Michael Musitronic amp from the 1950's - Anyone ever hear of it?

[bobpaolucci]

I have this amp circa 1955 and is from an unknown, defunct company named Michael Musitronic and I’m trying to determine if I should fix it, sell it or modify it.  It was in bad shape so I tried to fix it up but there is absolutely no info on this amp...tube compliment, schematic, old advertisement, nothing. I was able to piece together what I had and sketched out my own schematic of what was there but have no idea if it’s been modified.

It had a voltage doubler so no power transformer, 2 octal and 3 9-pin tubes, a small 6.3v 1.2a filament transformer, and is split between an upper chassis containing 2 9-pin preamp tubes and a lower chassis which has the phase inverter and 2 6V6’s.  They are connected via an umbilical cord. The circuit is old school and makes sense in some ways and not in others. I’ve determined that it used 2 6V6's and 3 12AX7's (or similar) tubes and uses a cathodyne phase inverter. No circuit boards, just parts that fly across each other or mounted to what looks like a flat wire grid. Not very safe.  OT is a universal type and I’m using a Fender Eminence 12” 8-ohm speaker.

I tried rebuilding the amp using a 300-0-300 CT transformer I had, got all the bias voltages correct and fired up the amp.  As expected, the grounding is horrible. The preamp doesn’t distort but the tone stack sucks, is extremely tubby with tons of midrange bite, it has this weird circuit that shunts the volume pots to the next stage plate which sucks all the tone out of it. Finally, the circuit has problems where values seem incorrect.  Everything works but is very distorted and nothing I tried seems to make any difference.  I tried using a 12AY7 in the preamp and 12AU7 in the phase inverter and it helped only marginally.  In general, this amp sounds bad. The more I modified to sound decent it the more it turned into a Deluxe which kind of tells me this early circuit is bad and probably why no one has ever heard of this beast.

Rather than redesign this circuit, I'm at the point of deciding whether to keep this original and sell it to let someone else have fun or have the fun myself and convert it to a Tweet 5G9 Tremolux, after all it does have all the right parts. One side of my brain feels bad about converting a piece of history, but the other side wants to have a decent sounding amp. 

Does it sound like this thing is worth keeping in its current form simply because it’s old or should it just be sold as is?  I’m inclined to build me a 5G9.   Any thoughts on this would be helpful.

[acheld]

With a writeup like that     we need pictures!

So, no I have no info on this amp. 

Once you've made sure there is no market for this piece -- it sounds like it is not a good design --   well, not everything needs to be preserved for posterity.   There just isn't enough space in the garage for all the good stuff, much less the crap.

So yeah, make something great!

[bobpaolucci]

Here are some pictures.

[acheld]

Interesting.   Here's one (similar) that looks in better shape.    These look like 1940's design to me.   But I've never seen anything like the circular perforated metal disk with the circuit attached.   

I was able to find a trademark filing for Stanley Michael dba Musitronic Manufacturing, Bloomfield NJ.  Not much information available, one could say.

https://reverb.com/item/61205557-musitronic-tube-guitar-amp-1950-s-60-s-grey

[TIMBO]

INTERESTING 

[bobpaolucci]

Yes I am going to do the conversion. I think it would have more resale value if it were a known, great sounding circuit that someone can relate to. I just don't think there would be a market for an amp that has unknown characteristics.

[Lectroid]

I'd rebuild it without a second thought.  Just looking at that so-called "circuit" makes me nervous. But it's a great vintage cabinet.  Good luck with the gender-reassignment. 

[sluckey]

I would restore it as is just because it is such a unique amp. Who cares what it sounds like or what it's value is?

[Platefire]

Yelp! That's an unusual amp in many ways and unheard of.  You've got a good example of what it should be by the linked pictures on Reverb. If it fell in my lap, I would want to get the circuit up and running right, see what I got with that and go from there>>>>>>>

[bobpaolucci]

I did the restore and got it running with the voltage doubler and got to about 280v. Several resistor values were wrong,especially in the cathode PI. Constant grounding hum, PI out of balance, nasty sound.  On the scope the signals out of the preamp are not distorting but sound awful. Took the voltage doubler out and replaced with a power transformer, changed some values/tubes and got it to sound a little better.  Problem was it was turning into a Deluxe which I did not want to do. This thing is mounted on a wire mesh, not grounded to it, very dangerous so I stopped and needed to decide to keep going or, since it was being converted any way in a sense, why not pick a circuit that I might like.  It will still be mostly original but with a well known circuit in it.  Also, very difficult to work on the way parts are just flown across each other.  Still, great input and is giving me something to think about.  The more input the better.

