[AMPS] SB1000 parasite question for Tom and Rich+

[measures]

>
>> Rich it seems to me that one way to reduce vhf gain would be not to use
>> tubes ( very expensive ones) that have max. ratings to 500 mhz  on
>> frequencies of 30 mhz or below. if it will amplify at 200 mhz it should
>> also oscillate.....yes??   HANK
>
>Hi Hank, 
>
>How do all those big fancy amplifiers with lighthouse tubes or big 
>tubes in cavities with extreme Q's not oscillate?
>
A fellow who works for Hughes assured me that they do.  Hughes builds a 
100kW HF amplifier that uses six antiparasitic devices.  

>Actually that is exactly backwards. The most stable HF amplifiers 
>use tubes that are good well up into VHF or UHF.
>
I know of an 8877 amp that has an incredible track record.  

>The LEAST stable amplifiers use tubes that marginally work at 30 
>MHz, like 811A's or 572B's.
>
I have tested around a dozen gold-sputtered 8877s.  This is a tube that 
you claim can not oscillate.  

>It's always been that way and will always be that way.
>
>It isn't difficult at all to check an amp for stability, almost anyone 
>can do it.

The method you previously described does not check for intermittent VHF 
parasites.  
>
>The real reason this nichrome thing has gone on so long is 
>because people want a single simple answer to any problem they 
>might have. 

Nichrome is not the only way to decrease VHF Q.  Add enough inductance to 
a copper-wire L-supp., and VHF Q can be lowered even more.  The trick is 
to keep from destructing R-supp. after L-supp is increased.   

>It makes anyone an instant expert, and they don't have 
>to understand a thing about tubes, oscillators, switches, radios, 
>relays or anything else.
>
>All they have to do is click their heels together and say...."there's 
>no problem but parasitics...there's no problem but parasitics..."
>
>Before you know it, your an expert on the sole cause of any failure 
>or abnormal operation....and you know the sole cure....a sprinkle of 
>something that doesn't hardly change the VHF Q.
>
You are the only person here who has claimed to be one of our recognized 
amplifier experts, Mr. Rauch.  (*QST*, 9-94, p.71)  // Sure, decreasing 
VHF 41% is no big deal.  This is why we supply glitch protection in our 
retrofit kits.  With the more resistive wire we now furnish in our 
suppressor retrofit kits, my guess is that VHF Q is c. 50% of that with a 
copper-wire suppressor.  note --  If anyone has access to an Impedance 
Analyzer, and would be willing to do some measurements, I would be happy 
to furnish a new low Q parasitic suppressor for analysis.  
>
Cheers

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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