Pacific Music Supply Company - Model M-100-3

[IrishMail]

Pacific Music Supply Company - Model M-100-3

I'm hoping to reconstruct this (6SJ7 - 6L6 - 5Y3GT orig RCA) bottom-mount amplifier. The alligator tolex, flat leather handle, and white chicken head knobs give this slanted TV-front cabinet a very nice early Magnatone vibe.

My images include a self-drawn schematic, a component layout, and a few photos:
http://i356.photobucket.com/albums/oo3/ ... EMATIC.png
http://i356.photobucket.com/albums/oo3/ ... LAYOUT.png
[note - drawings revised 6-22-11]
[img2555]http://i356.photobucket.com/albums/oo3/ ... binet1.jpg[/img]
http://i356.photobucket.com/albums/oo3/ ... binet3.jpg
http://i356.photobucket.com/albums/oo3/ ... rear95.jpg

The cabinet design and mounting, down to the cup washers, oval slot head screws, rear panel routing, and chassis slots, is very similar to a couple of Magnatone M-199-3 amps.
http://www.vibroworld.com/magnatone/M-199-3-O.html
http://www.vibroworld.com/magnatone/M-199-3J.html

I'm thinking 1948 is the year for several reasons: the 5Y3 is stamped 8-48; the 500k tone pot is stamped 134823., and the 8" Oxford(?) alnico speaker is stamped 395842. The speaker cone is stamped "W188-1". It has a Triad 6308 power transformer, hefty enough to feed the 6L6.

It was a package purchase, with a post-‘47 Rickenbacher NS lap steel (white-dot), bakelite Manoloff tone bar, and original GEIB leopard-print case. The volume and tone pots were frozen, so the previous owner snipped several wires. Upcoming service will substitute 9 caps (at original values), 2 pots, and a grounded AC cord. When this happens, I’d really like to hear what the original tone stack sounds like.

I intend to use it as a lap steel and blues harp amp. If I can't get the orig, or if it's not that great, I'm thinking of the tweed Princeton tone/vol design. Because the 6SJ7 pushes into a 6L6 rather than a 6V6, is there any mod I should look into?

I’ve dug thru Schematic Heaven, but was left unsatisfied. I appeal to the vintage amp gurus and anyone with a Magnatone, Dickerson, or Valco amp with a "Tone" control and the same octal tubes, for advice and assistance.

[Lonely Raven]

That looks like a fantastic project! I love the octal preamp amplifiers.

[Structo]

Cool look cabinet for sure.

Seems like the lines blur with the early 1930-1950 amps like that.
Valco, Supro, Dan Electro, Magnatone, Silvertone, etc. all seem similar.

I know Valco built amps for a large variety of companies.

If I was you I would find as close as what you have there, in other words a octal preamp, single ended output to figure out the volume and tone stack.

The circuit as you note is very similar to a 5C1 Champ.

http://www.el34world.com/charts/Schemat ... _schem.pdf

If your amp originally had a field coil speaker then you need to adjust values in the circuit, usually the power supply to compensate for the lack of that coil.
Frequently it is used in lieu of a choke coil.

On your schematic, you show a 25 ohm cathode bias resistor on the 6L6.
That is pretty low, usually it is at least 250 ohm or higher like the Champ has a 500 ohm resistor.

Those should be at least a 5 watt resistor due to the heat.

Good luck!

[IrishMail]

If your amp originally had a field coil speaker then you need to adjust values in the circuit, usually the power supply to compensate for the lack of that coil. Frequently it is used in lieu of a choke coil.

I'm pretty sure the alnico is the orig speaker- the date code is consistent with the other components, and (except for the knucklehead who snipped the vol/tone pots) doesn't look altered.

On your schematic, you show a 25 ohm cathode bias resistor on the 6L6. That is pretty low, usually it is at least 250 ohm or higher like the Champ has a 500 ohm resistor. Those should be at least a 5 watt resistor due to the heat.

