Well, two-and-a-half years in my basement proved to be just the right amount of time to let this amp age, to get me up to speed on several design quirks I had never seen before, to locate the correct schematic, and to get help at various points from the kind folks here at LTG. Thanks all.
The amp did indeed come back to life, the owner was ecstatic about it when I surprised him with it, and most importantly, it sounds great. Worth the learning curve and the time it took.
The major breakthrough occurred a month ago when I rebuilt the bias circuit. Yes, that major cluster in the northwest territory took a shot of courage to dive into, but it's my belief that the domino effect of many problems that this amp had all began with the bias.
It's not much less of a cluster now but here it is with correct values and dialed into the correct .300 VDC on the cathodes. Upper left >
Notice also the cigar-shaped resistor mounted to the right of the bias pots. After repairing the bias, my B+ voltage went from an alarming 626 VDC down to a near-manageable 595. Still 6% over the 560 volts that's required to energize the output transformer and the 6GF7 tube. This 10W, 500 ohm resistor brings down the B+ that last 6%. It works independently of the other B+ nodes, all of which were very close to spec.
My theory is that the amp has always been a little high on the B+ side, and when the bias circuit went out, voltages raised even more. Virtually all of the dropping resistors to the preamp tube plates had blown open. Needless to say, I became adept at removing the three preamp circuit boards.
Before I stumbled on the bias fix, I had replaced every coupling cap and the entire voltage regulator circuit. I rationalized later that while those changes did not fix the amp, they probably made the fixed amp sound better. Another such pair of changes were the massive voltage-doubler capacitors. Look for the pretty blue cans >
They fit in the same holes and are the same height as the originals. Nice screw terminals on the underside made for some clean work >
A casualty of the repair was one of a matched pair of 8714s, a GE. Here you see it replaced with an RCA - the tube tops are different. Thankfully each gets it's own bias control pot.
The amp got buttoned up nice and tight, some Meguiars brought a shine back to the tolex, and the amp should be good for another few decades. I gave the owner detailed instructions on how to set the bias, and also
when to set it - anytime a power tube is changed, even if it is one being swapped for the other. I also told him how to adjust the screen regulator circuit, but this is much more stable by comparison. It was built to be that way.
If you are working on a Quantum X, I encourage you to keep the amp running with 8417s. The really do sound good, and man does it blast. It's no accident that these amps originally came with cabs loaded with 15" JBL D130s!
Thanks Capn Juan for your encouragement and also Dr JayBee for rolling up your sleeves and melting solder with me.
RWood