[AMPS] B+resistors

Rich Measures measures@vc.net
Fri, 10 Oct 97 00:22:28 -0800

>On Thu, 9 Oct 97 10:24:41 -0800 Rich Measures <measures@vc.net> 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 
glitch resistor.  

>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
>them . 

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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