I was wondering if there were any examples of driving an output triode with
a cathode follower in which the cathode follower has a large inductor for a
cathode load instead of the usual resistor. I was thinking of using this
idea to drive an SV572 SE in Class A2 near zero bias such that I could
attach the cathode of the driver directly to the grid of the triode and
return the bottom end of the choke to a small adjustable negative supply.
That would save making a high-voltage negative supply and also cut out a
coupling cap. Then I could adjust the small negative supply so as to keep
the bias on the output tube at or perhaps just below zero volts. Assuming
this would work ok, would there be any different requirements for the choke
in this type of arrangement as opposed to the chokes designed for plate
loading of drivers? I know that good quality plate chokes of reasonably
high inductance can be pricey so I was wondering if one could get away with
something else in this application like a large power supply choke? Also -
I have some large SE OPT's laying about - can I use the primaries as cathode
chokes for experimentation purposes? If used in this way, do I have to do
anything with the secondary leads, or just leave them open?


thanks in advance,

Dave

Behold, David Forsyth signalled from keyed 4-1000A filament:

I was wondering if there were any examples of driving an output triode
with a cathode follower in which the cathode follower has a large
inductor for a cathode load instead of the usual resistor.

I have seen a few, but you're looking at inductances in the 100-300H
range.

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca

David Forsyth wrote:

I was wondering if there were any examples of driving an output triode with
a cathode follower in which the cathode follower has a large inductor for a
cathode load instead of the usual resistor. I was thinking of using this
idea to drive an SV572 SE in Class A2 near zero bias such that I could
attach the cathode of the driver directly to the grid of the triode and
return the bottom end of the choke to a small adjustable negative supply.

No problem.
The anchoring negative bias end of choke should be low impedance.
I have tried using choke coupled to the anode circuit of a triode driver
using a large cap, and the the choke replaces the R normally used for biasing.
Thus instead of an R load hooked to the driver, its an L, which can have much
higher impedance than the R.
Cathode followers driving grid current on output tubes need all the help they
can get,
and the choke avoids the need for a resistor from its cathode to a
negative voltage, say -200V.


That would save making a high-voltage negative supply and also cut out a
coupling cap.

Yes,

Then I could adjust the small negative supply so as to keep
the bias on the output tube at or perhaps just below zero volts.

Yes, maybe rectify and filter a heater voltage somewhere.

Assuming
this would work ok, would there be any different requirements for the choke
in this type of arrangement as opposed to the chokes designed for plate
loading of drivers? I know that good quality plate chokes of reasonably
high inductance can be pricey so I was wondering if one could get away with
something else in this application like a large power supply choke?

Ideally, the choke placed in a cathode follower circuit should have the same
high L
as in the anode circuit, to get away from saturation, if there were fewer turns.

The choke don't have to be too large, surely, and 10,000 turns of 0.15 mm wire
around a core of 1" x 3/4" may do the trick nicely.
The grid current still has to flow through the driver triode CF,
and all the choke does is supply some DC
for the CF for idle purposes, and help in DV bias for the output tube.

Also -
I have some large SE OPT's laying about - can I use the primaries as cathode
chokes for experimentation purposes?

Yes, although their finite amount of low inductance, maybe only
20H, could cause a bother at 20 Hz, where the ZL is only 2.5kohms.
100H, or more would be nicer.

If used in this way, do I have to do
anything with the secondary leads, or just leave them open?

Leave them open, lest you load down the primary.

The CF should give you a flat response, despite the
transformer resonances, since the CF is a low impedance driver.

At least with the choke biasing the output tube, if anything goes wrong
with the CF tube, and it shorts to the B+, the choke will anchor the
output grid to whatever bias you set it for.

The SE OPT you wish to try may have a larger than required air gap
for use in a driver amp where the idle current is lower than when used
in an output stage, so maybe reducing the gap will give you a higher L.
But you won't be able to get from say 20H to 200H.
At LF, and high drive voltage swings, the choke will try to saturate.
The trick is to make sure the CF driver triode does not itself become victim
to grid current on peak positive swings into grid current of the output tube
grid.
Maybe a 6V6/EL84 in triode, or paralleled 6SN7,
maybe even a 2A3, might do as CF drivers.


thanks in advance,

Dave

Patrick Turner.

Altec 1570B

_-_-bear

David Forsyth wrote:

I was wondering if there were any examples of driving an output triode with
a cathode follower in which the cathode follower has a large inductor for a
cathode load instead of the usual resistor. I was thinking of using this
idea to drive an SV572 SE in Class A2 near zero bias such that I could
attach the cathode of the driver directly to the grid of the triode and
return the bottom end of the choke to a small adjustable negative supply.
That would save making a high-voltage negative supply and also cut out a
coupling cap. Then I could adjust the small negative supply so as to keep
the bias on the output tube at or perhaps just below zero volts. Assuming
this would work ok, would there be any different requirements for the choke
in this type of arrangement as opposed to the chokes designed for plate
loading of drivers? I know that good quality plate chokes of reasonably
high inductance can be pricey so I was wondering if one could get away with
something else in this application like a large power supply choke? Also -
I have some large SE OPT's laying about - can I use the primaries as cathode
chokes for experimentation purposes? If used in this way, do I have to do
anything with the secondary leads, or just leave them open?

thanks in advance,

Dave

--
_-_- BEAR Labs
- Custom Audio Equipment, Cables, Mods, Repairs -
http://www.bearlabs.com