Fri, 10 Oct 97 00:22:28 -0800
>On Thu, 9 Oct 97 10:24:41 -0800 Rich Measures <firstname.lastname@example.org> writes:
>>>>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
>>>spirally cut, as are/were made for dummy loads?
>>Spiral-cutting would reduce the voltage-gradient in the resistive
>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
>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.
IF a 1000pF capacitor bypasses RF from the anode circuit, it seems
unlikely that much VHF energy could get past that bypass and invade the
>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
>I use a 6.3 Ohm 15W Dale WW in almost all amps since I have a bunch of
However, 6.3 ohms is likely to allow more than 200 peak amps - depending
on the ESR in the filter capacitors.
>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!
If an HF amplifier oscillates at VHF, the oscillation is parasitic by
>>> ... 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
>> How else could the VHF
>>parasitic-oscillation resistor be damaged?
>Too much mistuning on 10M will easily do that.
The 10m RF voltage drop across the VHF suppressor is a function of the
10m current flowing from the tank to the anode C of the tube. I don't
see how mistuning could easily increase the 10m circulating current
through the Rs/Ls VHF suppressor.
R. L. Measures, 805-386-3734, AG6K
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