To: <amps@contesting.com>
> Date: Thu, 14 May 1998 09:13:27 -0500
> From: Jon Ogden <jono@webspun.com>
> Subject: Re: [AMPS] Re: Parasitics
> To: w8ji.tom@mcione.com, amps@contesting.com
> Tom,
>
> How can a series resistance of 100 Ohms or so be greater than the K-Ohm
> impedance of the anode circuit?
I assume you mean the time varying resistance of the anode as
modified by stray reactances.
The tubes anode system, so far as that time-varying resistance, does
not come into play. We are only looking at a termination for that
system, so source impedance does not concern us greatly.
The requirement is, if you could stand at the anode and look outward,
that you would "see" a load resistance that is primarily resistive
and dominates the lossless reactive part of the load. That guarantees
controlled phase shift, and the high resistance reduces VHF gain (the
mechanism for gain reduction is the shunting anode capacitance).
There are two mechanisms at work.
One is the reduction of Q because the resistive part of the anode
load impedance external to the tube dominates, and resonance (and
uncontrolled phase rotation with frequency) is reduced. To meet
this criteria the system, looking from the anode outwards, must
look like a resistance is a major portion of the load impedance.
The second criteria is the resistance must be large enough
that the shunting capacitance of the anode can reduce high
frequency gain.
There are other methods ( such as moving anode and grid resonances
apart, that was the method selected in the AL-1500 which is
unconditionally stable at VHF with NO suppression. I also use that
method in multi-kw PA's that operate at HF and VHF). But I was
speaking only in the case of PA's that can not be easily stabilized
by lossless methods.
The case of the system requiring a tuned or inductor loaded low Q
suppressor system applies mostly to tubes with
self-neutralizing frequencies near or below the operating frequency,
or tubes with poor grid design.
811A's, 572B's, 3CX1200A and D7's, and 3 and 4 -1000's fall in this
category. The 833A is the flagship of unstable tubes, as well as old
QRO bottles from the 30's and 40's (when they HAD to use resistive
leads and lower the Q at HF as well as VHF).
> The conventional concept of a supressor is that the coil chokes the VHF
> signal and the resistor then absorbs it. However, if you think about
> impedances and such, that doesn't make sense.
No, it makes absolute sense. The resistor dominates the system at the
frequency of oscillation, because inductor impedance is high. It's
tough to build a TPTG or TPTK oscillator when the anode tank is a
resistance.
If the resistor "absorbs" anything the system is less than ideal.
It's function is to load the system, not absorb energy. Sometime we
can't help having it absorb a little "good" power, but it certainly
doesn't have to do that.
No more posts til Tuesday...
73, Tom W8JI
w8ji.tom@MCIONE.com
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