>Hello All
>
>The TL922 manual says "DO NOT under any circumstance OPERATE your exciter in
>CW mode when the linear is in the SSB mode. Failure to head this warning may
>cause immediate, extensive damage to the amplifier""
>
>"REPEAT: DO NOT OPERATE CW IN THE SSB MODE"
>
>I always wondered about this comment. The amp runs in CW mode at about
>1700V and about 2700V on SSB (Under load). On CW, the amp barely puts out
>as much power as my single tube AL80A. The statement above is remarkably
>emphatic but I wonder how accurate it really is when the SSB tune up
>procedure allows key down in SSB mode.
The 922's HV-xfmr is ok with voice mode on the SSB tap and CW mode on the
CW tap, however, since key-down/A? mode has about triple the current
draw, the I^2 x R Cu-loss in the windings is substantial. Thus V-sag is
also substantial - which changes RL. To get an accurate SSB-tuneup with
any 1500W-out linear amplifier that does not have a 2500W PS, tune it
with 50wpm dits or a tuning pulser.
>
>My question is what is the weak link that prevents operating CW at the
>higher voltage and is there a mod to get around this?
The mod: A higher-speed temp-controlled DC-brushless cooling fan and a
Peter Dahl Hipersil? HV transformer. The difference at the Rx-end would
be about a quarter of one S-unit.
>
>As long as I am asking questions, the AL80A manual says to tune up to 550 mA
>of Ip and then back off to 400. I think the 3-500Z max Ip is 400 mA.
> Is it really OK to drive it at 550 mA?
The 3-500Z is rated at 400mA. The "expert" who came up with the "550Ma"
rating assured us that he had a letter from Eimac authorizing 550mA and
that he would post the letter for all to see. It has been several years,
and we are still waiting. Maybe he's been too busy?
It can't hurt the 3-500 due to its thoriated tungsten cathode, however,
tuning at 550mA is a pretty good idea because it results in a mistune --
i.e., less efficiency -- when the drive is backed off to 400mA without
retuning. The benefits are: since less efficiency means less RF-out and
that means less QRM on the Ham bands, And, as an added benefit, in the
winter months, the extra heat in the radio-room feels mighty nice.
>If so, is the amplifier still linear at
>this level?
The characteristic curves indicate that the 3-500z rapidly looses
linearity above 420mA.
>Is 400 mA the destructive limit or is it the point at which
>tube aging will accelerate?
Neither. Aging is due to filament running time and filament temperature.
>Also, in a grounded grid configuration like
>the AL80, at what current would linearity be lost? Is there a reference
>with the appropriate curves that I can use to answer this question?
>
The curves to look at are peak anode current, especially around lower
anode voltages. The more bend bend in the curve, the more loss of
linearity. [note: the relationship between peak anode current and
average anode current is roughly 3 to1. Thus, the 1.2A-pk curve
corresponds to c. 400Ma avg.
>Thanks in advance
>
ur welcome,Tom W2SC
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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