In my experiences, parasitics are iffy because not all tubes of the same
type have the same vhf-gain. The 2 x 3-500Z amplifier [a friend's
factory-stock TL-922] that produced the toasted bandswitch on p.33 of
October, 1990/*QST* [Figure 23 on my Web site] had a pair of 3-500Zs that
were from downtown Squirrelsville. They would oscillate at the drop of
hat at c. 119MHz. They also required 15 fewer watts drive at 7MHz than
other 3-500Zs.
-
-- A caveat: One of the most common misconcepts about vhf instability
is that it can be tested for in the same way that one tests an amplifier
for instability at the (hf) operating frequency -- i.e., by adjusting the
tank caps throughout their ranges with no drive and watching for the
presence of grid-current. This method is specious because the vhf
circuit that can sometimes support vhf oscillation is determined mainly
by the anode-C [typically 5 - 10 pF] and the inductance [typically 50 -
150 nH] of the conductors between the anode and the Tune-C. Nothing can
be done to substantially change this resonance other than adding R to
decrease the vhf ringing V produced whenever anode-I changes. The
trade-off of adding R is c. 2% less P at 29MHz. Note - the
aforementioned 922 would sometimes take off as the amplifier was unkeyed
-- i.e., as ZSAC dropped abruptly from 160dcmA to 0mA.
>Hi John,
>-
>Regarding the "no parasitic suppressor" portion of
>your story below...
>-
>I would think the potential for RF amplifier parasitics
>are from quite the mixed bag of sources. Each chassis
>layout and potentially each varied tube combination
>as an example...
>-
>As some of the reported VHF unstable SB-220's
>were tamed with "updated suppressors", some
>same circuit, different tube - amp Combinations
>were for the most part, always stable with the
>same basic physical layout.
>-
>I've found what I belive to be unofficial similar results
>with SB-220 and other amplifiers. My BTI LK's
>included. One LK wants to sing like a dog while
>the other has always been pretty much rock
>solid stable.
>-
>The point is you might have just gotten lucky not
>including VHF suppressors. Although you had
>good results, could you be sure there would be
>no surprises in various tube circuit combinations..?
>-
>I would expect the only real proof of a stable circuit
>would be a track record over time. I would also
>consider the addition of suppressors a practical
>addition and some measure (no pun intended)
>of prevention. Pictures of damaged tubes make
>me nervous.
>-
>cheers
>skipp
>-
>-
>From: "John T. M. Lyles" <jtml@lanl.gov>
>
>I got the RFPP HFS1000G converted to 5 MHz a week back, at work. It
>was at ISM freq 13.56 MHz. Now it is driving a dedicated experiment,
>easily providing a kW output. Efficiency is about 69% not counting
>drive feedthru. The 8877/3CX1500A7 takes plenty of abuse without even
>flinching. It self biases via cathode resistor only, at about 200 mA
>idling. Dumping about 600 watts in the anode in this case. With drive
>it is better.
>
>Changed the input to a simple pi, low Q, with no tuning needed.
>Output L was replaced with 3/16 refrig tubing coil, 12 turns, about 7
>uH. Added 150 pF padding with 15 kV ceramic doorknob caps, one NPO
>and one N750, for the C-tune. C-load padded with a stack of heavy
>duty surplus Erie RF chip caps at 670 pF. Used Hp impedance meter
>looking at the anode, with a 50 ohm load on the output. Set the
>controls for 2500 ohm loadline at the tube, and then it came on first
>time, hardly needed any tuning. By the way, the 8877 doesn't have
>parasitic suppressor. Its very stable - no surprises. Using an ENI
>240L 40 watt solid state broadband amp as the driver for the 8877.
>
>73
>John
>K5PRO
>
>
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- Rich..., 805.386.3734, www.vcnet.com/measures.
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