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Thread: Music Man RD50 distorted output

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    Music Man RD50 distorted output

    Amp arrived with two burned screen resistors, burned diode in bias circuit, and bulging 2uF filter cap. All components replaced, and that got me up and running again, BUT with early and ugly distortion on the output.

    On the scope, my sine wave is clean at the outputs of IC-4 (pins 1 and 7). But the wave becomes ugly after the FETs Q5 and Q6...but only when power tubes are installed. With power tubes removed, the signal at pins 8 of the power tube sockets looks good: half of the sine wave, with silence in between the humps--I assume that's correct, because the FETs only conduct for one phase each.

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    So why does installing the power tubes make the sine wave disintegrate? Screen and grid resistors are OK, voltages are OK, tried two pair of output tubes.

    Made me think the OT is bad. But I disconnected the OT leads and tried the Neon Bulb test, and it passed.

    Any ideas?

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    No FETs, those are bipolar. Are both of those healthy? Burnt screen resistors means shorted tubes, which means the transistors were ssubject to excess current themselves. Are the two emitter resistors OK? The 6.8 ohm resistors.

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    So why does installing the power tubes make the sine wave disintegrate?
    Without tubes inserted the 2 driver BJTs don't conduct as there is no collector load and collector supply. Very simply put, the tubes are the collector loads for the BJTs and the BJTs are variable cathode bias "resistors" for the tubes. What are the DC voltages at the bases?

    See also here: https://music-electronics-forum.com/...l=1#post438320

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    At best, there's always a visible - and audible - crossover notch on these amps. I've spent many an hour trying to eliminate it, but never successful. So I've come to accept, that's the way they are. And they'll always show you some dreadfully low bias current figure, maybe 6 milliamps or so. That's "normal" too. Sounds like it's way worse in your case. I'm saying this so you'll know, there's a point beyond which they will not go and not much point in trying to venture beyond there. Good thing the overdrive channel sounds so good - to me anyway.

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    Quote Originally Posted by Enzo View Post
    No FETs, those are bipolar. Are both of those healthy? Burnt screen resistors means shorted tubes, which means the transistors were ssubject to excess current themselves. Are the two emitter resistors OK? The 6.8 ohm resistors.
    The 6R8 resistors measure to spec with my meter. As do the other resistors connected to the transistors. I will measure voltages as @Helmholtz suggests.

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    Please post base to ground and base to emitter voltages.

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    Quote Originally Posted by Helmholtz View Post
    Please post base to ground and base to emitter voltages.
    502mV and 521mV base-to-ground

    487mV and 508mv base-to-emitter

    This is at idle, with power tubes installed.

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    Power tube voltages, WRT ground:

    plates: 560v 560v
    screens: 240 245
    grids: 31.3 31.3
    cathodes: 56.7 57.9

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    Here's what I see on the scope. From left to right, at pins 1 and 7 of IC4, at cathodes of power tubes, and and at output jack:

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    Note vertical scale at lower left. This is with 250mV p-p input signal, and Volume on the clean channel at about 1.5 on the knob. Showing about 5.25 watts at the output into 8-ohm resistive load.

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    All failures in the power amp come mainly from defective output tubes. Before any service power amp put new output tubes (V2/ V3) .
    When a repair the power amp never uses output tubes of type as.
    - check transistors Q5 / Q6 (MJE1692) If any defective replace both (match)
    - check R51 / R54 resistors (6.8 Ohm / 1W) If any defective replace both
    - check R69 / R72 resistors (75k)
    - check for leaks (replace) C25 / C26
    - check voltage + 30V on D9 (Zener 30V/ 1W 1N4751A)
    - check voltage + 30V on V2/ V3 pin 5
    Is the signal (clean ch) at point R42 / C24 clear?

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    http://pacair.com/mmamps3/sites/default/files/docs/MM%20Service%20Bulletins%20All.pdf

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    Quote Originally Posted by xtian View Post
    Power tube voltages, WRT ground:

    plates: 560v 560v
    screens: 240 245
    grids: 31.3 31.3
    cathodes: 56.7 57.9
    All DC voltages look ok. Screen voltages are somewhat low.

