Two Mark III's on a Stereo 70 chassis ? Well I though I would give it a shot and attempt to create a 60-65 watt per channel amp (an ST-125 ??) on a Dynaco ST-70 chassis. It seems to have worked out OK.
The power transformer instead of having the 360-0-360 volt secondaries of a normal PA-060 power transformer has 420-0-420 secondaries and is rated at 425 milliamps. A normal PA-060 has a stack lamination of about 1 1/2 - 1 5/8" while this power tranny has a stack lamination of a little over 2.7 inches. The new power transformer puts out about 500 volts on the first section of the quad cap and about 485 volts on the plates of the output tubes.
The output transformers are interleaved/layer wound with M6 grain oriented laminations and are 3 7/8" tall, 3 1/4" wide and 3 1/8" deep (about 1/2" taller, 3/8" wider and 1/4" deeper than a stock A-470 output transformer). Each output transformer weighs about 2 pounds more than a stock A-470 output transformer.
A Dynakitparts (80, 40, 30, 20 @ 550 volt) quad cap was used for power storage but was supplemented on the second section of the cap by two banks of 195 Mfd @ 800 volt caps mounted on terminal strips. I am measuring about 495 Mfd on the second section of the quad cap with my cap meter. I am presently using a Weber WZ68 Copper cap solid state rectifier which is equal to two GZ34's and is rated at 450 milliamps. The Weber has a short built in delay of about 2 seconds before it applies the high voltage and gives a higher B+ than any of the Mullard GZ34's that I tried in there.
A VTA driver board was used with some minor modifications. Instead of a 2200 ohm resistor to drop down the B+ to the driver board down to 400 - 410 volts, a 6800 ohm resistor was needed to drop the B+ about 70 volts. .27 Mfd @ 750 volt Vitamin Q coupling caps are used. These caps have the outside of the case conductive so I wrapped them in tape. The triode/pentode switches that I usually put in these amps have taken the extra voltage in stride. I flipped each one back and forth about 100 times while the amp was playing with no problems.
The amp is dead quiet, plays very well and is noticeably more dynamic at higher volumes than a comparable VTA board driven ST-70. I am presently running a matched quad of the new Genalex Gold Lion KT88's in this amp. An A and B comparison with my pair of Mark III's shows pretty much equal power with maybe a slight nod (according to my friend Paul who listened with me) at higher volumes to the newer amp built on the ST-70 chassis . This was due undoubtedly to the better power supply on the newer amp.
Photo of this amp and internal wiring at link below ...
http://i9.photobucket.com/albums/a78/Bo ... osite2.jpg
Bob Latino