Negative feedback from a transformer winding applied to output tube cathodes seems to do the most good when the OPT is not the best design.
I've had good results improving push pull PA amplifiers having center tapped secondary windings (such as 0-4-16 ohms or 25VCT) where the peak voltage from the winding is about half the peak grid drive voltage. More drive voltage to the output tubes is of course required to overcome this and get the same power output. I usually up the plate supply for the driver if that's the limiting factor.
I've done it on a few small AM table radios as well. It cost nothing to route the cathode through the 4 to 16 Ohm speaker winding as shown and the few mA through there won't hurt a thing.
This is all very general, one has to try it to see if the results are worthwhile.
A caveat is that at high frequencies and with square waves, there can be some slight ringing that must be dealt with. Nothing's perfect!
I've had good results improving push pull PA amplifiers having center tapped secondary windings (such as 0-4-16 ohms or 25VCT) where the peak voltage from the winding is about half the peak grid drive voltage. More drive voltage to the output tubes is of course required to overcome this and get the same power output. I usually up the plate supply for the driver if that's the limiting factor.
I've done it on a few small AM table radios as well. It cost nothing to route the cathode through the 4 to 16 Ohm speaker winding as shown and the few mA through there won't hurt a thing.
This is all very general, one has to try it to see if the results are worthwhile.
A caveat is that at high frequencies and with square waves, there can be some slight ringing that must be dealt with. Nothing's perfect!
You can do that, use R6 as the feedback resistor, not to gnd but to the OPT.
With another resistor to the cathode the bias of the tube change.
Mona
The beast that started this thread (not me..the radio). Went through it. Cleaned all the controls. Did a recap. Alignment. Checked and replaced a couple tubes. Replaced the rotted power cord with a modern one with a ground. Applied Tubelab's neat trick to keep the DC off the speaker.
The white cable you see is hooked to the turntable input. The owner (who is about the same age as the radio 70 yrs. or so) wants to play his CD's through it. I wish they made an air freshener that smells like an old radio firing up for the first time in 30 years
Thanks all
The white cable you see is hooked to the turntable input. The owner (who is about the same age as the radio 70 yrs. or so) wants to play his CD's through it. I wish they made an air freshener that smells like an old radio firing up for the first time in 30 years
Thanks all
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