Bob Duckworth wrote:
>
>What is max screen voltage for a 4cx250??
>Eimac doesn't have a value for screen voltage under
>max ratings.
>As a refresher, I'm wondering if there are an el-cheapo tetrodes that will
>work similar to 6F6 audio amp with grid driven and HV applied to
>screen. Screen and plate tied together with the tube side of the output
>transformer.
Oh... that's what you were asking...
No, that certainly doesn't work with any RF tetrode I've ever heard of.
In these tetrodes, the screen is generally designed to run at about 20%
of the anode voltage (or less). If you try to apply too much voltage to
these tetrodes, the cathode current will skyrocket and the screen will
burn up.
That's before you could even start to think about linearity.
Returning to the other branch of this discussion, the 4CX250B/R is
*capable* of good linearity, but there are two things wrong with the
operating conditions recommended on the EIMAC data sheet:
1. They are too greedy for output power. With good modern standards of
linearity, the output capability at 2000V is only about 250W PEP per
tube - that's only TWO hundred and fifty - and it applies to either the
4CX250B or the 4CX250R.
The 300W figure for the 4CX250B is almost believable, but ham
expectations are too conditioned by the 470W claim for the 4CX250R.
You can get that many watts, if you bias and drive the tube as EIMAC
indicates, but the signal sounds foul.
2. They don't recognize the benefits of a stabilized screen supply. For
very clear evidence of the linearity improvement, see my web site for
measurements made by GW4FRX.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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