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

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

    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!

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    What's the pot value?

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    Quote Originally Posted by gtrplayr1976 View Post
    What's the pot value?
    22k

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    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?

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    Quote Originally Posted by Helmholtz View Post
    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.

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    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.

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

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    Quote Originally Posted by Greg_L View Post
    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.

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    Quote Originally Posted by Helmholtz View Post
    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.

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    Quote Originally Posted by Helmholtz View Post
    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?

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    Quote Originally Posted by Greg_L View Post
    22k
    Try 50K. That's what I have in my JTM45.

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    Quote Originally Posted by gtrplayr1976 View Post
    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!

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    Quote Originally Posted by Greg_L View Post
    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.

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    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.

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

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    Quote Originally Posted by Enzo View Post
    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.
    I've soldered parts. I've soldered the proper resistors all over this thing. I'd just rather not change that pot. That requires ordering parts and waiting. And frankly, these tubes are so far off I don't even know what pot I'd need...and it might not even work!

    I'm gonna try a different measurement method. I'll be installing the 1 ohm cathode resistors this evening.

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    Quote Originally Posted by gtrplayr1976 View Post
    I've had to increase pots ,or resistors more than once to get the bias where I need it.
    Which resistor would I need to change? The 47k? Would I need to decrease it? Can I jumper in a parallel resistor just to see what happens?

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    Quote Originally Posted by Greg_L View Post
    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.
    Were these parts all there when it was running EL34's? If not, which did you change?

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    Last edited by vintagekiki; 10-15-2019 at 12:24 AM. Reason: Upload Bias Charts
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    Quote Originally Posted by g1 View Post
    Were these parts all there when it was running EL34's? If not, which did you change?
    The grid leaks were 220k for EL34s. The bias power resistor was 27k. I piggyback paralleled resistors to get them down to 6550 specs. All readings have been verified.

    When I got the amp the conversion was halfass complete. I put it to proper EL34 bias specs and it worked perfectly. Now I'm putting it back to it's original 6550 specs and it isn't.

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    Update -

    I soldered in a 1 ohm precision resistor from pins 1&8 to ground on one power tube socket. Took a reading across the resistor. I get the same reading as with the bias probe, so the probe is telling the truth.

    Just for kicks, I jumpered a 150k across the 47k that lives in series with the bias pot. That dropped the 47k to around 35k. My bias reading went up at the tube. Not much, but it did go up more in the right direction. My light bulb limiter got brighter though. It went from a very dull dim glow to noticeably brighter, so something is drawing more amps. Maybe these tubes are just bad.

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    Yes, the hotter you bias the tube, the more current it draws. That current comes ultimately from the wall outlet mains supply, hence the bulb brightens.

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    If you bias hotter, you are going to draw more mains current. The additional current comes from somewhere.

    Edit: Enzo beat me!

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    Quote Originally Posted by Enzo View Post
    Yes, the hotter you bias the tube, the more current it draws. That current comes ultimately from the wall outlet mains supply, hence the bulb brightens.
    Quote Originally Posted by The Dude View Post
    If you bias hotter, you are going to draw more mains current. The additional current comes from somewhere.

    Edit: Enzo beat me!
    Ha, yeah, it just seemed to get brighter than I thought it should. Maybe? I don't know. It's a 300w bulb.

    I had a rectifier wired backwards one time. That thing got really bright!

    I'm gonna put it back to EL34 configuration and get readings with the EL34s.

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    Taking bias readings with the light bulb or not?

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    Quote Originally Posted by mozz View Post
    Taking bias readings with the light bulb or not?
    Both

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    Quote Originally Posted by mozz View Post
    Taking bias readings with the light bulb or not?
    Good point! Bias should be set without the limiter at full/normal line voltage.

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    Quote Originally Posted by Greg_L View Post
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    Don't use the limiter to bias. You need full power.

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    Quote Originally Posted by gtrplayr1976 View Post
    Don't use the limiter to bias. You need full power.
    I have checked it without the limiter. Still the same.

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    The bulb limiter is a very useful tool, but you cannot rely on voltage readings taken while on it.

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    Quote Originally Posted by Enzo View Post
    The bulb limiter is a very useful tool, but you cannot rely on voltage readings taken while on it.
    Noted, thanks. But I did take my readings without the limiter. I started with a variac and the limiter so nothing would blow up. Once I was confident nothing would go up in smoke, the power came straight from the wall.

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    I'm gonna put it back to EL34 configuration and get readings with the EL34s.
    The resistor values for the EL34 bias circuit (except the 220k grid resistors) seem to be more appropriate for your extra "cold" 6550s as well.

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    Quote Originally Posted by Helmholtz View Post
    The resistor values for the EL34 bias circuit (except the 220k grid resistors) seem to be more appropriate for your extra "cold" 6550s as well.
    I was thinking that myself....not that I know for sure, but thinking it was worth a try. I'm going to put it back to EL34 spec and try the 6550s.

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