On Aug 27, 2006, at 9:47 AM, Harold Mandel wrote:
> Here is what I understand from the consensus of opinion:
>
> 1. VHF Parasitics may develop sufficient amperage
> such that filaments may physically distort.
For tubes with no midpoint filament support, I concur, however, in
most cases the HV-PS can not deliver enough Joules during a single
event to bend the filament helices of a 3-500Z far enough out of
vertical to cause a filament to grid short. - note - the Henry 3K-
A's 2500W CCS HV-PS is an exception.
>
> 2. Some sort of oscillation can happen as a result
> of mis-tuning, etc., that may be called "parasitic."
Mistuning apparently does not cause instability. The circuitry that
generates the VHF signal that starts the oscillation comes before the
tank circuit. This source is the series resonant circuit that
consists of the anode-C's X (reactance) to gnd, the anode-lead's X,
the VHF suppressor's X, the DC-blocker's X, and C-Tune's XC to gnd.
Since the X of the anode-C is many times higher than the X of C-Tune,
adjusting C-Tune from max to min typically changes the resonant freq.
only a few %. Whenever anode current changes quickly, said resonant
circuit rings and generates a damped wave signal. In a TL-922 this
freq. is c. 120MHz. Since the V that is developed across a tuned
circuit is proportional to Q, decreasing the Q of the VHF suppressor
decreases the amplitude of the damped wave, and increasing the Q of
the VHF suppressor increases the ringing V. Thus, when designing a
VHF oscillator, one must be very careful that the Q of the tuned
circuit is high enough to generate sufficient ringing V at turn-on to
reliably start oscillation.
>
> 3. The Q of the Lsup device may or may not dampen
> oscillations or events that may cause filament sag.\
L-supp in combination with R-supp serves to reduce VHF Q and lessen
the amplitude of the VHF ringing V. L-supp and R-supp also lowers
the parallel-equivalent resistance (Rp) presented to the anode at the
anode-circuit's VHF self-resonance - thereby decreasing VHF gain.
>
> 4. Some 8877 amps have used Lsup devices, some
> have not.
AL-1500 amplifiers do not use any R-supp/L-supp VHF suppressor even
though a XC=5100-ohms feedback path exists within the 8877 at it max
freq. rating. I know of no other 8877 amplifier that does not use a
R/L VHF suppressor.
>
> 5. Some commercial manufacturers use Lsup
> material(s) that are higher resistance than
> silver strapping with no attempt at an added
> series inductance.
>
> Why then, is there anyone using Lsup circuits, and
> conversely, why is there anyone not using Lsup's?
>
> I can understand how a tube like a 3-500Z or 4-1000A
> can experience filament sag with huge amounts of
> inrush current. Is the energy conveyed during a
> "grid excursion" enough to distort a filament?
From what I've seen, it is.
> (Let's say we feed the RF drive through the center tap
> of the filament xfmr)
too much L, Hal
>
> With the amount of energy required to deform a filament
> where does the power come from, the filament supply?
No, the stored energy in the HV-PS filter. Adding a sufficient glitch-
R usually ends grid-fil shorts.
>
> Again, respectfully,
>
> Hal
> W4HBM
>
> [snip]
>> Parasitic
>> oscillation MAY be the cause for SOME grid-to-filament shorts.
> [snip]
>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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