KM1H said:
"The way I was taught is that you key the amp with no drive and vary the
caps all around and look
for a kick in the plate or grid current. Do this on all bands and into a
dummy load. If all is quiet then hook up the antennas and do it all over
again."
ADDITIONAL COMMENT:
This works for a class A, AB1, AB2, and B amplifier where there is some
iding current. In cutoff operation, such as class C, where the RF drive
swings into conduction, you have to move the bias up (more positive grid V
on a grounded cathode tube) to begin to draw current. Then contine as Carl
said.
Class C amps will usually sit there doing nothing, not even peep a
parasitic, until some noise is introduced, such as the electron beam, or RF
drive. Also, the gain is diminshed in these circuits, and it takes a bigger
grid voltage to drive the tube into saturation.
"Arcs during tuning or operation are purely a result of high voltages in
the tank circuit, poor tank circuit design/components or another
oscillation ( NOT a parasitic in the tube) caused by stray LC in the
tank ONLY. These arcs cause switch and capacitor damage but rarely, if
ever will cause tube or grid choke damage."
COMMENT:
Big tubes do introduce internal parasitic circuits, such as the
circumferential modes between the screen to plate path radially inside the
tube. In these, the tube is indeed the culprit. Even coaxial cable has
these modes, which are normally in the GHz, beyond which the coax exhibits
suck outs and losses. I think the present "Wireless" generation of
engineers have to relearn this fact.
But in a power tube they are problematic due to being in the beam itself.
But tubes the size of your fist or even your cat usually have these modes
so far up in frequency, that they are beyond where the tube can support it
as an oscillator, due to transit time. Larger tubes have these in the UHF
ranges, where they still can amplify it. The solutions are not as
straightforward as installing a parasitic network in series with the anode
or grid. They are usually solved with various ferrite or resistive
materials placed in locations around the tube, the socket, or the
enclosure.
I just wanted to add these conditions to the general comment. It depends on
the size of the tube, besides the plate inductance, tank cap value, tube
cap value, and all of the normal stuff that can cause parasitics.
It was nice to hear Frederick Terman's quote about it. Right on.
John
K5PRO
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