Thanks for the heads-up on the old dog-bone resistor. Here's a photo - to me, it really looked like 25 and an Ohm symbol. It probably really DOES read 250. I'll put it under the magnifier when I do the re-cap, and update the drawings as needed. ((wiring HAS to be some of the ugliest layout I've seen in a while))

[img2576]http://i356.photobucket.com/albums/oo3/ ... cuit80.jpg[/img]

Might the tweed Princeton vol/tone circuit work w/o modifying its values?
<replacement parts are enroute from Weber>

[David Root]

That is a super nice cabinet for that vintage!

I have a Magna Electronics '47 195J SE 6L6 with a really ratty cabinet that needs restoration. It's waiting until I retire. And a '47 Gibson BR-9, ditto, which is cosmetically really nice.

Tom is right, that has to be a 250 ohm cathode resistor. 5W minimum, sandbox or other wirewound. Except for the caps in these old amps, if it ain't broke, don't fix it. But you need to check all the values because they drift all over the place in 50+ years! Including pots. If the tubes are original keep 'em!

[Structo]

That does look like a 25 ohm resistor as far as the printing on it goes.

Are you sure that it connects to the power tube socket, pin 8?

[IrishMail]

Thanks again, Tom, for your keen eye on the resistor. I managed to get the meter-probes into that rat's nest of wiring, and it read 195 ohms, which is 22 pct short of the 250 ohm you mentioned.

One lead is soldered to pin 8, the other through a ground lug into pin 1 on the socket for the 6L6, which is the original RCA metal envelope. A friend has a tube checker, and all three will be checked out.

Each pot is very close to 500K, but they don't even turn with vice-grips(!). Replacements are on the way - rotary switch on the vol, and push-pull DPST on the tone, so I can experiment with caps.

Drawings revised. They were done with the primitive tools in MS Power Point.

[IrishMail]

That is a super nice cabinet for that vintage!
I have a Magna Electronics '47 195J SE 6L6 with a really ratty cabinet that needs restoration. It's waiting until I retire. And a '47 Gibson BR-9, ditto, which is cosmetically really nice.
Tom is right, that has to be a 250 ohm cathode resistor. 5W minimum, sandbox or other wirewound. Except for the caps in these old amps, if it ain't broke, don't fix it. But you need to check all the values because they drift all over the place in 50+ years! Including pots. If the tubes are original keep 'em!

Thanks for the info. I wonder if the vol/tone used in the Magna M-195 is what I'm looking for. It has an addl tube (6C5), but it DOES have a tone control. hmm...

[IrishMail]

Ordered parts from Weber and swapped out all the known caps. One of the replacement pots wasn't what I ordered, and its replacement arrived late today.

I guess I'll just play around with/without the tone stack using alligator clips to swap parts until I hear what I (more or less) like.

Also thinking to 'heat' up the instr-2 jack by switching resistors, for the harp-mic feel to suit me.

So much for original restoration . . . .

[IrishMail]

Pacific Music Supply Company - Model M-100-3

After swapping the caps and the AC line cord, I hooked up the vol-pot like the Champ 5C1 and skipped the tone pot for the time being. Plugged in the Rick-NS lap steel that came with the amp, and all I can say is WOW ! ! ! -- Dimed, I swear it's shoulder-to-shoulder with any stock Champ I've ever heard or felt. Easy enough to use the guitar's vol/tone, and leave the amp on full. With the 6L6 for its power-hitter, I'm truly at a loss for adjectives.

For blues harp, it's just a bit too brittle when playing through my Turner 256 or Shure 444 (with a CM cart). So, between gigs, I'll play with a few tone stack alternatives. BTW, I chose an audio 1-meg for volume and a linear 1-meg for tone.

[Cliff Schecht]

I think you're hearing the magic of a single 6SJ7 in the preamp. I bought my '54 Silvertone 1331 (6V6 power amp) for the same reason, it just sounds magic when pushed. I use it onstage at acoustic shows for electric slide songs and anything that needs some grunt. Also it takes overdrive pedals really well, as should your amp.