[Alex Pogossov]

"John Byrns" wrote in message
...
RCA published a paper describing the 6L6 beam power tube and its
development in
the Proceedings of the IRE and also the RCA Review back in the mid 1930s.
Besides describing the development of the 6L6, they showed the op amp like
connection you are speaking of and what the characteristic curves of the
6L6
look like when this connection is used, basically triode like IIRC.

I don't think the connection was very common although I believe I have
seen a
couple of amps that used it. Much more common is a variant on the
connection
where the resistor from the output tube plate is feedback to the driver
tube
cathode, I have seen this connection quite a bit over the years.

Interesting to calculate will such feedback be deeper tham the op-amp
connection? Possibly, if Rfb is relatively small (and here you start loosing
AF power) and driver plate load resistor is large. As J.L.Stewart mentioned,
a pentode in the driver will be an advantage.


The problem with either of these schemes is that it destroys the pentodes
inherently good power supply rejection. That means that we either need to
provide greater power supply filtering to reduce the noise on the power
supply
line, or go to parallel feed to improve the power supply noise rejection,
either
solution adds to the cost.

Yes, but all 300B / 2A3 lovers are bound to face the same issue.


If I were doing this and was free to spec the output transformer I wanted
I
would go with the cathode feedback scheme in the output stage. Taking the
feedback from the plate doesn't improve the low frequency stability
issues, it
only helps with high frequency stability, so what I would do if I couldn't
spec
the output transformer I wanted, would be to take the negative feedback
from the
secondary at low frequencies and from the plate/primary at high
frequencies.

I did the two branch feedback exactly in the same manner. One solution is
just to throw a small capacitor (10pF) from the plate to grid. It will work
as a compensation cap in the op-amp, creating a dominant HF pole. In other
words it makes 6V6 work as integrator at HF.

Later I realised that far better is to throw a larger cap from 6V6 plate to
unbypassed cathode of the previous driver stage. (To the same cathode a
second resistive (regular) NFB is fed from the OPT secondary.)

Regards,
Alex