400 SB220 that's quite a record or was it one repaired 400 times?
R L Measures wrote:
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On Oct 23, 2006, at 12:07 AM, pentalab wrote:
> --- In ham_amplifiers@..., R L Measures wrote:
>>
>>>
>> RICH SEZ...W8JI has expressed disdain toward oscilloscopes and
>> dipmeters. His problem with oscilloscopes was when one measured
>> the worst-case tank circuit potential in a SB-220 at 3600V
>> instead of
>> the >9000V he predicted.
>
> ### agreed. Per Tom... if u underloaded an amp... or over drove
> it... or had a wide open /dead short on the output, tank V
> should soar to as much as 5 x normal. He also claims that V
> across the
> active part of the tank coil is 12 % higher than the V across the
C1
> tune cap.
112% of Tune-C pk V across the tank L makes some sense, but we are
talking about a different point in the circuit.
> I figured with my #88 switches rated at just 13 kv..
> and 6900 V underload on the tube... that peak V across the C1 cap
> would be a little less than 6900V. The tap being used on the
> coil is
> always at a 50 ohm point.... the unused taps [high bands] will be
> higher V. IF tank V doubled, I'd be in trbl... if it increased 5
> x, everything woulda blown to bits...... but doesn't. So what
> gives here ?
Tom was trying to blame occasional parasitic arcing on anything but
parasitics. The laugher is that following my last article on
parasites in *QST* (1990) he told me that over half of the 400
SB-220s he had repaired had signs of VHF parasitic arcing . . . but
apparently now they didn't.
"Oh what a tangled web we weave ... ...
...
>
>
>> Tom's problem with dipmeters was when one showed that grid-
>> resonance Decreases in frequency when a grid is grounded with
>> heavy duty Cu straps instead of caps (SB-220, TL-922. L4B)
>
> ### IT does decrease.... by just 1 mhz if I remember. What I
> don't understand is.... why does stability improve with grids
> strapped
> directly to chassis ??
It may and it may not. Moving one resonance a bit may appear to be a
sure cure, however, removing the perforated cover from a Henry 2K-4
will usually make it more stable but in my experiences parasitic
oscillations are inheriently on the ragged edge, so it's easy to be
deluded into thinking you made a slam-dunk by making one change and
seeing no fireworks during a few minute test.
Since one can do nothing about a tube's internal feedback-C,
IMO, the place to go after parasitic oscillations is where they
begin:
in the anode circuitry.
> That alone will solve 90+ % of
> parasitic problems.
I do not believe that parasite problems are solvable -- but as I
see it, they are somewhat controllable.
> On the remainder, use more turns.. and a
> globar.... or ur nichrome alternative.
Resistance-wire (nichrome) does no more than decrease a R/L
suppressor's VHF-Q without increasing dissipation in R-supp.
The combination of both resistance and inductance is very effective
in limiting parasitic oscillations to a negligible value of current.
- - F. E. Handy, W1BDI (1926 Handbook)
> I'd still grnd the
> grids directly.... just cuz the drive requirements drop by an easy
> 20-25
> watts.
That the 2 x 600pF of grid bypass on the pair of 3-500Zs in a 922, L4-
B or SB-220 dissipate 20 - 25w is hardly likely.
>
>
> later... Jim
VE7RF
>
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> Yahoo! Groups Links
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R L Measures, AG6K, 805.386.3734
r@..., , rlm@..., www.somis.org
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