[Amps] Grounded Grid(Screen) 4CX250B power gain

[Bill, W6WRT]

W6WRT comments interspersed below:



On Tue, 21 Jul 2009 06:15:46 -0700, "k7fm" <k7fm at teleport.com> wrote:

>
>I thought that somewhere on my amateur license, or in the rules, there was a 
>clause that you had to be able to build amplifiers using cheap tubes.  I 
>cannot find that language right at the moment.

I don't see an emoticon but I assume that was an attempt at humor. There never
was such a requirement. 




>  But, if an 8877 costs $700 

Brand new Chinese 8877s are $550, but your point is taken. I'm using one in my
homebrew amp and it has been 100% satisfactory so far. eBay item number
260425826378. Equivalent power level tetrodes, brand new, are in the same price
range.


>and you see a box of 4CX250 tubes sitting at a swap meet with 50 cents per 
>tube, that seems to be a good reason to think about using the 4CX250.

No argument there, especially if they are brand new. I have had nothing but bad
luck with "pulls". They are "pulled" for a reason - they are approaching the end
of their useful life. I am far from a wealthy individual, but I will only use
new tubes in any amp I intend to keep. If I am just fooling around, that might
be different.


>The reason to use them in some sort of grounded grid configuration, rather than 
>grounded cathode are numerous:
>
>1.  You can eliminate the necessary stabilized screen supply and bias 
>supply.
>2.  Grounded grid is more stable
>3.  Grounded grid is cleaner than grounded cathode (lower imd).  This is 
>relevant since the 4CX250 is not especially clean when operated grounded 
>cathode.
>4.  The extra drive power of grounded grid is easily handled by hams with 
>100 watt rigs.
>5/  Some hams like to experiment with unconventional ways of doing things, 
>especially if it saves lots of money.

Reasons 1-4 are reasons to use GG, not necessarily reasons to use tetrodes. I
would like to see some data on reason #3.

Reason 5 is a good one, no argument there either. 

So your arguments boil down to cost and availability. That is basically what I
said in my original post - "If you have one lying around". 

73, Bill W6WRT