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Thread: MusicMan HD130. Tube PI. Upward ramping Bias

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    MusicMan HD130. Tube PI. Upward ramping Bias

    After months of psychological counselling following the rampant Marshall bias issues of last year, my heart sank when I see this MusicMan HD130 2575 with the same problem.

    This amp has the tube PI in what looks to be a Cathodyne configuration. Oddly, the 10 ohm Cathode resistors are missing, with the cathodes connected directly to ground. As it appears that these are only used to monitor cathode current, I have left the resistors off and monitor the individual currents with my bias probe.

    The bias voltage at the junction of the two 330K resistors is relatively stable at around -42 volts. Its what happening on the pin 5's of all the tubes that is wild. Over the course of a few minutes, the voltage on the pin 5's ramps up from -42 to -22, with the cathode current going from 12mA to a staggering 70mA - with a plate voltage of 700V!! Meanwhile the bias supply is relatively stable at -42 to -40. I thought leaky coupling caps so I replaced the 0.047uF caps.

    I pulled out the still good USA Sylvania's and tried a fresh set of JJ EL34L's. Same. Same. I'm stumped as to what else it could be. There doesn't seem to be anything else in the circuit that would effect the bias voltage at pin 5 that doesn't effect the actual bias supply. The Bias supply is stable.

    Have you seen anything like this before? Any suggestions on where to go from here are much appreciated. Meanwhile, I will try to find that psychologist's phone number. I can recommend him for therapy following Triple Rectifiers, 410C's, SVT's, and more! Only problem for you is that he's in Sydney. :-)

    Click image for larger version. 

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    Maybe leaky (carbonized) tube sockets. Do all pin 5s behave the same?

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    Quote Originally Posted by Helmholtz View Post
    Maybe leaky (carbonized) tube sockets. Do all pin 5s behave the same?
    i think so too,if it goes less negative and the tubes are good,probably there is leaking voltage from pin 4,some dust,try cleaning with alcohol,monitor the screen voltage too

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    Old Timer J M Fahey's Avatar
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    . As it appears that these are only used to monitor cathode current, I have left the resistors off
    Wrong.
    They are not to "monitor" cathode current but to *set* it.

    And forget "voltage bias" ; grid voltage is set **strongly** positive so tube on its own passes all the current it can and then some, and cathode transistors decide how much.

    You remove the sensing resistors, drive transistors have no clue on actual tube current and it drifts anywhere it likes.

    Usual "tube amp knowledge" does not apply much here, because this game uses very different rules.

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    Yup... They used a system that can squeeze every last bit of life out of the power tubes; Peaveys of the era did the same. I'm not familiar with them so I pass on them if people ask. Plus my meter only goes to 630... I know when I'm beat.

    That said, their system is quite good at doing its intended job.

    Anybody care to explain it to us?

    Justin

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    Are the output tubes fixed biased? If there are just check for a defective tube measuring any voltage over each grid leaks resistors. Check also with the standby sw off to see if any voltage is present over grid leaks resistors. I suspect one of the power tube is broked

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    This is the 2575 model. It is a conventionally grid one fixed bias driven design. The schematic is almost unreadable but it looks like the two 10 ohm resistors might be connected in parallel at the node marked "y". The note "1/2VAC" hints that the purpose is to monitor AC operation but they handy for checking DC bias current too, provided the tubes are matched. I'd leave in place. Not that that is the problem.

    I suggest you monitor the g1 voltage with the EL34's out.

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    Thank you for the suggestion. I have given the bases a good scrub and the sockets, too. No luck yet

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    Its fixed!! There was a couple of things, and I'm not sure which one was the cause in the end:

    1. I measured the resistance of each of the grid resistors. All were up a little from the 1K5, to say 1K6, but one had drifted up to 2K2. I pulled them all out and replaced with 1K5 metal film.

    2. With no tubes in place, but with my bias probe connected I was dropping 3v across the 330K resistors (from -43 to -40V), where I would expect no voltage drop with no tubes installed. I appears that my bias probes were conducting. That's unexpected.

    3. Because of this, I stopped using the Bias Probes and installed a 1ohm from Cathode to Ground on each tube (remember, someone had removed the 10ohm resistors at some stage in the history of the amp).

    I powered up the amp, and it seems rock solid on the bias. I set the bias around 20mA +/- at 695v. So in the end, because I did so many things between tests, I'm not 100% sure if it was leaking coupling caps, crappy grid stoppers or the missing cathode resistors. Either way, my mouth is smiling and my ears are ringing.

