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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Parafeed without a cap

Hi, folks!
This circuit is not finished, just shows the idea.


The main trick to avoid coupling cap before OPT is bias servo (lowpassed comparator in this case) which keeps DC Vdrop across the tube equal to B- so we have 0VDC at OPT's input.
Also the tube is loaded with CCS which delivers voltage swing to OPT when signal is applied.
I'm wondering if it all will work or this is a bad idea?

I would be concerned that the transformer will still see the low frequency that the servo works at as an AC signal and depending on the range of variation, that might translate to some high flux levels. Can you inject a .wav file of some music and monitor the the servo to see how it reacts?


The typical servo is tuned under 1Hz... it moves slowly... you could tune it down to seconds (fractions of a Hz.) if you wanted to...

there is a bigger issue as to the stability of the two power supply rails WRT output level and maybe even more importantly variations in AC mains. Might call for regulated supply rails. One would need to do a little bit of experimenting.

Of course some minor offset and flux in the xfmr that does not occur regularly and gets nulled out for the most part might not be much of a problem. And it does open up the potential for DC coupling of the stages or stage before the outputs.
This is the grey area between AC and DC. If you tune it down to under 1hz and net a 10V swing, the (temporary) flux levels can be huge in the core.

I'm not sure how much of an issue this is but down at those low frequencies the transformer has basically the only DCR to prevent current flow.


I wrote a nice response and then it got lost in internet space...

If well designed I think it will be a limited issue. The realities when tested may prove it to be a problem or not.

Seems like the stability of the rails with AC line dips and/or draw by the circuit could cause momentary offsets. But if for example the rails were regulated, then the amp might be pretty stable without the servo, and the servo would only adjust for the tube's change in bias point over time.

I think the idea has some merit, but in practical terms I think I'd want a bit of output iron that was gapped anyhow... :D

And also, not sure that the basic parafeed with a cap feeding the OT is much of a deficit. Any way it seems like having a cap with a charge on it is better than a cap that swings through ground.

random thoughts...