[acheld]

INTERESTING 

Wait, it gets better.   

US patent US468274A was applied for in 1954, and granted in 1957 for Stanley Michael's design.   In essence, the circuit is mounted on the perforated disk to save weight AND to reduce the distortion caused by sound.   

https://patents.google.com/patent/US2812382A/en?oq=US+2812382+A

In the spirit of the amp, maybe you should make up a perfboard circle, and mount your circuit on that and attach it to the speaker as it was originally, with the umbilical unit and all. 

I do get it that you might not want to use a metal perfboard, but what the heck, it's no worse than the metal chassis we use every day.   And that metal perfboard "reduces distortion."

What fun!

[Platefire]

I checked the Patent out, pretty neat. However I didn't see a schematic? Would be nice to know the original design of that "Power Pack"

[bobpaolucci]

I made a schematic so I could figure out what was going on. Let me know what you make of it especially the "Depth" control.  I first though it used 6L6's as shown in the schematic but I later determined that they were really 6V6's based on the 1.2a filament transformer that it has.  Can barely run 3 12AX7's and 2 6V6's and the transformer has signs of overheating.

[sluckey]

V2B grid has no DC path to ground.

[Platefire]

No value on the volume, tone and depth pots. I'm assuming all 1M?

[bobpaolucci]

500K on the volume, bass, treble pots.
25K on the Depth pot.

I have found a few things strange, possibly incorrect in this circuit. The tone stack has tons of mid-range and not pleasant sounding. It does work, however, and I've checked the schematics against the circuit again and the only thing I can see is that the treble pot has an extra lug that is physically grounded to the chassis. Actually, all the pots are physically touching/grounded to the chassis. Could that be the ground reference you are expecting? What should happen without a DC path to ground? Are we just talking AC after the coupling cap or is the ground reference needed regardless? Interesting catch!!

[sluckey]

the treble pot has an extra lug that is physically grounded to the chassis... Could that be the ground reference you are expecting? What should happen without a DC path to ground? Are we just talking AC after the coupling cap or is the ground reference needed regardless? Interesting catch!!

That extra lug on the treble pot would be providing the dc ground reference for the grid. Otherwise, there may be an error in the tone stack schematic.

If the grid has no dc resistance path to ground (and ultimately the cathode) the tube cannot be biased properly. It's the dc voltage ***BETWEEN*** the grid and cathode that establishes the tube bias. Easy to check. Just measure the resistance between the grid and cathode. I suspect there really is a dc resistance path for the grid to ground. My point was aimed at a likely drawing error.

[PRR]

The blue "NFB" line has a resistor in it. And the OT secondary has a common/ground, which should be explicit on the drawing.

[bobpaolucci]

Yes, the OT does indeed have a 27K resistor soldered directly on it and is also grounded.   I have made the fixes on the new schematic. These are great catches! Thanks to all for helping shed a little light on this relic and making this schematic better and possibly make life easier should another one of these pop up. What does anyone make of the "Depth" control? Seems to pull the whole tone stack down and sounds thinner.  Best when full up but the bass gets very tubby when the bass pot is full up.

[PRR]

Depth control may be a mid/high cut, for a "rhythm guitar" sound.

I don't like the 22K in the phase splitter plate. I'd be expecting 47k or 100k in both plate and cathode. With 470k grid resistors it would be fine.

[bobpaolucci]

Originally the plate resistor was 22K and the other was 100K. I tried 22K/22K and 100K/100K but made no audible difference but did balance everything out.  I did leave them at 100K so I will change the schematic.  The output section has a .005mf cap between the plates and if I removed it I got no audio output. Any idea why you would want a cap between the plates or why output would stop? Prevent oscillation?