On Thu, 9 Oct 97 10:24:41 -0800 Rich Measures <measures@vc.net> writes:
>>Rich,
>>
>>>Special, high peak energy-absorbing resistors are best,
>>>although the cost is high.
>>
>>Have you any comments about using big solid carbon or metal film
>>resistors - things like 50ohm, 50 watt metal film tubular resistors
>NOT
>>spirally cut, as are/were made for dummy loads?
>
>Spiral-cutting would reduce the voltage-gradient in the resistive
>material.
Rich, Peter, et al:
I prefer the standard "Brown Devil" style of resistor. Here is why.
The resistance wire is made from that N Word stuff. Along with the
current limiting which is the primary purpose it also exhibits inductance
which can be a benefit.
Many commercial and homebrew amps use a secondary VHF choke in the B+
lead on the assumption that it helps stop VHF energy from entering the
PS.
In reality, and depending upon the layout, it can actually cause problems
by becoming part of a tuned circuit and enhance instability.
The SB-220 is a good example since RFC-2 is a parasitic enhancer IMO. It
also does not use a surge resistor. The same holds true with the TL-922.
A 5-10 Ohm 10-15W wirewound resistor will usually measure 6-10 uh which
is a good value to suppress VHF crud. If that is not enough R for surge
use then Rich's comments about adding more R in series is a good one.
Also the winding spacing on low value resistors minimizes internal
flashovers. Add a 500 -1000 pf bypass at the cold end of the first
resistor.
I use a 6.3 Ohm 15W Dale WW in almost all amps since I have a bunch of
them . They also measure the same L as a Z-50 choke. IMO, that is enough
R for 3-500 and similar robust tubes. Fragile tubes such as the 8877 and
ceramic tetrodes probably are better off with more current limiting R.
>
>IMO, the glitch resistor IS important because it limits peak current
>during an intermittent oscillation condition. I believe that limiting
>
>peak current during such an event can prevent the aforementioned types
>of
>grid/filament failure.
Agreed...as long as you dont say all oscillations are from parasitics!
>
>> ... as has been
>>suggested, because arcs per se don't occur. Unless of course, one
>>postualtes that the VHF parasitic gets going, and the voltage swing
>>achieved at the plate is enough to initiate an arc, leading to all
>the
>>results..........I dunno
>>
>To be sure, it's a 'whodunit'. It seems likely to me that the arc is
>external, and it occurs after the current pulse which apparently
>damages
>the tube during a VHF oscillation condition.
I suspect that many arcs ARE external and caused by the actions of the
output tank. The Pi-L of early Alphas which was partially copied into
Ameritrons is an example of potential instability problems. What looks
good on paper can become a disaster when some knob twiddling ham is let
loose.
How else could the VHF
>parasitic-oscillation resistor be damaged?
Too much mistuning on 10M will easily do that. An improperly selected
or sized suppressor will have the same effect.
73 Carl KM1H
>Rich---
>
>R. L. Measures, 805-386-3734, AG6K
>
>
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