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MusicMan 150 HELP

12/9/1997 3:33 AM
MusicMan 150 HELP
Anyone know about these amps? A quad of 6L6's, getting 150 watts... 670V on the plates at the high power setting, is this right? (the low setting is about 475V ) This thing has cooked one pair of tubes and has a good loud hum in the power section. Is this thing fixed or cathode biased? The cathodes run thru some kind of three legged flat transistor Thing (one for each set of tubes) The burnt set side seems to be shorted to ground. I can't even find the %#@& plate resistors!! HELP
12/9/1997 8:36 AM

I'm NOT a Musicman expert, but I believe that they use more or less ordinary power transistors in the cathodes of the power tubes to run the power tubes in cascode to drive the output transformer.  
It's not cathode or fixed bias in the normal scheme of things, at least not like other tube amps.  
It is likely that you have already found the flaw - the shorted "three legged flat transistor Thing"  
is probably a transistor, and if it's shorted, you just have the power tubes eating all that power, no way to bias or drive them. It would cook power tubes in that case, and possibly the output transformer.  
Get those shorted transistors replaced, then see what you have. I don't know if the circuit is self balancing/biasing or not.  
There aren't any plate resistors as I remember the glances I've had at Musicman schematics.
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12/9/1997 3:33 PM
Reid Kneeland

The Music Man amps use cathode drive on the  
outpute tubes; that is, the signal is applied to  
the cathode rather than the grid. There is  
a fixed bias voltage applied to the grids, which  
originates in the low-voltage winding of the  
power tranny (the same winding as the op-amp  
supply). You may find a trim pot (connected to  
a couple of diodes), but that's the the bias trim  
for the driver transistors, not the tubes. Those  
driver transistors are either JE1692 or 2N6488,  
depending on the vintage. The 2N6488 is still  
being made by Motorola. The datasheet is at  
Check the tubes for shorts before you put in  
a new transistor, or the new one will just  
blow, too. Transistors in a well-designed circuit  
don't (usually...) blow for no reason, but  
are much less forgiving of over-voltage and  
over-current conditions than tubes are (one  
of the reasons that fixing solid-state amps is  
harder than fixing tube amps - if you don't  
nail down the problem, the new parts you  
put in will get fried almost instantly).  
Good luck. Schematics can be found in  
The Tube Amp Book
12/9/1997 5:39 PM
Bill Bolton

> Anyone know about these amps?  
> A quad of 6L6's, getting 150 watts...  
> 670V on the plates at the high power  
> setting, is this right?  
It usually closer to 700V. These amps run in class B and use a solid state cathode drive.  
> HELP  
Get hold of a schematic before you do anything to the amp. It is NOT a standard Radiotron Handbook derived design.  
You can find some schematics in Pittman's "Tube Amp book" or Ernie Ball will send you a schematic if you ask them politely and tell them the EXACT model number that is shown on the back panel of the amp.  
12/9/1997 9:05 PM

What would I do without this BBS ?? The suspect device is the JE 1692, I've tried Mouser, Newark, Motorola , they all say....huh? As for the schematic my " Tube Amp Book" must be an earlier edition-- no MusicMan stuff. I'll give EB a call. This is the MM 150 head, when it works it sounds Great! I wonder how our modern 6L6's are going to stand up to those kind of plate voltages? Or would NOS tubes be the way to go? Or does Class B operation help the situation? This is interesting new stuff! Thanks for the help ( any leads on the part would be appreciated) Steve
12/10/1997 6:49 AM
J Fletcher
The plate voltage is very high,but the screens run at about half the plate voltage(at least in the RD100 they do),this reduces the stress on the tube.I put Sovtek 5881's in the Music Man I repaired a few months ago,and they worked fine,685 volts on the plates,360 volts on the screens as I recall.
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