Thanks to Jack for his willingness to 'experiment' with this build! Not being a 'tested' version, and with the costs involved to acquire those massive output transformers from Transcendar, there was a bit of a 'risk' involved, not just 'damage', but that sonic performance might be compromised as well. That said......the amp turned out beautifully!! Once again, the circuit Mr. Gillespie has designed worked wonderfully well, and making the transition to these alternative output transformers wasn't as difficult as it could have been. I might add a word of praise to the Transcendar folks for a great product as well. Not my first go-round with them and their stuff......and I've been 'completely satisfied' every time!!! Anyway, as per the few posts above, there was a bit of an issue in getting the proper voltages lined up. Due to the increase in DC resistance of these new transformers, the 'supply' voltage going 'to' them had to be increased. In the end, the Power Supply first stage dropping resistor ended up being about 550 ohms, which provided a B+ of about 275 VDC to the OTs. I used two 10 watt resistors to get to this resistance level, measured my voltage drop, and I think there's less than 2 watts per resistor of power being dissipated. This 175 'in' to the OTs resulted in a plate voltage of 255 VDC....right on spec! However, the increase in B+ also resulted in an increase of the B+1.....which was previously 'adjusted' in Dave's circuit by the 820 ohm resistor. To get this back to the proper 255 VDC a resistor value of 1.5K was used. And, again, 255 V...right on spec. The only other thing worth mentioning is that in using these OTs, the leads available included a 'non-used' Ultra Linear tap on the primary side, and 4, 8, and 16 ohm taps on the secondary side. If you note the pic above you'll see that there are 2 'red' speaker terminals for each channel. Jack had wanted 16 ohm outputs in keeping with some of his 'vintage' speaker selections. So, the 8 ohm tap was used in the circuit, including the feedback section, and the 16 ohm taps were used as secondary speaker connections on the back. This could have just as easily been the 4 and 8 ohm taps.....again with the 8 ohm taps following the schematic. In this case, the un-used 4 ohm taps were simply covered over with shrink wrap under the amp. Likewise with the UL taps. Care taken to assure not 'shorts' could occur ….ever!!! Beyond that, the new tubes supplied by Jack worked beautifully, all of which were JJ brand! I was quite impressed overall with how this whole project turned out. Again, thanks to Dave for the Circuit....and to Jack for the chance to build the amp for him!!! Jack, if you are so inclined, you might consider posting a pic of the underside so as to show some of the details of the wiring and layout.
Tom D.
