Bill Turner wrote:
> 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.
Part of the problem has been carbon comp resistors which were (still
are?) widely used. They are unsuited to the application and there have
been better alternatives for at least 30 years.
>
> 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.
Being pedantic, it doesn't require any tuned circuits at all. The
condition is gain>1 and phase=0. How you avoid this doesn't matter. How
it arises can vary wildly, with very different optimum cures. What goes
on with circulating currents and stray fields outside the tube can be
more significant than internal feedback. A resonance gives you rapid
phase changes over a narrow frequency, which increases the likelyhood of
getting the right phase for oscillation, but it's not a prerequisite.
>
> 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.
It might be gain reduction rather than damping resonance that's working
here. The capacitor from cathode to grid acts as a divider with the
plate to cathode feedback C. For tubes with VHF grid structures, I'd
guess that a low inductance 100pf grid to cathode would go a long way to
killing all vhf parasitics in a hf amp.
>
> 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.
Your dip meter tells you there's a resonance. It doesn't tell you if
it's series or parallel, which matters, it doesn't tell you it's the
frequency of oscillation, and it doesn't tell you the plate impedance at
that frequency. Don't get me wrong - plate suppressors do help stop
oscillation, but copying something used in a another design using the
same tube without considering differences in all the external circuitry
and/or targetting something picked up by a dip meter means that success
might be pure luck. How many amplifiers, like yours, don't need
suppressors but have never been run without one?
Unfortunately few have access to equipment which allows them to
investigate why an oscillating amplifier does so. Common factors in some
tubes lead to the 'I did this to fix it' folklore, which is fine - but
then it can start being seen as a universal cure, when each situation
and/or tube type needs individual consideration.
Steve
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