I just joined the forum and posted my introduction on another thread. This thread is a question I have regarding my MK III rebuild and bias problem.
Here's a pic of said amp, please note the weird coil choke between the 100uF - 160volt cap off the bias pot...
Sorry, can't post an outside pic for seven days...
Ok here's the story: After 10-15 years sitting on a shelf in my closet I finally got the gumption to do something with these MK 3s. I did a lot of googling, read a lot of threads here and there and ended up calling Chad at Triode Electronics. I'm no electronic circuitry wiz, but I have built a few kits over the years, so know how to solder, follow directions and be safe. Chad talked me into the SDS cap mod, but thought I should keep the original driver as they usually don't go bad, and give it that vintage dynaco sound. My amps had been previously modded with odd size caps taped in here and there willy-nilly along with the original quad cap. I was able to proceed on removing extraneous stuff and installing the new stuff with Chad's help, found my old tubes and started them up one by one.
Both amps came up to bias (1.56 volts) and after warming up sounded great in my system, but I had one issue with my rectifier tubes. I had eight GE 6550 tubes, one Sylvania GZ34 and two smaller unmarked tubes that could only be rectifier tubes, but I didn't have the courage to try them out, so I used the one GZ34 for both amps, trying it in one than the other. That, plus one amp took full pot defection to come up to 1.56 volts. I ended up sending all my tubes off to RAM Labs to be tested, one of my 6550s had their name on it so I called them up. they test all the tubes, found one bad one, the others tested ok, plus those two smaller ones were strong GZ34s. I put the tubes back in and biased them again, one amp first then the other. I left the one amp on while I biased the other, so it was on about 30 minutes. Then I shut them both off, hooked up preamp and speakers and turned them on again. When I turned on the left one it immediately started humming louder and louder till I shut it off, 4-5 seconds.
I unplugged everything and turned the amp over to discover a bad solder joint broke between that cap and weird coil choke off the bias pot. I re-soldered it and turned it back on to re-bias, that's when the bias just keeps climbing past 1.5 volts no matter where the pot is set. After reading a 2009 thread on here where you suggest testing pin 5 and 6 on the output tubes, I test both between -44 and -76 volts. I tested the bias pot, it's ok. I checked the precession 11.2 ohm resistor, it was exactly 11.2 ohms. The caps look ok, I don't know how to test them though. I also don't see any other broken solder joints.
Any suggestions? Thanks in advance.