Jon Yaeger wrote:

OK, I admit that my understanding of tube electronics is rudimentary at
best. But one thing really puzzles me.

On a Williamson UL PP amp, the screen is at about the same DC potential as
the anode (as measured with a DVM). I thought that the screen had to be at
a lower potential for normal PP operation.

The screens don't have to be at a lower than anode potential.

But with pentode/beam tetrode operation its best that Eg2 *is* no more than
the makers recomendations, so an 807 should never have more than Eg2 = 300v
regardless of the anode supply voltage.

But 807 were amoungst the first to be used for UL amps with 450v
applied as the anode supply.

With UL, the screen gets signal volatge applied from the anode circuit via the
OPT,
and the dissipation of heat in the screens is much reduced because of the
signal present.
With triode op of 807, the G2 diss is at its minimum.
With pentode op, when the anode voltage swings low, the G2 current peaks,
and when anode swings high, G2 current is reduced; the electrons seek the
most easy and attractive target.
But with UL or triode, the G2 voltage swings down with anode voltage,
so its current input is lower, and when anode swings high, the is still g2
current reduction.

Screen current input is about proportional to anode current input though.
So in class AB UL amps which are pushed hard with a load which is too low,
the screens are probably at some risk.

UL operation limits the B+ which can be used because of screen limitations.

A pair of 807 can be cajoled into making 80 watts in beam tet mode,
with 600 v anode supply but only 300v G2 supply.
EL34 can make 100 watts with 900v anode supply, but only 450 G2 supply.
Neither of these tubes can be used at the anode voltages for UL, unless
a special G2 winding is wound onto the OPT to give the wanted UL
signal voltage and supplied with an appropriate G2 supply.
This works OK with 6146 type tubes.

But if I was trying to
make a high voltage output stage I'd either stick to
beam T or pentode op, or use cathode feedback windings, which are more
effective
at reducing the thd and Ro of multigrid tubes, and you then have the
option of setti9ng g2 at the lowest efective value which will keep the tubes
happy,
and the screen current at its minimum, as well as reducing the wanted G1 bias
to a minimum.

EL34 and EL84 were designed to be able to run with equal anode and G2
voltage supplies, with 450v limit for EL34 and 350v limit for EL84,
but I have seen each used with 20% higher voltages for UL amps.

Screens are grids which draw current, and they run warm, and will run red hot
if their current is eccessive, and then start emitting electrons of deforming
in shape and alignment
and ruining the tube function.

Patrick Turner.



What's up with that?

TIA,

Jon