Musicman power section

[cxx]

The musicman high voltage power section from the 75/150 watt amp looks kind of strange. It looks like the plates are at 700 volts the screens are at 350 and the grid is held at 22. The cathode goes through a transistor to ground which controls current through the tube. I haven't seen many schematics for a ss pre to a tube power amp. Is this a standard architecture? Is it something that musicman designed?

When someone uses one of these musicman amps as the basis for a dumble type amp do they just use the hardware and implement a standard power section?

[ToneMerc]

When someone uses one of these musicman amps as the basis for a dumble type amp do they just use the hardware and implement a standard power section?

Amp is usually gutted using only the PT and OT. With 700V on the plates the grids don't require as much negative voltage. All that stock MM s/s cathode biasing is junked, in favor is one leg of that stock bias supply feeding a conventional Fender style bias supply.

TM

[rdjones]

The very earliest MusicMan amps from 74-75 used a more typical cathodyne PI grid drive circuit with grounded cathodes and negative grid bias.
Everything else after had the solid state cathode 'current source' drive output section with a positive grid bias voltage.
This includes 50, 65, 75, 100, 130, and 150 watt models.
Tom Walker was the MM designer but I suspect it was derived from a hybrid TV video drive circuit.

Peavey used a similar hybrid output section in their VTX series around the same time period, and interestingly, with an Ultralinear output transformer.

Hybrid circuits with solid state preamps and tube output sections (of a variety of topologies) are fairly common with many brands including Marshall, Vox, and Fender having examples at some point.

rd

[Gaz]

I just fixed a MM combo with such a power section. It did deliver 100W clean with 2 x EL34s. I have no comments except for that it is a bad ass design and should be copied.

[rooster]

The MM output is Class B, not a common amp in my world, YMMV. I have worked on a few different models, my favorite being the tube PI 50 watt versions. As to the 130 watt versions, to have one survive it is all about the tubes. The originals were Philips 6CA7s, an uncommon tube in today's world. They are being made but, eh, so are many versions of tubes. Are they as good and of the same quality? Not. For example, I sent a customer out with a new quad of J/J E34Ls and he lasted two nites playing his show using his fuzz pedals and high volume. The Philips would have survived, I know this.

Anyway, if you want to see something entertaining, go to YouTube and search for a Johnny Winters video where he is playing a MM. Here you will see a 4/10 130 watt combo light of fire. ON FIRE! Eh, that's always encouraging to see as you consider owning something.

[JazzGuitarGimp]

A year ago, I stopped buying anything with the JJ name on it. Microphonic tubes and lousy customer service sent me over the edge. The new Tung-Sol and Mullard reissues from Russia look interesting. I've had the opportunity to try some Mullard 12AX7's in my Tonemaster head, and they sound amazing. I have TAD's in it right now, but when it's time to order tubes, I'll be buying the Mullards for it.

[Gaz]

I think the Musicman runs the tube pretty easy with the screens at only 350vdc. I think you just had some bad luck as did the guy whos went up in flames. How many of these amps did they build, and how many did not catch on fire? I agree of course that old tubes are better. As far as JJs, they are fine, and like any new tubes need to be bought from a reputable dealer. I guarentee you won't get microphonic one from Eurotubes or Doug's Tubes, for example. FWIW, I think the Chinese stuff from Shuguang is the best right now in terms of quality.

[ToneMerc]

In the past I've used SED EL34 and my lastest HV build has JJ's 6CA7, supposely the Sovtek 5881 are pretty robust as well.

TM

[Gaz]

Yeah, those "5881s" are really tough, and in fact based off a Russian military tube. You can find the Russian spec shet, and the max voltages are low, around 350v. Someone explained that the numbers, however, were not applicable to guitar amp use, as these tubes were intended for other types of circuits. Soldano has always used them, Naylor, and a few others. Ken Gilbert used them in his famous BAGA. Problem is, they've got no Mojo and are inexpensive

[leadfootdriver]

I had 2 quads of the Sovtek 5881's back in the day, and while they had crusty tone, they were stout. I even tried to kill them, but they never died.

[Gaz]

See: only benefit of the Cold War