Music Man RD50 distorted output

[xtian]

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.



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?

[Enzo]

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.

[Helmholtz]

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

[Leo_Gnardo]

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.

[xtian]

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.

[Helmholtz]

Please post base to ground and base to emitter voltages.

[xtian]

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.

[xtian]

Power tube voltages, WRT ground:

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

[xtian]

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:



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.

[vintagekiki]

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?


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

[Helmholtz]

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.

[vintagekiki]

http://pacair.com/mmamps3/sites/default/files/docs/1650-rd.pdf
check voltage on + C34 (+290V)

[xtian]

http://pacair.com/mmamps3/sites/default/files/docs/1650-rd.pdf
check voltage on + C34 (+290V)

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?

[xtian]

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.

[xtian]

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.