Potential MusicMan Conversion

[donvan]

I am the original owner of a Musicman 100 watt RD112 from the 80s. It has never had any issues and I nor anyone else have been in it ... yet.
In the past I've been hesitant to do anything to it since it is a perfectly working amp. However, it doesn't sound anywhere near as good as the #183 that I built - clean or overdriven.
So, I'm considering opening her up and modifying to perhaps a TAN Ultrasound type circuit.

One thing that is really confusing me is that it only has 2 6L6GC power tubes yet it is a 100 watt amp. Anyone care to look at the schematic and explain how you get 100 watts from this circuit? I am baffled.

[sluckey]

What is the value of R31 and R76? Perhaps you have the 65W version. I suspect the 100W version has four 6L6s.

Another possibility is that the 100W rating is not RMS power, but only peak power. Back in the '60s lot's of consumer manufacturers advertised peak power ratings to confuse/fool the consumer into believing their amp was more powerful than it really was. Probably not the case with this Musicman.

[martin manning]

See here: http://pacair.com/mmamps3/model-chart

[donvan]

See here: http://pacair.com/mmamps3/model-chart

So that chart confirms what I'm saying - a 100 watt RMS amp with 2 6L6GC power tubes. Generally speaking, you would only get 50-55 watts from 2 of these tubes in class AB operation. How are they doing it? Does it have something to do with the transistors driving the cathodes?

[donvan]

I found another site that says the answer is because of the high voltages and because the amp is biased in class B. And the fact that it is a hybrid cascode circuit.

[martin manning]

Output power is determined by the plate voltage swing and the OT primary load. The power tube control grids are fixed at 22V, and they are driven from their cathodes by BJT's. It must be running in Class AB2 to make an honest 100W. How does it sound with a distortion pedal in front of it?

[donvan]

Output power is determined by the plate voltage swing and the OT primary load. The power tube control grids are fixed at 22V, and they are driven from their cathodes by BJT's. It must be running in Class AB2 to make an honest 100W. How does it sound with a distortion pedal in front of it?

I don't care for the built-in distortion mode but a tube screamer in front of it sounds really nice.

[donvan]

I got the number stamped on the power supply transformer - 200-130. That seems to be good news. According to posts here that is the one used by Dumble.

I found the wiring diagram but so far no spec. Does anyone know how many amps the heater windings are rated for?Seems odd that musicman would use a transformer capable of powering 4 EL34s for an amp that only uses 2 6L6GcCs.

[martin manning]

For 4x EL34 it would be at least 6A.

[donvan]

From the following posts it looks like this PT was rated at 6 amps. Once again, HAD is going over the spec by almost 1 amp when you add 3 12ax7s.

This comes from the ampage archives:
------------------------------------------------------
7/26/2000 3:06 PM
Ray Ivers
email Re: Music Man 130 (model 2275 4-10)
Mike,

Just in case it will do you any good, here are the power transformer specs for that amp, from Schumacher:

Primary 120v

High Voltage Sec #1 Sil Voltage Doubler (F.W.) (2) 100mfd
Brn - R/Y 710vdc @ .100a 660vdc @ .275a
Red- R/Y 440vdc @.275a

Low Voltage Sec #2 org - yel- org
Sil. Bridge (2) 150 mfd cap + - 42 V 0A 60V CT @ .1 RMS

Filaments (2) 6.3V @ 6 A grn- grn/yel

Ray Ivers
R.A.G.E. Electronics


7/26/2000 5:33 PM
Mike
email
Thanks Ray,
It's a big help.

If the filament is rated @ 6 A, then the amp is already exceding that with the 4 EL34's and 12ax7 right?

And if that's true, to add 2 more 12ax7's I'd need to either install a seperate filament transformer or remove the 2 EL34's to free up 3 Amps.

Mike

7/26/2000 7:24 PM
Ray Ivers
email
Mike,

This transformer (200-130) was specified for use in all the Music Man 150 watt amps - both the 4-6CA7 and 4-6L6 ones. The heater winding is way overrated for the 6L6 amp (total draw 3.6 amps; this one never used the 12AX7 driver) and as you noted in your post, slightly underrated for the 6CA7 amp with 12AX7 driver. The GE Essential Characteristics book list the 6CA7 as drawing 1.5 amps heater current, so the transformer is right on the money for the transistor-driver version and .3 amp underrated for the 12AX7 driver version, which really is not a concern (I have seen 1.6 amps quoted for EL34 heater current draw, though). To add more tubes, though, I think I would recommend adding another filament transformer.

As Chris noted in his post, this is a quite unusual power supply design, and I definitely agree that I would leave the screen/plate voltages as they are, unless you are going with KT88's or KT90's for output tubes. I'm mainly familiar with the cathode-drive version of this amp (which I personally would not modify, at least not the power amp), so I can't really say whether or not you should go ahead with your mod plans; however, I have always felt that the preamp is the weak point of these amps, for what that's worth. The (kind of hard-to-read) schematic is available at http://www.ernieball.com/musicman/techi ... 75-130.pdf.

Ray
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[beasleybodyshop]

Keep in mind it holds the screens at half of plate voltage, which is part of how it's keeping things stable at 700+ volts DC. I've recorded approximately 90 watts clean sine wave, right before break up on one of these that was on my bench for repair. So 100w? not quite but close enough

[talbany]

Music Man 700v 350 Scr setups are tight and bright! 4X El-34's gives virtually unlimited headroom Increased touch sensitivity along with an open and clear midrange. It is punchy with a faster response time with little compression at high volumes. Those that have played those old Blackface AB 763's 50's where it hits the wall (around 4) bass begins to fart out and go soft. Not these amps they just get LOUDER
IMO a real players output section with a big sound where every note is clear and jumps out from the speaker, (a light touch is all you need) unlike other amps where you have to muscle it to get it to respond. Not with the Music Man set up.Also great for slide, out of all the ODS models Dumble built the Music Man's are my favorite.

ODS on Steroids

Tony

[donvan]

Here is a photo of the PT in my music man amp. Can anyone confirm if this is the type 1 transformer and that it is the one that HAD used?

Thanks!

IMG_5003.jpg