Advice on this amp schematic please

[martyh]

This is another attempt at a circuit modification for the Altec 1568A amp. I’ve kept the operating point on the outputs per the original, tweaked the operating point of the driver stage and swapped out the input /phase splitter stages for a direct coupled ltp.

The output feedback resistor is my biggest sticking point. I’ve read what I can find on the subject and I am still not sure what constitutes “too small” per Broskie. As it is drawn (if I am doing the math correctly) f3 in this drawing should be about 40K. I believe that the current swing of the driver stage across the feedback resistor will swing the output grid past the point of clipping but not sure if I am missing something.

Another area of concern for me is the current sink on the ltp. I can add filtering to the original negative voltage supply but it is only half wave rectified. Is there any problem with a somewhat noisy supply for this duty.

I would rather not carve up the chassis so I am stuck with the original tube count. The only way I can see to use pentodes drivers is something like a pair of 6bm8 and the layout is not great for that. Hopefully the triodes with unbypassed rk will do.

The original schematic can be found at The Free Information Society - Altec Lansing 1568A Electronic Circuit Schematic

Thanks in advance,
Marty

[Tubologic]

I'm not sure what you call "output feedback resistor" ? The (6CG7) driver tube 24K plate resistors should be connected to a separate (well-filtered/decoupled) 375V HV supply rail , not to the output tubes plates. Then you can add local feedback resistors from the ouput tubes plates to the driver plates if you feel so inclined but it's not strictly necessary as there is already a (adjustable) global feedback loop. These feedback resistors will need to be of a much higher value (probably in the order of hundreds of kilohms) and some experimentation will be needed to find their correct value. Improvement will be very small, if any.

[martyh]

I think I made a couple of mistakes with the first version. I don’t think there was enough swing left for the plate in the first version I posted but since I am eliminating a tube rectifier I have a 5v winding I can use with a small transformer to generate a full wave rectified -220v supply. Also I used the wrong capacitance and gain to calculate the hf limit of the outputs so a larger feedback / driver plate resistor is possible. Any thoughts on this version?

Tubelogic,
Thanks for the reply. I am trying to achieve most of the feedback in the small loop between the output plate and grid so as to lower the effective impedance driving the transformer. My guess is that a small amount of feedback will help only a little and if I am understanding the original circuit the outputs are biased only a small bit above cutoff so operate nearly in class B. For most of an output tubes swing it only sees an 850 ohm load so reducing the driving impedance to some fraction of that seems like a good idea.

Thanks again.
Marty

[payitforwardeddie]

Your 6CG7 has -30v bias to the grids and -120v on the cathodes. That's a lot of grid current flowing. They may live about 5 minutes, less if Chinese.

[martyh]

Thanks Eddie , I screwed that up in the schematic. The 6cg7's should be running at 5mA per section.