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I picked up a M-192-5 faceplate off of ebay a while back, and this is what became of it.
The amp is a tribute of sorts to an original M-192-5. The criteria I set for the project was as follows:
Used M192's are available and cheap enough, so there was no reason to pursue a clone of some sort. I studied a lot of period schematics of para-phase PI cathode biased amps. I settled on a design that might be considered a blend of several. mostly the Fender Deluxe 5C3 and the Gibson GA-20. I made a few tweaks that came not from my engineering prowlness, but from these great people that generously share their expertise freely:
I searched ebay for a long time looking for a suitable old Triad or Peerless PT. The only candidates were complete crap-shoots, sometimes with frayed leads, or unknown amperage ratings. I've had a lot of good luck with the Chicago made Magnetic Elec. Corp. transformers so I ordered the 40-18038 OT and the 40-18016 PT, both designed for higher voltage, higher wattage Deluxe circuits.
I wanted to try a trick with the power tube proxity to the output transformer (OT) that Magnatone used in the 1964-1966 M-series "suitcase" amps. Engineer James Evans found that locating the tubes equidistant from the OT and using a twisted pair for the plate leads was the best arrangement to reduce noise.
These are cool phase inverters that don't get much respect. Once nice thing about them is they are old tube friendly, since both stages are low-current gain stages. These PI tubes are not likely to wear out. If a twin triode is used (like this circuit) and the cathodes are tied together with a common resistor, there is no need for a bypass capacitor. (See Valve Wizard link below).
I had this idea to isolate the filament wiring from the signal and HT circuit by running the filament wires on the outside of the chassis and have them enter the chassis as close as possible to the tube socket solder tabs. Of course, it is impractical on many levels, but why not try it?
As you can see, the green wires first route to a tag board, and then fan out individually to the tubes. The 8BD basing (6SL7,6SN7) put the heaters on pins No.7 and No.8, so a single hole could be placed nice and close to the tube socket. The 6V6's however, put the heaters on No.2 and No.7. To keep the twisted pair twisted as tightly as possible up to the socket really required a compromise (I ran them inside through a single hole (the tap board) on the No.2/7 side of the tubes).
Does it work? I was happily surprised to find the amp to be very quiet! It was extra work, and isn't extremely practical. besides the extra holes, extra thought, the extra panel above to conceal the wires, I also had to build a "stand" for the PT so the wires can exit the PT above the chassis.
I used F&T lytics for the filter caps and some cool NOS El Menco .012uf's for the coupling caps. These El Menco's were actually the first thing I decided had to go in the amp after I got the faceplate. The 5C3 used .05uf/600v. Since I was shoe-horning these .012's into the circuit, I set the grid-leak resistors and the resistance to ground that followed the coupling-caps so that with the .012's the C/R pair would provide somewhere around a 60Hz bass roll off. The El Mencos were popular capacitors in the 1940s and early 1950s. One draw-back is that they are lower voltage (350V?). In coupling cap application, that isn't a problem in regular operation, but I suppose if the phase inverter tube fails, the caps might over-voltage.
For the first three filter caps, I had an odd idea of putting the F&Ts in a cap-can. Inside that giant can is a 1"x3" eyelet board with three caps. This was fun, but I realize that it's sort of dopey.
If you plug into the MIC jack, you get a single pre-amp gain stage and then its on to the PI stages. This mimics the 5C3, GA-20, and the M192. If you plug into either Inputs No.1 or No.2, an extra 6SL7 gain stage is added to the front of that MIC input, and both volume controls can be tweaked to adjust which tube you overdrive.
Because the total gain from both 6SL7 triodes would be too much, I put a 470K ohm resistor (R114) on top of the 500K pot to cut gain from the first stage to something between 0 and 50% before sending it to the 6SL7. 470K might be too low, it might really need to be 1M (however, it sounds great as is). I also made the grid leak resistor of the MIC stage to be 4.7M so it won't change the taper characteristics too much of the No.1 & No.2 volume pot. (I think the datasheets say 2M max, but it seems to be generally accepted that 2M can be exceeded in low-current draw gain stages like this one. Plus Ampeg did it with the 6SL7's in the B12 !!).
For my tastes, typical single knob controls roll too much high off after 50% of the knob travel. Here I've just used the tone knob as an adjustable treble-bleed cap.
The HI/LO switch places a cathode bypass cap on the MIC channel of the 6SL7. This is a very drastic tone altering control. It is similar to the difference between the bridge and neck pickup on a guitar. Its common on tube-rolling forums, but I'm surprised amp manufacturers don't do it more. Joe Naylor did it on his Hellhound and Kingsnake amplifiers, and if that's not a vote in favor, I don't know what is...
There were lots of cool forties octals to pick from, like the 6SJ7, 6SC7, and 6N7. The tube compliment I settled on is a 5691 (6SL7) preamp, 6SN7 phase-inverter, two 6V6GT's, and a 5V4. I chose a Philco 5V4 rectifier tube over some of my 5Y3's. One reason is this 5V4 is so big, it doesn't fit any of my other amps. The 6V6s are non-matched RCA's (one black smoke, the other ...not). The 6SL7 I chose is among the creme de la creme of pre-amp tubes. A genuine red base RCA 5691. What a handsome tube! It sounds great too!
This first picture shows the top plate with 1/2" stand-offs, as well as the raised PT. To raise the PT (this was a bit of a pain), I removed the bottom "bell cover" and MIG welded three brackets to its sides, and reinstalled it on the PT.
That crazy pot with the white plastic tube is the trimmer pot to set the signal level going into the PI's lower gain stage. The pot is over-kill and really isn't needed: In a para-phase, this balancing/matching is not important.