[AMPS] B+resistors

km1h @ juno.com km1h@juno.com
Fri, 10 Oct 1997 09:54:51 EDT

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


>>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 
>>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 
>>PS.   In reality, and depending upon the layout, it can actually 
>>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 
>glitch resistor. 

In the SB-220 that 1000pf bypass cap is highly reactive at VHF. It is not
RF rated. However replacing with a 1000pf disc ceramic with very short
leads will do the job. 
As I tried to allude to earlier, not all VHF energy flows thru the
primary RFC path in that amp due to the physical layout.  
>>It also does not use a surge resistor. The same holds true with the 
>>A 5-10 Ohm 10-15W wirewound resistor will usually measure 6-10 uh 
>>is a good value to suppress VHF crud. If that is not enough R for 
>>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 
>>them . 
>However, 6.3 ohms is likely to allow more than 200 peak amps - 
>on the ESR in the filter capacitors.  

That is assuming a catastrophic failure and dead short. A momentary arc
may not have all of that  energy. 
Anyway, I said I use them since I have a few hundred....not that they
were an ideal value. At least they appear to do the job in 3-500Z and
572B amps.

>>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 
>>ceramic tetrodes probably are better off with more current limiting 


>>> 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.

Wrong choice of words. I should have said excessive key down tuning on
10M when the suppressor design allows a bit too much 10M RF to be
In the Clipperton L it is even worse, the manual even warns about this. 

73  Carl   KM1H

>R. L. Measures, 805-386-3734, AG6K   
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