On Wed, 20 May 1998 18:16:48 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>We seem to be getting multiple threads again.
>
>There are three, I think, and muddling them up causes
>misunderstanding.
>
>1. Are parasitic suppressors of nichrome more effective than those of
>copper?
>
>2. Do parasitics lead to high enough currents to bend filaments?
>
>3. Can parasitics boil gold off grids?
>
>
>I'd suggest that Rich has presented more than enough evidence to
>suggest
>that the answer to 3 is YES.
YES but as only one of the reasons...not the only.
>
>To 2, I am still totally unconvinced that you can get enough current
>from
>the space charge and the filament in 3-500Z to do this. So I'm a flash
>arc
>believer.
I suspect that Rich is the only true believer of that idea Peter. Put me
in the totally unconvinced column also.
>
>To 1, I'd say 'It all depends'.
>From a purely engineering viewpoint leading to a manufactured amp the
traditional approach may be the better way. At least it gives the
engineer something to do.
>From an independent service center or individual owners viewpoint the
aftermarket versions certainly appear to work, and come in several tube
type varieties.
>
>My idea of the parasitic problem is that the lead inductance
>(including that
>of the tune capacitor) resonates with the stray capacitance to give an
>anode
>tank circuit at VHF, which resonance is at some frequency where the
>feedback
>is enough to cause oscillation. Increasing the amount of stray
>inductance
>has been known to stop the parasitic by moving the resonant point of
>the
>'stray' tank circuit to the right point in relationship to the self
>neutralising frequency of the tube.
Funny you mention that. It appears that most participants in this
discussion fail to mention poor neutralization as a path to instability.
I wonder how many have actually checked the neutralization of their GG
amps?
I suspect that's why some
>amplifiers get
>away without a parasitic suppressor - until the tube is changed.
I recently sold a new Eimac YU-158 to an AL-1500 owner. He had parasitic
problems until he installed one of Rich's suppressors; that amp is
produced without a suppressor. He also reported it drove to 1500W out
with 45W of drive as compared to about 70W with a standard new 8877.
Dick Erhorn has previously spoken about the gain differences in the
3CX1500A7 series.
SNIP
>
>After all this, I don't disagree that a good suppressor can be made
>with
>nichrome tape.
A tape wound inductor is the way I designed my suppressors. I use .25 or
.5" diameter resistance strap depending on the power level. Key down on 6
or 10M for a full minute, power off and quick bleed the PS and the L and
R are barely warm to the touch. No wasted heat, no parasitics, no
hassles.
I do not like Rich's use of #18 nichrome all the way from the anode
connector to the blocking capacitor for much over a 811A size tube.
>By the way, has anybody experienced holes in the envelopes of 3-500Z
>caused
>by parasitics? I've seen it in other tubes, where a component close to
>the
>tube has enough capacity to allow a VHF current at the parasitic
>frequency
>to flow from the anode to the component through a localised area of
>the
>envelope: the dielectric losses are enough to heat the glass, and a
>pinhole
>appears. Seen it in 6146s and 572s.
I have had that happen to me with 572B's and sweep tubes Peter. The sweep
tube occurance was 30+ years ago and I dont recollect the details.
The 572B occurance was with a Clipperton L that I was experimenting with
plate choke designs to permit 12 and 17M operation. It only happened
twice and always to the same tube socket position and then only on 15M.
There was no suppressor damage and re-doing the plate choke cleared it
up. What was the cause....I cant believe parasitics ??
The Cetron and Chinese 572B; 6146's, etc all use soft glass. The
Svetlana 572 uses hardened glass as do all Eimac glass jugs so a melt
down would be less likely.
73 Carl KM1H
>
>
>73
>
>Peter G3RZP
>
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>
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