by sorenj07 » Wed Oct 03, 2007 7:17 pm
First thing - reconnect the chassis/bus ground to earth. I'd cut it but measured 10-15VAC between the two, and touching the amps increased hum a bit. This is my left monoblock by the way, the one that had buzz on top of the hum and noise. I've since installed RCA brown-base 5R4GYB's.
Then I measured plate voltage on all the tubes and found a stupid wiring mistake, where the 47K plate resistors in the driver stage weren't in parallel - I had 47K going to one plate, then 47K off that PLATE to the plate of the other tube - in effect, one tube had 94K. I'm surprised the amp worked at all the way it was set up. I fixed this but didn't notice much difference in buzz.
Then, another "doh!" moment. It turns out, that I stupidly had left the negative bias tap off the B+ connected, thinking that with the tubes disconnected, no current would be drawn. Of course, there's a bleeder resistor after the rectifier to ground. One of the capacitors was only rated for 160V and was seeing 300V on it, and was bulging. I disconnected the coupling cap from B+ and let that wreckage sit on the ground bus - can't do any harm the way it is. I had hoped that this would have fixed my problem, but I think it only decreased buzz a bit.
I'd been using a portable CD player to test the amp but when I connected it back to my laptop, I had the most horrible hashy noise on top of the buzz. I tried cutting earth ground, which made my laptop have only as much buzz as with the CD player, but had 15VAC on the chassis again!
Damn, this ain't very easy. The only nice news is that with a B+ of 420V, and a 30V drop on my 470 ohm bias resistors for the KT77's, I'm dissipating a comfortable 25W (64mA per). I'm considering just keeping cathode bias because this lower plate voltage/higher current seems a bit closer to class A - am I right?
Anyway I have work to do but I'll keep plugging away at this amp (the left one) until I get it shipshape. I wish I had my workshop - I'm soldering on the floor in my dorm hallway!