Music Man HD-150

[Gibanez]

Hi Everyone,
This is a first post for me. I've been looking through the forum
and I see a great community of amp builders. I have a Music Man
HD-150 and would like to convert it to a Dumble Circuit. Not sure
which one but the amp has a solid state pre-amp whith a tube
power amp. (4) 6L6s and it also has reverb. Does any one think
that it would be feasable to drop in 3 pre-amp tubes and a hand
wired board and still use the transformers and power tubes and
also use the reverb tank? This head is just laying around and I
figured I'd do something with it.


Tony

[skyboltone]

Sure Tony:
Anything's possible. Is the OT multitap? Or perhaps a better question, do you really need 85-100 watts?

Your PT will be weird unless they have a separate transformer for the SS side of things and somethng like a 335-0-335 for the plates.

Take your time, study up on the schemes and read every post you can. There is so much information here that once you get an idea to surface on what you want to do, there are folks that will help you get there.
Good luck
Dan

[Gibanez]

Thanks Dan, I'll look in to what the PT is and do some searching
to see what I can come up with.

Tony

[chris_sanford]

Hi Tony,

Many great sounding Dumbles used Music Man iron. The extra pt winding can be used to power the relay circuit... which is what Dumble did. The B+ is high, so you will need to drop it down a bit.

Hope this helps.

chris

[odourboy]

The B+ is high, so you will need to drop it down a bit.

Just looking at a few schematics of some Music Man amps - many seem to have a similar PT and it offers a lower voltage tap off the HV winding. The B+ is marked on the schematic as delivering 700v (yikes!), but the schematic shows the standby switch is three-way, and can select the lower voltage tap (labled 'Lo', presumably to provide a lower output power). It appears your amp uses this scheme, so maybe you could use that tap to get a lower B+. However, the voltage is not marked, so it may not be a whole lot better, but it's something you might check out.

[Bob-I]

I have an old MM 65 head that I almost converted. After I saw some of the prices on e-bay I decided to keep it stock. It works perfectly and I use it with bass so I'll keep it.

[heisthl]

You can just wire half of the "solid state" secondary out of phase and in series with the main secondary to achieve a workable output voltage.
see
http://ampgarage.com/forum/download/file.php?id=1215

[skyboltone]

You can just wire half of the "solid state" secondary out of phase and in series with the main secondary to achieve a workable output voltage.
see
http://ampgarage.com/forum/download/file.php?id=1215

Hi Heisthl, I wrote a long winded explanation complete with pictures with the circles and arrows on them sometime back in the archives that explains buck/boost principles. Not many takers back then.

For those who care, search Buck/Boost or maybe just buck with my name. It may come up. It's a good way to adjust voltage on an otherwise great piece of iron to suit a project.

Remember the buck or boost winding ONLY CARRIES THE CURRENT OF THAT PORTION OF THE TOTAL VOLTAGE IT CONTRIBUTES. Seems weird but that's what all the references say. I've done it a hundred times in practice. It can also be done on the PRIMARY side.

[heisthl]

Yeah buck/boost is cool but when you try and knock 700 volts down to 580 the buck changes the heater voltage by too much.

[skyboltone]

Yeah buck/boost is cool but when you try and knock 700 volts down to 580 the buck changes the heater voltage by too much.

Then you add a filament tranny. It just depends on how badly you want to save the PT.

[heisthl]

Well luckily the MM PT has the other taps so you can "buck" on the secondary side and not worry about heaters.

[Gibanez]

Thanks for all the posts. I appreciate all the
suggestions.

[skyboltone]

Well luckily the MM PT has the other taps so you can "buck" on the secondary side and not worry about heaters.

Absolutely. I never meant that a primary side buck was the solution in this case.

[FUCHSAUDIO]

While I'm sure bucking is a possible solution, the easiest way (IMHO) is as shown by the attached diagram.

Dumble used the full 700 V on the plates of the EL-34's and the 350-V tap for the screens, just like Music Man did. The only change I made (your option) is adding a simple zener string to limit the preamp voltages while the amp is cold. If you use series caps (like Dumble did), and the voltage total is 700-V or so, the large resistor is all you need. If you use 450 or 500v caps the zener diodes protect the preamp filters from overvoltage, until the tubes all start drawing current.

This is what we did in our Music Man mods. The bias winding is about 70-V total, with center tap. You can use the winding with the center tap grounded, and make dual 35-V supplies with a bridge rectifier and a couple of filters. The negative side is adequate for bias (the EL-34's with 450-V screens needs low bias voltages), and use the positive side (through a dropping resistor) can power a 12-V regulator to run the relays. The winding has enough current that the bias does not change when the relays are turned on and off, as long as use you low current relays.

[heisthl]

While I'm sure bucking is a possible solution, the easiest way (IMHO) is as shown by the attached diagram.

Dumble used the full 700 V on the plates of the EL-34's and the 350-V tap for the screens, just like Music Man did.

The circuit you posted was nicely done and would be a definite improvement.
Isn't 700 volts across an EL-34 output tube, or even on a good old 6CA7 a bit scary?
Maybe a little less scary if you used cathode bias.

Music Man used npns connected to the cathodes on the output tubes as both cathode bias and input. Some models even ran the grids as high as +30 (not a typo +30!) because the tube was limited in it's conduction by the transistor. Not sure what the voltage ended up being on the cathode at idle. But whatever it was it would be subtracted from the total B+ for voltage across the tube.