Music Man lesson

[Enzo]

I learned something new. I never thought about it before - the special case that is Music Man amps.

I was over at my friend's shop, and we worked through the topics he had questions about or wanted to learn about. We came to the Music Man. It was a 2475-65 or something like that. I don't think we ever did come up with the true schematic. His had the trem and reverb and the print we used was just preamp and power amp but no FX. No matter, we had enough.

COmplaint was the amp wasn't loud enough and not quite clean. OK, plug it in...

It certainly made loud, but maybe not all the loud? And maybe a bit gritty too. MM amps are known for LOUD and clean. So my guy is wailing that solid state is a mystery - looking at the op amp preamp. And how would we approach this amp. I said start at power. The amp made plenty of sound, but we check voltages anyway, don't we? Sure enough the power tube screens had about a volt and a half on them.


Um... without really thinking about it, in my mind the rule is: no screen voltage, no tube conduction. However in the music man, there is +16v on the control grid, and with the cathode drive for signal, apparently the positive grid is enough to allow conduction. Enough conduction to make pretty loud. SO some basic exploration to find an open resistor, and BOOM, now it is REAL LOUD, and plenty clean. The two power tubes do not have screen resistors, the screens are tied together, then a 1k5, 7 watt resistor feeds them from the B+ node. Basic work to finish up.

But I had no idea Music Man amps acted that way - USUALLY a open screen results in no sound, but not here.

Never assume.

[nosaj]

Don't some Peavey's act that way also? Classic Vtx maybe?

nosaj

[Jazz P Bass]

"Never assume. "

It makes an ASS out of U and ME.

[nevetslab]

"Never assume. "

Assumption is the mother of all F**KUPS.

[Enzo]

Yes, the PV VTX amps all are very close to the same circuit. I just never found myself passing signal with no screen voltage there. Maybe they do, maybe they don't.

[J M Fahey]

Sounds weird on first take but if you think power tubes need -50 to -40 V bias on normal grids just to make them pass a relatively important idle current, already enough to make them dissipate half of maximum plate power .... and then you see MM applies 16 V POSITIVE to them , well .... I guess grids will TEAR electrons from nearby red hot cathode and hurl them towards very high voltage plate (which makes it hungry for electrons) like stones on Davidīs slingshot ...

I bet even triodes will pass a lot of current with such high grid positive bias ... even lacking screens.

Also that unbiased screen pentodes wonīt be too far from triodes.

[Enzo]

Juan, you forgot the cathode sitting there at 75-90 positive As far as the control grids know, they ARE at -50 to -75v of bias.


I got no trouble understanding this, it is just that in all my decades I never noticed this aspect.

[g1]

Current wise they are running around 5mA at idle? Very cold.

[Enzo]

Yes, they do run cold. 5ma to 10ma sounds right. These things run close to class B.

[J M Fahey]

Juan, you forgot the cathode sitting there at 75-90 positive As far as the control grids know, they ARE at -50 to -75v of bias.

I got no trouble understanding this, it is just that in all my decades I never noticed this aspect.

No, of course I didnīt forget it

But cathodes (at idle of course), sit at +70/90V with fresh tubes and passing 5 mA or whatever the cathode transistors inject into them ... while new.

Nobody cares to measure this (I have) , but when tubes are quite worn, and "natural" emission decreases, cathode to grid voltage decreases.

"How much?" : "whatever is needed to make cathodes pass current injected into them" by transistors, which behave as perfect current sources.

Thatīs why normal tube circuits slowly wear out every hour they are ON while MM extracts down to the last drop of juice from tubes ... as long as there is still some cathode coating that is. (thorium something if I donīt remember wrong).

My point being is that IF needed, +16V are already there, always, and more important, that supply can inject significant current into grids (which a conventional Phase Inverter can not) .

And since cathodes can swing (again: IF needed) down to +2V, (drive transistor saturation voltage), tube can get +14V net grid to cathode which gives huge current out capabilities to power tubes or allows quite tired ones to perform almost as fresh ones.

Yes, fully agree with you that what matters (to the tube) is grid to cathode voltage, and also that it doesnīt know/care where that cnmes from.

What I am emphasizing is that here we have a "Class AB2" circuit, "hidden in plain sight", reaching (if driven hard) both positive grid voltage and injecting current into grid, the telltale signs of AB2.

Like other AB2 circuits, at idle grid to cathode voltage is, as you say, negative, "that cathode will float as positive as needed to make grid negative to it" and "it will not pull grid current".

Besides, since at idle it passes very little current, (5 mA), cathode will rise very high, near whatever is needed to switch tube OFF.
As high as +90V? ... definitely.

Not sure who designed it, was it Leo working with MM or some other Tech/Engineer? ... but it was a stroke of Genius.

Current wise they are running around 5mA at idle? Very cold.

Yes , very cold ... for a conventional voltage driven tube.

Normal amps *can* be set that cold, no big deal, but transconductance at such low currents, near cutoff, is very low, so gain is very low, and to boot response in that area is very nonlinear so combined audible effect is equivalent to a poorly adjusted noise gate: choppy, ugly sounding and sustain killer, the opposite of what Guitar players want.

But MM can get away with that, because tubes here are NOT voltage driven but cathode current driven instead.

"Grounded grid" stages have unity current gain, output stage will be as linear as driver transistors are ... and bipolar transistors are way more linear than any tube, specially at very low current values ... what we have here.

Again, a Designerīs stroke of genius

[Helmholtz]

Thanks JMF, excellent circuit analysis!
Yes, AB2 and current drive are the key.