Music Man 112-RD-50 has fizzy clean sound

[Coop]

I’m working on a Music Man 112-RD-50 amp. It had weak sound and hum…. I discovered it had bad MJE1692 driver transistors. I changed those out for 2N6488G. And that solved the hum and weak sound. But on the clean channel it has a fizzy sound. All electrolytic caps have been replaced, all of the big wattage resistors changed. All of the TL072cp op amps have been replaced. All voltages on the output tubes looks ok, compared to what’s shown on the schematic. I have not changed the MC14016B. I’ve tested all tubes, and tried different tubes. This amp has no bias adjustment pot. I’ll try to post a link to the schematic. Any advise will be greatly appreciated. Thanks
Coop

[glebert]

Have you checked the voltage across R51 and R54 to see if the tubes are both conducting and somewhat similar current?

[xtian]

Did you eliminate the speaker as a possible source of fizz?

[Coop]

I tried a different speaker. Same issue.
The voltage drop across R51 is .025 VDC, R54 is .022 VDC

Is that good?

[glebert]

I tried a different speaker. Same issue.
The voltage drop across R51 is .025 VDC, R54 is .022 VDC

Is that good?

That seems to me to be really cold (25mV/6.8V= 3.8mA), but this cathode driver circuit is kind of an oddball, maybe someone else can chime in what the idle current should be. This thread (https://music-electronics-forum.com/...man-rd-50-what) suggests 5-6mA would be normal. Also, that same thread says that some crossover distortion is unavoidable (?) so maybe it is about degrees of fizziness?

[g1]

That seems to me to be really cold (25mV/6.8V= 3.8mA), but this cathode driver circuit is kind of an oddball, maybe someone else can chime in what the idle current should be.

Agree it is a bit low, maybe below the threshold where it becomes quite audible.
But before speculating about bias, is it possible to set the other channel for a clean sound, and does it not have the fizziness? If you can get a non-fizzy clean sound out of the other channel then you can look for an issue in the clean channel.

[glebert]

Agree it is a bit low, maybe below the threshold where it becomes quite audible.
But before speculating about bias, is it possible to set the other channel for a clean sound, and does it not have the fizziness? If you can get a non-fizzy clean sound out of the other channel then you can look for an issue in the clean channel.

I'm wondering if you could inject a very small signal into the reverb circuit return to avoid both preamp channels. Maybe even inject at the reverb footswitch?

[Leo_Gnardo]

OH man have I ever been through this one! Spent many many hours fiddle faddling to no avail. Unfortunately the correct answer is "that's what they do, get over it." I've tried altering bias voltage but with the transistor drive/bias circuit nothing I tried brought success. There's always that twinge of crossover distortion that makes what should be clean into always a bit fuzzy. Clean-ish is the best you get out of an RD-50. Good thing the overdrive channel sounds pretty dam' good, so there is some merit to these amps.

IF anyone does come up with a solution, be sure I'll be paying close attention.

[Enzo]

I think you are trying to adjust bias for a conventional push pull circuit like any Fender or Marshall. But these cathode drives are not that. They operate close to class B and are in fact pretty cool at idle. I don't have the Music Man guide handy, but the Peavey VTX system is almost identical, and on a one pair circuit (50 watts) their resistor is 5.6 ohms rather than 6.8, but close, They want about 30mv across that. Actually they say 20-40mv. 30mv across 5.6 ohms is about 5ma current.

I'd say your numbers are pretty darn close.

[Coop]

Thanks for all your replies. Very interesting stuff here. My ole tube amp mentor would have thrown this amp out his garage door, after he pulled the tubes out. lol….It’s kinda sad that these amps are designed like this.

And the Limit (distortion) Channel never gets truly clean either, with its gain knob on 1 or less. Honestly, the distortion channel sounds pretty rad!

Would it be worth changing R51 & R54 from 6.8 ohms, to 5.6 ohm /1 watt resistors? Maybe a little less fizz in the clean?

[Helmholtz]

IF anyone does come up with a solution, be sure I'll be paying close attention.

I never experimented with the circuit, but here's what I'm seeing:
BJT and tube are wired in series and constitute a cascode circuit.
Common current is controlled by the BJT, more exactly by its collector voltage which defines tube cathode voltage and bias.
BJT collector voltage depends on the ratio of R68/R50 (resp. R69/R53).
As lower collector/cathode voltage increases tube current, idle current could be increased by either increasing R50 or lowering R68.

If R68/R69 have drifted to a higher value, that could explain unusually low idle currents.

Are we even sure that the problem is in the power stage?
Would like to see scope pics.

[Helmholtz]

Would it be worth changing R51 & R54 from 6.8 ohms, to 5.6 ohm /1 watt resistors? Maybe a little less fizz in the clean?

No, will not make a noticeable difference at idle as the voltage drop is much smaller than the base-emitter voltage.

[Enzo]

Would it be worth changing R51 & R54 from 6.8 ohms, to 5.6 ohm /1 watt resistors? Maybe a little less fizz in the clean?

Just a thought. Before throwing parts at it, have we scoped a clean signal leaving the amp to determine if the fizzy you hear is really crossover distortion or not?

[Leo_Gnardo]

My ole tube amp mentor would have thrown this amp out his garage door, after he pulled the tubes out. kinda sad that these amps are designed like this.

And the Limit (distortion) Channel never gets truly clean either, with its gain knob on 1 or less. Honestly, the distortion channel sounds pretty rad!

Would it be worth changing R51 & R54 from 6.8 ohms, to 5.6 ohm /1 watt resistors? Maybe a little less fizz in the clean?

Would be a shame to jettison a solid built amp like this - it's just not meant for a player who needs clean. Some don't. FWIW late 80's I got to witness Ace Frehley use one to record a demo track for his song "Remember Me." On the OD channel of course! I'll admit I'm not a huge Ace fan, but he was getting an awesome tone from his little RD50.

Believe me, I tried swapping R51 & 54, changing the bias zener, swapping in all sorts of 6L6 including RCA black plate 6L6GC. Raised the bias current to maybe 7 or 8 milliamps for all the effort, which didn't get us out of the fizzy fuzzy zone on clean. Watching on a scope, there was an easily seen small - but very audible - crossover notch where upper half meets lower half of a test sine wave. Nothing I did made it go away.

[52 Bill]

What is the voltage reading on the output tube input grids? If you don't have 30 volts there, the amp will fizz like you describe. Check the Zener diode supply.