My MK 3 Bias problem...

Hello,
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.

John

peterh wrote:change output tubes.

Thanks for the quick reply. I forgot to mention that I already swapped all the tubes including driver and rectifier tubes from the good amp to the bad one, still had runaway bias. Just for fun I'm going to double check the "good" amp biases up correctly with the tubes out of the bad amp. I'll report my results.

Thanks,

John

Ok, I just swapped the tubes out of the bad amp into the good amp, they worked fine in the good amp, biased up nice and stable. Is there a way to check a capacitor with a multimeter? The only component that I worked on was that bias cap, maybe it's damaged.

I just replaced that bias cap but it didn't make any difference, the bias still climbs out or control.

peterh wrote:

GlacierJohn wrote:I just replaced that bias cap but it didn't make any difference, the bias still climbs out or control.

Strange.  Recheck the groundings of 11.2ohm, recheck solderings and tubesockets

Well of course it turns out to be a bad solder joint, duh! It was the little ground wire off the back of the new SDS power supply. Thanks for your help, I learned a lot more about my amp in the process.

John

Everything is sounding great, but I have a few questions;

1) Is there a way I can test power at home with just a multimeter? I don't have an occiliscope to see where clipping starts, so I'll have to use my ear, but it would be interesting to see if what kind of peaks I was getting at comfortably loud levels.

2)It's been a long time since I ran the MK IIIs, so can't remember how much heat they put out, do the MK IIIs run pretty hot? My MC225 with the much smaller 7591's runs much cooler. Also there is a "hot amp" smell that could be all the skin oil and latent stuff burning off the amp after sitting for fifteen years and a few weeks of handling.

2a While I was trouble shooting my bias problem I found some discrepancies on values from the original manual. Voltages were very consistent between both outputs tube in both amps, but high:
        - Pins 3&4 were 566-567 volts, manual calls for 475
        - Pins 5 and 6 were within limits ranging from -50 to -78 volts variable with the bias pot
Are the high voltages on pins 3 and 4 bad? Could that be why the amps are running hot?

Thanks,
John

peterh wrote:

GlacierJohn wrote:Everything is sounding great, but I have a few questions;

1) Is there a way I can test power at home with just a multimeter? I don't have an occiliscope to see where clipping starts, so I'll have to use my ear, but it would be interesting to see if what kind of peaks I was getting at comfortably loud levels.

2)It's been a long time since I ran the MK IIIs, so can't remember how much heat they put out, do the MK IIIs run pretty hot? My MC225 with the much smaller 7591's runs much cooler. Also there is a "hot amp" smell that could be all the skin oil and latent stuff burning off the amp after sitting for fifteen years and a few weeks of handling.

2a While I was trouble shooting my bias problem I found some discrepancies on values from the original manual. Voltages were very consistent between both outputs tube in both amps, but high:
        - Pins 3&4 were 566-567 volts, manual calls for 475
        - Pins 5 and 6 were within limits ranging from -50 to -78 volts variable with the bias pot
Are the high voltages on pins 3 and 4 bad? Could that be why the amps are running hot?

Thanks,
John

If your multimeter is right, yes 566 V at 6550 plates is high, it would correspond to 75w output power. It would also put the electrolytic caps in danger!

Thanks again for the quick reply, I won't use them until I figure out how to get those voltages down. My wall voltage measures 123 volts. I found the 1,000 ohm resistors between pins 5 and 6 on the output tubes actually measure 750 ohms, could this be the problem? Not knowing much about how the circuitry works, are these pin voltages controlled by the driver board? I kept the original drivers and am thinking about upgrading them with either the Poseidon or Tubes4Hifi boards.

The one time I had to deal with bias rising too high it turned out to be because the ground on my upgrade input jacks were isolated from chassis ground. Make sure you measure continuity between the input ground and chassis.

daveshel wrote:The one time I had to deal with bias rising too high it turned out to be because the ground on my upgrade input jacks were isolated from chassis ground. Make sure you measure continuity between the input ground and chassis.

Thanks for the reply. My issue, I discovered, was similar to yours, a broken ground connection from the new SDS board to the common speaker lug.