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Old 26th September 2013, 04:21 AM   #1
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Default Need help identifying a phase inverter

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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Old 26th September 2013, 07:39 AM   #2
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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
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Old 27th September 2013, 03:50 PM   #3
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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....
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Old 27th September 2013, 03:51 PM   #4
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I meant 6SL7
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Old 27th September 2013, 04:00 PM   #5
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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!
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Old 27th September 2013, 05:18 PM   #6
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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.
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Old 27th September 2013, 11:59 PM   #7
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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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