Help please! 1973 Marshall Super Lead - EL34 to 6550s bias question

[Greg_L]

Hello friends, I have a 73 Marshall Super Lead. This is a USA model originally outfitted with 6550s.

Somewhere along the line, someone converted it to EL34s. It worked fine, sounded good, etc. I've been thinking of converting it back to it's original 6550 spec, and I've done that...to the best of my knowledge. I have a set of Sovtek 6550WE power tubes.

My problem....I can't get the bias hot enough. Not even close. With the bias pot maxed, I'm getting about 24ma current through a bias probe at each tube. At the middle setting, about 17ma. The tubes seem fairly well matched - same readings at each tube. My negative bias voltage at the grids is about -40vdc at max pot adjustment. Plate voltage is approx 395vdc.

I'm using the figures off of an old Unicord schematic with figures for USA 6550s.

15K before the diode
15K with the 10uf bias caps
47k in series with the bias pot
150k grid leaks
All checked and verified with an ohmmeter.

So what gives? Why can't I get these tubes to bias? Help please!

[gtrplayr1976]

What's the pot value?

[Greg_L]

What's the pot value?

22k

[Helmholtz]

Real 6550s should run VERY hot at -40V bias. Marshall spec was -44V with 395V plate voltage.
What are your screen voltages (pins 4)? Should be around 390V.

How did the EL34s measure at -38V?

[Greg_L]

Real 6550s should run VERY hot at -40V bias. Marshall spec was -44V with 395V plate voltage.
What are your screen voltages (pins 4)? Should be around 390V.

How did the EL34s measure at -38V?

Screen voltage is hovering around 388vdc.
Grid voltage with the bias pot midway is about -48vdc.

I didn't take any EL34 measurements because one had blown. But in the past I could bias EL34s no problem.

[Greg_L]

Should I try to measure the bias a different way?

I have some precision 1ohm resistors I can bridge from pins 1/8 to ground.
I can also try the OT measurement method I guess.

[Helmholtz]

So if your bias probes and meter (battery) are fine, your 6550s are out of spec.

[Helmholtz]

Should I try to measure the bias a different way?

I have some precision 1ohm resistors I can bridge from pins 1/8 to ground.
I can also try the OT measurement method I guess.

If there's any doubt about the bias probes, I recommend to try alternative methods as mentioned. Always useful to have 1 Ohm cathode resistors installed.

[Greg_L]

So if your bias probes and meter (battery) are fine, your 6550s are out of spec.

That's what I'm thinking. The meter and probe has worked fine for literally everything else. My voltages seem to be where they should be. Nothing weird is happening, except the tubes won't bias hot enough.

[Greg_L]

If there's any doubt about the bias probes, I recommend to try alternative methods as mentioned. Always useful to have 1 Ohm cathode resistors installed.

With 6550s would I still tie pins 1 and 8 together?

[gtrplayr1976]

22k

Try 50K. That's what I have in my JTM45.

[Greg_L]

Try 50K. That's what I have in my JTM45.

Thanks, but is there another way to increase range? This is a later 73 amp with a PCB. I'm fine with soldering on PCBs, but I'd rather not if I don't have to!

[gtrplayr1976]

Thanks, but is there another way to increase range? This is a later 73 amp with a PCB. I'm fine with soldering on PCBs, but I'd rather not if I don't have to!

I honestly don't know any other way. Maybe hotter tubes? Helmholtz would know.

[Enzo]

You alter the amp to accommodate your tubes, or you alter your tubes to accommodate your amp. Those are your choices. Altering the amp requires soldering parts.

[gtrplayr1976]

I've had to increase pots ,or resistors more than once to get the bias where I need it.