    So how do you recommend I charge the customer? If I went straight to it and did those tasks, I would have taken 2 hours with caps and resistors. With the testing and searching it would be double that. How do you charge for this? Do you charge for the testing time, or just the time on the actual repair?

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    Old Timer J M Fahey's Avatar
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    Sorry, I had thought you had the "normal" MusicMan which is cathode driven, now I searched for that spcific model and found it´s a "conventional" Tube driven one, so yes, you can have leaking coupling caps, etc.

    Tubes are abused like on the other one, so I guess it will be very hard to get modern over the counter tubes to work there reliably.
    Oh well, glad you solved it

    EDIT: to each his own but in this case I would straight charge the "real" 2 hour repair time.
    Anything extra, assign it to improving your knowledge so next time it takes "just" 2 hours.

    In fact, next time same problem "should" be made within 1 hour

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    Thank you for your advice on how to bill for such repairs, Juan. I can knock out most repairs in good time, but sometimes I will spend a day, sometimes more, on a single job which I can't charge a customer for. I write that time off as my apprenticeship, although at my age I am closer to retiring than being a young apprentice.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by christarak View Post
    Its fixed!! There was a couple of things, and I'm not sure which one was the cause in the end:

    1. I measured the resistance of each of the grid resistors. All were up a little from the 1K5, to say 1K6, but one had drifted up to 2K2. I pulled them all out and replaced with 1K5 metal film.
    A drift to 2.2k won't cause the problem you saw. OTOH that was just a one sample data point, the resistor might be open at other times.

    2. With no tubes in place, but with my bias probe connected I was dropping 3v across the 330K resistors (from -43 to -40V), where I would expect no voltage drop with no tubes installed. I appears that my bias probes were conducting. That's unexpected.
    No, it should be expected. Your meter has a finite input resistance and that current causes a drop across the 330k.

    3. Because of this, I stopped using the Bias Probes and installed a 1ohm from Cathode to Ground on each tube (remember, someone had removed the 10ohm resistors at some stage in the history of the amp).

    I powered up the amp, and it seems rock solid on the bias. I set the bias around 20mA +/- at 695v. So in the end, because I did so many things between tests, I'm not 100% sure if it was leaking coupling caps, crappy grid stoppers or the missing cathode resistors. Either way, my mouth is smiling and my ears are ringing.
    Glad it's done but if you don't know why, what do you think you learned?. What would you do differently next time?

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    Thanks Nick. This self-apprenticeship is very hard and frustrating at times. Imagine if I had learned from people like you! I never suspected my meter (Fluke 87V) to offer sufficient load to cause a voltage drop. By the end of the day, I was frazzled and just wanted to get the job finished. But I hear what you're saying. With 3 strategies tried between tests I don't know for sure which one was the final villain in this case. In the future I would change-test-change-test. Having read you comments again, I am now left wondering if I have really solved the problem.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by christarak View Post
    I write that time off as my apprenticeship, although at my age I am closer to retiring than being a young apprentice.
    Oh, in this job you can NEVER stop learning,or you are left behind like yesterday´s paper.

    I am legally retired since December, yet I will keep the shop open until physically impossible.

    Worst case, will close it but stay in a couple Forums, at least to keep the mind working.

    Sure beats playing cards and Bocce, and feeding corn and bits of bread to the pigeons at closest public square.

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    Juan Manuel Fahey

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    I wonder if a tube is just running away?
    The reason I ask this is the vast difference in idle dissipation between the grid drive and cathode drive versions of this amp.
    Both versions have around 700V plate, 350V screen.
    Grid drive version idles (spec) at 25mA per tube (17.5W or 70%)
    Cathode drive version idles at 3mA per tube (2.1W or 8%)
    I don't think the big difference would just be due to cathode drive? Maybe they were finding that the tubes didn't like the high voltage, and I think it would be an even bigger problem with modern tubes?
    I sure wouldn't feel comfortable running the grid drive version that hot.
    Attached Files Attached Files

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    Juan, its thanks to people like you and the other regular contributors to this Forum that makes the repair life bearable when you are alone in a workshop day-after-day. The incredible knowledge and experience that you guys contribute is amazing. I think the day you guys go, or this Forum closes down, I'll hang up my soldering iron, turn off the light and feed the pigeons.

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    Nope. Then it'll be time for you to pass all you've learned on to the next generation of MEF'rs.

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    The cathode drive tubes do indeed run much cooler, a lot closer to class B.

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    I'll try Dude, but I won't ever be a Dude, Nick, Juan, Helmholtz, Justin, G1 and the usual guys that offer their experience. And God knows, I'll never be an Enzo!

