasing before proceeding too far.
The OPTs also have "weird" color coding, other than their B+ CT is red.
Of course, murphy's law struck, and on my first setup, the amp acted like a power oscillator, so I had to exchange the OPT primary connections to reverse the phasing. on the output side, yellow is Zero, white is 4 ohms and brown is 8 ohms.
FWIW, the blue (A1) and its associated SG black lead (2) connect to the plate and SG of the "front" EL84s, and the orange (A2) and its associated black SG lead connect to the "rear" tubes.
Once I got it all figured out, the amp sounded pretty good.
So my final approach will probably use the PT for a 300B GSG and use the OPTs and a PA774 or Hammond for an ST-35 design, or perhaps just try the set in a new design later on...
/ed B in NH
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