• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Organ amp conversion

A few things wrong with the tail as drawn. The 12K tail resistor needs to be somewhat larger and not bypassed.
You would also want a gain stage before it.
The earlier stage as a concertina splitter was probably a better choice unless you want to add another gain stage.
 
Ok, attempt #50. Still working on el34 grid leak size, and completely lost with the feed back values. But getting there. Added load line for 6sn7 first stage, cathodyne is similar.
 

Attachments

  • 20250228_114703.jpg
    20250228_114703.jpg
    403.1 KB · Views: 40
  • 20250228_114645.jpg
    20250228_114645.jpg
    419.1 KB · Views: 38
The EL34 will not have a grid leak resistor. They call it a stopper resistor in this case. It is meant to limit very high frequencies from getting to the power tube and causing stability issues. Should be about 1K. The resistors on the phase splitter should match 30K should work.

One reason to drop 50 volts between the phase splitter and the first gain stage to to any coupling between stages as far as plate supply.
 
I originally had the first stage at 300. But the +/- voltage swings look better at 350. Maybe I shouldn't worry, but I was trying to get to 200v peak to peak for the el34s. Also knowing I'll loose a little in the phase splitter stage. Also, I was thinking the 2 risisters on the cathode would add together. Is that not how it works? So 29.5, and 30.
 
The feedback is shorted to ground by the 100 uF bypass capacitor. Split the 2k resistor into 1.8k and 220 ohms. Bypass the 1.8k with the 100 uF, and below that have the 220 ohms. Feedback connects to the junction of the 1.8k and 220 ohm resistors.
 
Ok made some changes. Still trying to wrap my head around all this. Should the feed back line have a serious resistor and bypass cap? Oris the 1.8k and 100uf cap on the cathode all ready doing this?
 

Attachments

  • 20250228_161504.jpg
    20250228_161504.jpg
    422.5 KB · Views: 34
The feedback resistor should be fine. Remember to change the 27K cathode side of the splitter to 30K. You might try without feedback at first and add about 5-10K for feedback and see how many DB was dropped. You might need a tone generator which can be done with any computer. Also REW can run frequency response and distortion plots easily.

Also you want to add a pair of 270K bias resistors and tie them together and send to the bias pot.