Apologize for the crude dwg, cannot find a schematic anywhere. It's an old "Manatt King" PA from the 40's. Has 3 12SJ7s for a preamp, 6SL7 PI and 2 6L6s. Lots of interesting stuff - DC filaments for the preamp, two eyelet boards full of resistor and capacitors, and a weird PI. Three inputs, two for MIC going thru one 12SJ7 each (#1 and #2 in my dwg), going to a third 12SJ7 (#3) which also gets a PHONO input. Note all three tubes' plates and screen grids are tied together (after various resistors) to the plate of one side of the PI tube (6SL7) thru a 220K. I know it's not an LTP (no common-cathode), or a Cathodyne (single triode PI) or a Paraphase (inverted outputs from the PI plates). I am stumped. The amp works fine, plenty loud, but I want to play with it, see if it needs biasing, get some distortion out of it (for guitar of course!). Any ideas? Thanks!
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Not sure if the PI is drawn correctly... but if the signals are taken from the plate and the cathode of the same tube, then it is a split-load/cathodyne type. In any case, here is an example of what can be done with it.
Fender 5C8 Mod
Fender 5C8 Schematic
Fender 5C8 Mod
Fender 5C8 Schematic
You know, with one half of the 6AL7 taking all the preamp output, it looks like another gain stage, with the other half looking like a triode Cathodyne PI. How bizzare is that?? Like I said, this PA has a lot of weird features....
Thanks for the links. Have seen that utube of a King PA turned into a 5C8, but it would mean a total rewiring.....not sure I'm ready to gut mine right now....but I do dig the amp case...way retro!
Yes, the chassis is way cool! I would keep the look exactly the same, just change the gain structure (you sure got enough tubes to work with!) to suit geetar ;-) Keep us post on this project.
There is no plate resistor indicated on pin 2 of the 6SL7. If this plate resistor is actually 220k, then the phase inverter is a standard cathodyne.
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