>
>On Sat, 6 Sep 1997 00:44:51 +0000 w8jitom@postoffice.worldnet.att.net
>writes:
>>> From: km1h@juno.com
>>
>> Directly grounding the grids in an otherwise unmodified 220 is
>>asking for fireworks and possible tube destruction. I also suspect that
those
>>stock parasitic suppressors are junk by now.
>>
>>Nonsense Carl.
>
>I guess we are both entitled to our opinions Tom.
However, Tom's opinion carries the weight of 'expert' status, Carl.
>Anyway I never said to install a surge resistor in the grid, that would
>be a mistake since a true surge resistor should absorb the energy and not
>act as a fuse.
good point. However, not being a self-recognized 'expert' gives you the
luxury of being free to fess up to mistakes, Carl.
>What I did say was that the stock choke acts as a surge suppressor due to
>its 25 Ohms resistance. That was probably a wrong choice of words for
>this group. Due to the wire size the absorbing ability is very short
>before it opens.
OTOH, the choke's apparent fusing current of c. 15a does not appear to be
able to prevent the filament helix from being bent out of line during an
event, which apparently destroys most 3-500Zs.
>
>I STILL prefer the idea of the choke acting as a fuse in the absense of a
>true protection circuit.
>
>I know of at least one 2x 3-500Z amp made down in the land of cotton that
>directly grounds the grids. When the tube arcs, all sorts of expensive
>damage results.
>
In order for a tube to arc, the envelope needs to contain gas. I do not
subscribe to the Rauchian theory of disappearing gas. If a 'big bang'
was related to a gas discharge within the envelope, gas would be detected
with a high pot test at 8+kV. From my experiences, 90+% of kaput 3-500Zs
have normal vacuums.
>The SB-220 and similar designs generally survive an arc with the tube
>intact and require replacement of a few inexpensive parts. ( Assuming the
>220 multimeter has back to back protection diodes or was not in the Grid
>position when the bang occurred)
>
However, if one measures the grid to filament voltage-withstanding
ability in a SB-220's tubes after an event, one typically discovers that
the filament helix is somewhat bent, although perhaps not bent enough to
produce a short during operation.
>>The last thing you want, if the tube faults, is to have the grid fly
>>up to 3000 volts.
Flying up is somewhat unlikely, Mr. Rauch. One or more of the six grid
to ground, 200pF grid caps typically shorts during these events. Perhaps
the tube did not fault. If the tube subsequently exhibits a good vacuum,
and the VHF suppressor resistor has been damaged, in my opinion, a VHF
oscillation condition is indicated.
>>If the grid, because of series resistance, reaches anode voltage
>>levels during a fault... the next stop for the arc is the filament
>>and everything connected to the filament including the exciter.
>
>I have never heard of that happening in any 3-500Z amp that uses grid
>chokes! Your comment may be technically correct but a real stretch IMO.
>Maybe we should ask Rich what the Grid- Filament breakdown voltage
>is.....preferably with a non bent filament.
Typically, it is between 7000v and 9000v with a new Eimac tube. The best
we have been able to achieve in our tube-centrifuge is 8000v. This tube
that had roughly 500v of grid/filament withstanding ability when we
received it.
>If your statement were true Tom, then the thousands of Heath, Kenwood,
>Drake amp owners have lost thousands of rigs and kept silent about
>it.......
>
>>A HV series resistance is the way to go. Sticking a surge resistor
>>in the grid path is unwise, to say the least. Anyone doubting that
>>should ask Eimac or Svetlana.
>
I don't see any mention of this subject in either's literature, Mr.
Rauch. It seems to me that anything which can interrupt the flow of
15-plus amps of grid current would be good. Since transistor-switch
interruption reportedly often results in a C/E shorted transistor, which
then provides zero protection, something less elegant just might provide
better protection, even though it lacks 'expert' approval. .
- BTW, I'm somewhat eagerly waiting for you to publish the Eimac
Technical Bulletin numbers on the topic of VHF parasitic-oscillation
suppression - or perhaps they just don't exist, like the official formula
you apparently fabricated for calculating grid dissipation. - - - And
I'm still waiting for you to publish the letter which proves that the
person who Eimac's personnel dept. said was an 'Engineer B' was instead
the 'R+D Engineering Manager'.
- Does anyone know what 'R+D' is?. I've heard of R & D, but not the
plus variety.
Rich---
R. L. Measures, 805-386-3734, AG6K
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