Converting a Music Man HD150 into an all-tube combo

[crazytooguy]

I've had a Music Man HD150 2-12 combo, 150 W since the early 80s, when I used it for acoustic and electric folk-rock gigs. Since then, I've moved to all tube setups. Well, one day I realized that I had the most expensive parts for a tube amp build all in one unit - cabinet, speakers, chassis, output tubes (Sylvania 6L6GCs), power trans, and output trans. And I really didn't like the sound of the op-amp front end in this amp at all. So, here we go!

The attached photo shows the chassis, with the new heater wiring and a few holes drilled for 12AX7s. One challenge with this amp is that the power supply used a capacitive voltage doubler for the B+ voltage, and there's not enough voltage out of the power trans without it. The only actual limitation of that is that I can get only 1/2 of the PT's available current, but I think that'll be enough, even with a good number of 12AX7s in the preamp section. We'll see! This is all an experiment. Output section will be 2 EL34s paralleled by 2 of the original 6L6GCs connected as tetrodes in class A operation (similar to M*sa's SimulClass operation). Pre section will be based on a modified D*mble Overdr*ve, with DC coupled cathode follower at the end of the OD section driving the tone stack (a la Marshall). And it all may blow up in my face!! Should be interesting, though. Stay tuned!

[J M Fahey]

I guess that power supply and output transformer are optimally tuned for a, how should I say it? ....... Music Man type amplifier.
Have some doubts about its efficiency in a "regular" type power amp where, to boot, voltages would be either too high or too low.
And that filament winding powering a couple EL34's (I guess they expect 4x 6L6) plus a few 12AX7's, not to speak about the lack of a bias winding, leave me with a couple doubts, but, hey, it's yours, you can do as you please.
Just my 2 ¢.

[crazytooguy]

Yeah, those are definitely issues I've thought about. The voltage doubler produces 473V unloaded, so using a resistor divider (like most tube power supplies) will give me the right B+ voltages. Plate currents aren't very much for additional 12AX7s, a total of 5 extra mA. I will, though, probably have to add an extra filament transformer. I have no idea what the PT capacity is for the filament windings, but going to 2 EL34s adds 1.4A, and each 12AX7 adds 0.3A, so I doubt the stock PT has that much additional capacity. I measured the OT impedance (at 1kHz) and it's right about 2k ohms, which is OK for the four output tubes. I'm going to draw load line plots just to be sure, though (and because I need the practice...). I expect I'll have to tweak B+ voltages, but that's not too tough. It's the current capacity I'm concerned about. Just have to measure as I go, I guess. It'd be a big help if I could find specs for the power trans, but no luck so far.

[olddawg]

Since everything else is there, unless you have time to burn and are really cash straped with a lot of junk parts laying around, I would just start with the "right" PT to begin with. I would also probably simply gut the amp and start over except for the chassis, controls, (maybe)tube sockets, and the OT. If you don't like the sound of the OT when you're done you can always replace it later. Trying to force a square peg in a round hole or making a silk purse out of a sow's ear is always time consuming and problematic with usually lack luster results. Start from scratch. Make a new turret board from new components from a known good design, then mod/tweak it afterward if you must. Use a PT that is appropriate for the build so you don't have to add a filament transformer, fake a bias tap, and then have ground loop, hum issues, and othe idiosycracies to beat the band. IMHO anyway.

[Dave Curtis, dB AudioTech]

The voltage doubler produces 473V unloaded,

Actually, it should be well over 700 volts unloaded (stock). Maybe that had something to do with why you didn't like it.

so using a resistor divider (like most tube power supplies) will give me the right B+ voltages.

B+ will end up low; around 350. B+ ampacity will double.

Plate currents aren't very much for additional 12AX7s, a total of 5 extra mA. I will, though, probably have to add an extra filament transformer. I have no idea what the PT capacity is for the filament windings, but going to 2 EL34s adds 1.4A, and each 12AX7 adds 0.3A, so I doubt the stock PT has that much additional capacity. I measured the OT impedance (at 1kHz) and it's right about 2k ohms, which is OK for the four output tubes. I'm going to draw load line plots just to be sure, though (and because I need the practice...). I expect I'll have to tweak B+ voltages, but that's not too tough. It's the current capacity I'm concerned about. Just have to measure as I go, I guess. It'd be a big help if I could find specs for the power trans, but no luck so far.

IMHO, you'd have been better off selling this amp to someone who'd appreciate it for what it is, and use the proceeds to get an amp or a kit that would do what you want (my '76 212-65 sold for almost $500 ten years ago). Too late now, the damage is done.

At any rate, it's your amp; have fun!

[J M Fahey]

Being realistic, you have a PSU that supplies 700V (way too much) or 350V (too low), that can't power many filaments, and an output transformer that expects tubes fed very high voltage, which means low current, which means high impedance; in short, not quite fit for regular tube power amps.
Anyway, a 2x6L6 power amp, fed 350V and driving that OT *will* work, no doubt, only don't expect much power from it.
Imagine you are running your Music Man on the low power setting and you won't be much below it.
Why below?: because regular grid driving is not as efficient as that ruthlessly controlled original cathode drive.
Will it sound more "tubey" at least?
Yes it will.

[ThermionicScott]

Yeah, if you're going to change *everything*, why not punt it and start your dream amp from scratch?

Were it mine, I'd keep the output section *exactly* as it was, add a filament transformer for the preamp tubes (maybe a 12.6V unit for less hum), and go hog wild with that.

- Scott