    Do both cathode signals look alike? The signal at the cathode is not AC but pulsed DC (to view switch to DC coupling) having a huge amplitude of over 64V - much more than necessary for full power and will cause severe grid conduction artefacts. This may be the reason for the strange clipping at the positive tops. As soon as the cathode voltage gets higher than the grid bias, grid conduction starts.
    But where is the power going? Are you sure that the OT is ok? Can you get decent signals at lower levels, e.g. for a peak cathode voltage of 30V or lower?

    What happens if you lift C30/31 one after the other?


    Please post complete schematic.

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    Last edited by Helmholtz; 10-05-2019 at 10:36 PM.
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    Quote Originally Posted by vintagekiki View Post
    Voltage is 292 at that point. Voltages at pins 4 of power tubes (new tubes) are 248 and 254. That's 93mA and 81mA...across the screen grids! WTH is happening here?

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    Quote Originally Posted by Helmholtz View Post
    Screen voltages are somewhat low....Can you get decent signals at lower levels, e.g. for a peak cathode voltage of 30V or lower?
    Yes, signal is OK at very low input volume. I think you guys are on to something with the screens.

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    Quote Originally Posted by Helmholtz View Post
    What happens if you lift C30/31 one after the other?
    Already did this when I was evaluating the OT, and left C30/31 disconnected for recent tests.

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    That's 93mA and 81mA...across the screen grids! WTH is happening here?
    Sorry, but are you sure you installed 470 Ohm screen resistors? 47k resistors could be an explanation.

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    Last edited by Helmholtz; 10-06-2019 at 03:45 PM.
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    @Helmholtz, that was exactly the right question. Nope, 470K.

    Excuse me while I was the shame off, and put in the proper screen resistors...


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    Yep, that was it! Amp Tech error. Thanks for taking this time wasting trip with me, gentlepeople. Amp is sounding good.


    Is there a way to edit the title of this thread?

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    Okay, this explains a lot: Low screen voltage, very low power tube gain and output. The distorted cathode signal was probably the result of heavily overdriving the grids.

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    This is the second time I've received this amp in the same condition: blown screen resistors and blown 30v bias voltage Zener D9. Is this a common failure, because of the 560v plate voltage? Can I prevent this from happening again?

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    The plate voltage should be no problem. EL34s/6CA7s can take up to 800V plate voltage. Maybe use screen resistors and zener with higher power ratings.

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    This amp uses 6L6GC tubes, not EL34.

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    Quote Originally Posted by xtian View Post
    Yep, that was it! Amp Tech error. Thanks for taking this time wasting trip with me, gentlepeople. Amp is sounding good.
    Now that it's working right, do you see/hear that little remnant crossover notch?

    As for the error, had a similar one in a friend's Fender he had to have fixed by the local self proclaimed "hot hand" tech whilst I was out on tour. It came back, sounding like poopoo, and with a big bill besides. Had a pair of 4700 ohm screen grid R's. Got me to thinking, does this "genius" techy guy work in the dark? Or is he color blind? In either case, there is such a thing as an ohm meter. And his ears should have told him his "fix" was crapola. So much for the competition... Fooey!

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    Quote Originally Posted by Leo_Gnardo View Post
    Now that it's working right, do you see/hear that little remnant crossover notch?
    After the fix, I gave it a quick workout with the 1KHz sine wave, saw that it was making 20vAC RMS against an 8R load (50 watts). The sine looked smooth, no notch.

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    Lifetime Member Enzo's Avatar
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    Can I prevent this from happening again?
    Your screens sit at half the plate voltage, voltage was never a problem for the tubes. Tubes fail because tubes fail. They are the least reliable thing in an amp. The amp is 20-25 years old. If it takes another 20 years to blow up, you will be OK.

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    Quote Originally Posted by xtian View Post
    This amp uses 6L6GC tubes, not EL34.
    You're right. I mixed up MM models. Was too late for me last night.

