Anode choke is about 1.3mH, but its not mounted away, its pretty
close, about 2 inches, although its in the opposite plane to the tank
coil. Is that OK?

As for driving with a fixed generator first, it would be a good thing
to try I must admit - but its a big effort. I have no geny that will
go to 25MHz and to make one capable of providing the 20 watts or so I
would need would put me considerably back - I know shooting straight
for the goal in a case like this is harder of course, but seeing as I
have the thing running already at low power, I am hoping still...

As for the 'two grids' this seems to be bad documenting (the original
russian docs I think). There is definatly one grid - the tube has a
single grid between a pair of connections for the heater, one of which
is commoned with the cathode, and also a plate connection of course
(the heatsink). The docs showing 2 grids confused me but I assume that
its just that they couldent be bothered to redraw the picture! As I
say, theres simply not enough connections... Unless theres something I
dont know about here?

Re is it buzzing at UHF and I simply cant see it I did wonder that
too. But I guessed that there would probably be signs, like more heat
in certain caps, and corona and such. But I cant discount it. I guess
I could test this with a filament lamp indicator or similar.

And as for ARVs concerns, quite right to inquire. The test design is
currently as screened as is practical, mostly boxed in an aluminium
box, but the base is off, which is where the grid circuitry is. I
think that at this level, there is negligable radiation from this side
of the circuit, but at the desired power, maybe there would be some.
However the base will be on as soon as I have it working properly, as
much for the sensitive electronics I have in the workshop as for
reduction of spurious emissions. The several computers I have have
demonstrated their sensitivity to RF at a low power level by all
deciding to wipe their hard disks when I used a new HF striking TIG
welder a while ago - a sad incident as I am sure anyone who has gone
through the same thing will agree.

When in final use, the output will be capacitvley coupled to a quartz
glass tube that passes through the aluminium case. Gasses at low
pressure will pass through this, be excited by the discharge and will
glow, and this glow is viewed through a wire mesh screen for
spectrographic uses.

I may need to add a second outer case depending - I am unsure yet
whether induced currents in the case will reradiate a significant
amount of RF or not, so we will have to wait and see.

As you say, one option would be to fix the frequency and then tune
with a cap - the main reason I dident shoot straight for that is that
in the UK you cant radiate anything at all, so irrespective of my
chosen frequency, I have to screen to a high degree of attenuation.
And now that I am begining to think that I need to include a variable
cap to tune the grid circuit to the tank (this is my latest hypothesis
why I am not getting the power I need), there seems to be no
disadvantage to fixing the frequency. So we will see.

Could anyone comment on that idea by the way? That perhaps, the
capacitance between the grid and plate or cathode is resonating with
the feedback coil, but is tuned to a different frequency to the main
tank, and so limiting my drive? I did wonder about changing the Q of
the tank to make the amp more broadband, which I think would reduce
the impact of such a difference in frequency. Also I could try adding
a tuning cap to the grid coil to tune it to the tank coil maybe.



On Jul 26, 7:58 pm, MarkAren wrote:
The APC is intended to be resistive (damps Q) at high freq and very
low reactance at operation freq. I should have mentioned that the 4R7
should be a carbon resistor (not wire wound).

I hope that the anode isolation inductor is mounted away, so can't
form part of the tuned circuit... What value is the inductor you are
using ?

Just for getting the anode tuning sorted out, have you considered
driving the grid with a crystal oscillator source and a suitable
buffer - this splits the problem in two. A few volts of drive should
be a good start, maybe a CMOS oscillator driving some paralleled
74AC04 buffers, that will give you 5V P-P drive into a 1:2 torridal
transformer, 10V P-P may be useful dive ?