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    Hi G1, I set the amp to 20mA at 695V, about 56%. I hope that's OK. I have left in his original Sylvania EL34s, 3 of which are still close, while one is a bit down at 16mA compared to 20mA for the others. I decided to leave them all in, because I don't trust a modern tube to sit happily with 700v on its plates with a grounded cathode.

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    Quote Originally Posted by christarak View Post
    I'll try Dude, but I won't ever be a Dude, Nick, Juan, Helmholtz, Justin, G1 and the usual guys that offer their experience. And God knows, I'll never be an Enzo!
    Thanks for the guilt by association, but don't put me in that crowd! Everything I know I pretty much owe to them, and a healthy dose of hacking up my own junk (not always for the better) and blowing some stuff up, and in one case setting my amp on fire.

    I just find I have a pretty good memory & a knack for translating concepts.

    I used to spend literally all night several nights a week reading every single post when this site was still Ampage. Most of it I didn't understand. But I got there. Sorta. I'm usually pretty good until you bring in Impedance. Then I just scream "TELL ME WHAT TO PUT WHERE & FOR THE LOVE OF PETE DON'T ASK ME TO DO ANY MATH!"

    Justin & Jusrin

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    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    You're a funny man, Justin. You have my respect because you offer generous help to your fellow techs without any judgement or negativity. Everyone has their own online personality that I like, from Juan's fiery Latino, but spot-on, comments, to Enzo's "how can one man possibly know so much" incisive advice. Wouldn't it be so cool, if one day we could all meet up?

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by g1 View Post
    wonder if a tube is just running away?
    The reason I ask this is the vast difference in idle dissipation between the grid drive and cathode drive versions of this amp.
    Both versions have around 700V plate, 350V screen.
    Grid drive version idles (spec) at 25mA per tube (17.5W or 70%)
    Cathode drive version idles at 3mA per tube (2.1W or 8%)
    I don't think the big difference would just be due to cathode drive?
    My take on this is that power tubes are very nonlinear at low current, so when cold they sound choppy ugly "no sustain", like a poorly adjusted noise gate, so biasing them hot brings the mellow smooth quality out.
    Problem is that by ear musicians bias them crazy hot.
    You have noticed that most all Factories (usully run by Engineers) bias amps quite cold (in my book, "properly") .
    While in cathode driven amps tube is VERY linear , plate current is exact same as cathode current (grid voltage is irrelevant) and this comes from a transistor, which is very linear even for very low idle current.
    So in this hybrid cascode combination you can bias for very low current, a few mA, and have no problems.

    Which in due time, since you lower idle dissipation so much, you can "safely" rise HV a lot.

    Now "conventional PI" plus crazy high voltage , in my book, is an accident waiting to happen, or at least requires *careful* selection of components ..... or else.

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    Lifetime Member Enzo's Avatar
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    Wouldn't it be so cool, if one day we could all meet up?
    That would be great, but I understand that down under the beer has its head on the bottom?

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    And yet most of these amps have been working fine for decades.

    Hmmm, maybe the screens running at half the plate voltage has something to do with it.

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    Quote Originally Posted by J M Fahey View Post
    ...and feeding corn and bits of bread to the pigeons at closest public square.
    You don't have to spend all your time feeding the pigeons


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    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by drewl View Post
    And yet most of these amps have been working fine for decades.

    Hmmm, maybe the screens running at half the plate voltage has something to do with it.
    You bet, I know the screen voltage definitely plays a substantial role. But I'm specifically asking about the 2 versions of the amp, both of which share the same plate and screen voltage, but are biased very differently.
    I've probably worked on at least a hundred MM amps, but have only seen the PI tube variety maybe a handful of times. If you're telling me the grid drive/PI tube type biased hot are no harder on power tubes than the cathode drive type, I'm all ears.

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    Don't be ridiculous Enzo. When you come to Australia, do what the locals do: turn the glass upside down so the head is at the top!

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    Lifetime Member Enzo's Avatar
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    Sorry, I don't know enough about your culture, I guess...

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    Hilarious!

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    Only one way to solve that, Enzo. Come on down and it would be my pleasure to show you around this beautiful city of Sydney. I'll show you the beer drinking methodology for being in the Land Down Under. We'll do a cruise past the Opera house and the Sydney Harbour Bridge and I promise I won't ask you to explain common grid bias or the effect of different screen voltages. The only capacitor leaking will be my bladder capacity trying to keep up with your craft beer sampling.

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    Enzo deserves a vacation I'd say

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