[BULK] - [Amps] NCL 2000?

[Ian White, G3SEK]

Steve Katz wrote:
>
>I've had the same set of Eimac 7203/4CX250Bs in my two meter amplifier since
>1981.  Run it with Ep = 2500V, Eg2 = 350V, lots of air, and 1 kW PEP output
>power for, oh, about 10,000 hours so far -- same tubes, still generating 1
>kW PEP output at 144 MHz.  Ip = 615mA, Eff = 65%, Pdiss = 538W, which is
>7.6% above continuous rating per the data sheet.  With proper loading, Ig1
><10 mApk and Ig2 = about "zero," plus or a minus a few mA.
>
That's certainly achievable for CW, but not for linear SSB operation - 
note the 10mA grid current.

With reduced drive to stay in class AB1, the increased anode voltage 
will give 600-700W PEP output with quite good linearity. As Steve says, 
the added stress on the tubes is minimal.

>I think substituting a pair of 8874s for the 8122s and converting the amp to
>GG would be a far better idea, but 8874s are expensive, too.  Never tried
>the Russian tetrodes (yet), possibly a far better sub, especially for cost.
>
Yes - a pair of 4CX400As will enjoy the extra voltage. If the anode 
supply can deliver 0.8A peak at 2500V *and* if the screen supply can 
deliver +80mA peak at 375V without losing regulation *and* if the grid 
bias stays constant when you run into a few MA of grid current, then 
you'll get 1200W output with good linearity.

I don't know if the NCL-2000 power supply is up to that specification. 
If it's not (and I'd bet good money that the screen supply isn't) then 
you'll see reduced power output and linearity. Even so, the 4CX400As 
would probably be the best value-for-money replacement for the 8122s.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek