[Amps] New SB-200 owner

[jeremy-ca]

> Carl said:
>>Peter, just what would you consider a good economical tube if building 
>>from
> scratch for 6M? Ive not seen any REAL verified IMD numbers published for 
> any
> Russian tubes except maybe the 4CX800 and have hesitated in even trying 
> one.<
>
> Do you mean if buying a new tube, a used tube, or as I usually manage, 
> getting for free?


I mean brand new, never mind the source.


> It IS possible to get good IMD figures from tetrodes, but it does require 
> negative feedback. See Pappenfus.  4-125 and 4-250 can be good with NFB, 
> and they'll work FB at 50MHz..

Most homebrewers couldnt even pronounce Pappenfus much less follow along.


> I'd likely look at the 3-500Z, as a good workhorse.
> For real low cost, a pair of 4CX250  with lots of feedback and an 
> electronically regulated screen and bias supplies. Even better 4CX350J.

The stringent regulation is the rub. Ians kit isnt cheap either. I saw a 
partially built one sell on Ebay for $186 recently!

If I was to use any ceramic tetrode it would probably be the Svetlana 
4CX-400A as it has a well controlled grid and screen structure and decent 
claimed IMD. The price hasnt escalated like the 4CX800A either. Since it 
uses the 4CX250 socket that expense is pennies. I retrofitted them into a 
NCL-2000 for Svetlana when they first came out but didnt have the test setup 
then.


> Part of the economy is what's available for a plate supply, too. And are 
> you building one for you, or designing for production. There's a lot of 
> difference, as I have no doubt Tom can tell us.
> Finally, unless you have exactly the same layout etc, feedback can make 
> things change so much at 50MHz that you may not necessarily reproduce a 
> set of IMD figures.

Ive spent most of my career in RF R&D from tech to Sr Engineer. Coming from 
the grunt side of development since the early 60's Ive always had a strong 
sense of manufacturability and maintainability. Ive worked for engineers and 
had them work for me that could make anything work......once. Usually they 
were hams which always baffled me.

Carl
KM1H




> 73
> Peter G3RZP
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