On Jan 27, 2005, at 10:52 AM, David G4FTC wrote:
Barrie,
I'm the builder of the 6 metre 8171 amp referenced in Steve G8GSQ's
posting.
Although the output capacitance of 23pF may be considered high for a
tube
operating at VHF, when I built my 8171 amp I didn't find this to be
the main
problem - in fact I used a very conventional PI output but with
flappers for
both tuning and loading simply as an experiment to see if it was
possible to
build a "tubes with handles" amp without needing to resort to vacuum
variables. The flappers really do work fine.
I built my amplifier in a grounded cathode configuration and I didn't
fully
appreciate the losses that I would find with a tube having a 120pF
input
capacitance.
Hello, David -- What is the grid termination's resistance?
Is this amplifier neutralized?
Consequently the drive requirements were over and above what I
had expected that I would need. IMHO it is the input capacitance which
will
cause you many more problems than the output capacitance.
The SK300A tube base isn't really ideal at VHF, and the tube
manufacturers
recommend the SK360 for frequencies above 30MHz. I understand the SK360
allows better RF grounding for the filament with a lower inherent
inductance
at VHF. However, these are expensive bases compared to the SK300A.
An alternative to the SK300A may be the SK300. The SK300A was
developed from
the SK300 to provide improved cooling to the tube but this resulted in
cutting out the "skirt" from around the base. This increases the
inductance
and makes providing an effective RF ground somewhat more difficult,
I have used both and I can not see why the SK300 should have less
inductance than a SK-300A.
and
results in less that optimum gain. However even with the SK300A base
which
I used, I'm still getting 16dB of gain at 50MHz which in reality isn't
too
bad.
How much anode-V and screen-V?
Currently I'm researching building a new amp for 2 metres, and at this
frequency I feel that the 120pF input capacity for a grid driven
configuration would be a real show stopper.
In days of yore, the 8170 and 8171 were used in high-band VHF TV
transmitters up to 216MHz.
However, the input capacitance
of a 8170 in grounded grid drops to about 56pF - not much more that
the 42pF
of the 8877, and I really feel that this is the way forward. However
to get
really low inductance grounds for the screen and grid will require a
certain
amount of metalwork where I will remove the collets from a SK300A,
discard
the frame, and mount the collects on individual plates. These plates
will be
chassis mounted with PTFE insulation to provide RF decoupling and also
the
DC feeds for the screen/grid biases.
A less inductive way to go is: direct-ground the screen with 8 Cu
straps, ground the screen PS-pos., connect the screen PS-neg and the
anode-PS neg. to the fil CT (cathode). With this configuration, the
anode-PS/cathode neg./bias-PS+ floats at the screen-potential above
ground.
•• photo showing the 8 screen-grounding straps on an SK-300:
http://www.somis.org/pb_grid.gif
...
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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