For years VHF parasitic suppression has been done in the anode
circuit of an amplifier. While it does work, I wonder if it might be
better to apply suppression measures in the grid circuit, especially
with a grounded-grid triode.
One of the chronic problems with anode suppression is getting the VHF
impedance high enough, but not so high that the suppression resistors
overheat when operated on 28MHz. This is a delicate balance and one
that not many amplifiers - even expensive commercial ones - get
right. Many of us have found commercial amps with the suppressor
resistors seriously overheated, having their values changed and in
some cases, burned open, making them totally ineffective.
Since it takes two resonant circuits for a g-g triode to oscillate,
one in the plate and one in the grid-cathode, I'm wondering why there
hasn't been more focus on the grid side? If the resonance in the grid
side could be dampened out sufficiently, the anode side could be left
alone with no suppressor at all. In fact, some amps such as the
Ameritron AL-1500 already take this approach. The power level in the
grid circuit is much less than in the anode circuit and components
will be stressed much less.
I recently built an 8877 amp using one of AG6K's parasitic suppressor
kits. While it worked ok on 28MHz SSB and CW, when using constant
carrier RTTY, the resistors burned up within about 4-5 minutes of
steady operation. One of the resistors got so hot it unsoldered
itself, cracked in two and fell down on top of the tube! Since I
operate almost exclusively RTTY, something obviously needed to be done.
Thinking about the AL-1500, which uses the same tube, I decided to
try my amp with no anode suppressor at all. Gingerly, I fired it up,
expecting massive fireworks, arcing and strange noises. Guess what?
Nothing. It worked perfectly. I ran the plate tune cap through it's
range, ran the input tune circuit through it's range, tried
everything I could to make it oscillate but it would not. Neat!
The only parasitic suppressor the amp has is the 33 pF in series with
a ten ohm resistor, connected from grid to cathode, which Rich
recommends. Since the un-suppressed anode circuit grid-dips at about
145 MHz, the 33 pF has a reactance at that frequency of about 33
ohms. That value in series with a ten ohm resistor seems to be enough
to dampen any resonance in the grid circuit. At least, I can't find
any VHF resonances in the grid area with a grid-dip meter.
There are probably other methods which could be used in the grid
circuit. For example, a 145 MHz series resonant circuit with a Q of
3-4 in series with a ten or twenty ohm resistor from grid to cathode
might work too.
Anyway, the point of this post is to stimulate some discussion on
eliminating the anode suppressor circuit. All comments and/or
experiences are welcome.
73, Bill W6WRT
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