    But I can't imagine that the 560V plate voltage is the reason for the failures.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Helmholtz View Post
    You're right. I mixed up MM models. Was too late for me last night.

    But I can't imagine that the 560V plate voltage is the reason for the failures.
    Depending on which 6L6 are installed. Those awful Russian cheapies and their Chinese copies, popular in decades past, don't stand up to much. Tubes better than "minimum quality" not so much of a problem.

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    Don't forget that the cathodes are sitting at around 60V, making the tubes see only 500V at idle.

    Actually the OP didn't mention blown power tubes - but only a tube malfunction could take out a screen resistor.
    Still it's hard to imagine how defective tubes could damage the zener in the bias circuit. Would need something like short between grid and screen.

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    Quote Originally Posted by Helmholtz View Post
    Don't forget that the cathodes are sitting at around 60V, making the tubes see only 500V at idle.

    Actually the OP didn't mention blown power tubes - but only a tube malfunction could take out a screen resistor.
    Still it's hard to imagine how defective tubes could damage the zener in the bias circuit. Would need something like short between grid and screen.
    Here's a photo of the 6L6 type I mentioned, a Russian 6P3 from the tubes-store site. These were also made in straight sided bulbs. Chinese versions also. Note tubes store quotes plate power at a whopping 20.5 watts - rather weak. Also a max plate voltage of 375. Despite their poor reputation, I occasionally do see 6P3 functioning OK in amps. OTOH over the years I've had to heave hundreds into the bin with breakdowns including all sorts of internal shorts.

    When Sovtek first started importing from Russia the tube they called 5881, I was cautious, considering the reputation of the previous 6P3. Instead, the so called 5881 turned out to be a hit, very tough and took high voltage well in spite of its published ratings. In fact I've used them in RD50 with no problem except for the small crossover notch I've mentioned. Perhaps 6L6GC or 5881 with a higher emissivity would solve that problem.


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    Here's a photo of the 6L6 type I mentioned, a Russian 6P3 from the tubes-store site. These were also made in straight sided bulbs. Chinese versions also. Note tubes store quotes plate power at a whopping 20.5 watts - rather weak. Also a max plate voltage of 375. Despite their poor reputation, I occasionally do see 6P3 functioning OK in amps. OTOH over the years I've had to heave hundreds into the bin with breakdowns including all sorts of internal shorts.
    Interesting, thanks.
    To me a tube that doesn't meet minimum 6L6 specs is no 6L6 and shouldn't be labeled and sold as such.

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    Quote Originally Posted by Helmholtz View Post
    Interesting, thanks.
    To me a tube that doesn't meet minimum 6L6 specs is no 6L6 and shouldn't be labeled and sold as such.
    In spite of failing to meet 6L6 min specs, these are what was offered for sale widely in the US from mid 70's through the 90's. Groove Tube, Peavey, other outfits. Some were relabeled and sold with major tube manufacturer's logos. Others sold through "jobbers" supplies, IC brand for instance. Many were paint marked with "West Germany" or "England". One fiasco of note, and it's been reported in MEF, Groove Tube sold them as a "miracle" 6V6 substitute, without warning buyers they had better set bias as if they were 6L6. So much monkey business!

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    Quote Originally Posted by xtian View Post
    This is the second time I've received this amp in the same condition: blown screen resistors and blown 30v bias voltage Zener D9. Is this a common failure, because of the 560v plate voltage? Can I prevent this from happening again?
    How hot is the idle current? Measure the DC across the 6.8 ohm resistors. Typically these MM amps are biased very cold, around 6mA, so you should set for about 40mV across each of those 6.8 ohms.
    You mentioned not seeing a crossover notch, and signs of tube failure, so idle current may be high (for this model).

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    Quote Originally Posted by xtian View Post
    After the fix, I gave it a quick workout with the 1KHz sine wave, saw that it was making 20vAC RMS against an 8R load (50 watts). The sine looked smooth, no notch.
    Bravo. Welcome to repairman club.

    Who knows and does not know that he knows - wake him Confucius)

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    Thanks, G1, but amp is gone.

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