It's a good idea to try and and trace it back but right off the bat, ž of the preamp except maybe the inverter (without errors) needs to be replaced with a guitar friendly circuit if you want to use the amplifier for the guitar.
There are regions in the re-drawing that I do not understand, some also doesn't make sense with missing B+ nodes etc... I would leave off any feedback circuits in general and I would scrap it and build a guitar friendly circuit instead if you do not wish to repair this organ.
Obviously your organ has issues (stuck key sound) leaking sound into the amplifier, probably the tonewheels and sometimes you might actually hear the faint sound of the whole keyboard at high pedal volume settings with the amp idling while not playing anything.
Realizing your amplifier still amplifies the sound of a dying organ with normal voltages indicate that it is working. Maybe weaker with old tubes but then, you will have to compare with a new set and find the bad ones in the old set with somebody that has a tube transconductance meter.
But even before trying a new set, your ongoing schematic (if really as is) is likely the product of someone who probably did subjective work inside that need to be corrected. It seems counterproductive without having an accurate original schematic, to try and re-interpret the circuit using images of other wrecks having also some loose wires in common. I would scrap it and build another, using creativity among the available cleaned strips, new caps, well definitely new e-caps, new resistors etc...
So you have big transformers and with the organ's continuous tone, chords and all, I think the designers put good faith into those 6973's. Playing guitar through them with the intermittent nature of guitar playing (finite sound, breaks) will be kind of a relief for them, unless someone is picking whole tunes with chords using a small fan. You still have a sagging 5y3 which is not to be changed unless you add some resistance. The power tubes are still operating within their range looking at the data sheets.
So yes, 430V on the plates with fixed bias is ok... Just make sure the sg2 has 300v max or correct it so that it does. This supro two-section LFO circuit is a good idea and better tremolo than a princeton but its paraphase inverter will not work Ťas isť with fixed bias. You can replace it with an LTP with a small tail 6k8,10k and without feedback, like the 5G9 tremolux. This will retain the sound of the supro I think while making it work fixed bias.
If you want reverb, you can also build one channel with the actual 3 double-triode preamp chassis arrangement but using different types, for adequate gain and overall functionality. A 12ax7 input stage followed by the oscillator as the first tube envelope. The osc. cathode's being directly tied to the input stage section's cathode and sharing a bias resistor early supro-like. Then you will have to keep the hi-pass thumping filter to keep the supro sound. If you want it sounding more fender-like, do not put this filter and wiggle the -25 volts instead at the power tube grids junction, it will cancel residual low freq thumping and you will have richer low-end. Then you need one tube reverb drive and recovery with a coupling circuit and there's lots of known good one tube reverb circuits on this forum. Then, you need to wire that third tube, a driver inverter with some recovery gain without any negative feedback from the output, best using a 12AX7 as an LTP or split load.
The reason I suggest fixed bias rather than cathode bias is I find fixed bias to sound better with high plate volts, lower current settings. You will have to tweak the bias circuit (use a diode) to have between -25 -30 v. on the grids and 25ma flowing through each plate.
Topic: Got a new project. Thomas Model "M" organ. PP 6973 power amp. (Read 7261 times)