>Oh boy...I really hope I will not regret this.....
>
> -----Original Message-----
> From: Rich Measures [SMTP:measures@vc.net]
> Sent: None
> To: yo3ctk@alltrom.ro
> Subject: Re: [AMPS] re:4-250A
>
>
> Rich wrote:
> <snip>
> - How many tubes does Eimac rate for Class AB2 grid-driven
>linear
> amplifier service?
>
> For instance, EIMAC 4CX1500B.
CONGRATS, Mike. The 4CX1500B is such a tube. Another Eimac tube with
very low grid interception is the 4CX600J. It can be operated with up to
0.5mA of grid current in linear amplifier service. However, I am not
aware of any other such tube.
>According to the manufacturer,
>this tube is "specifically designed for exceptionally low
>intermodulation distortion". Its typical operation is class AB2,
>grid-driven service for frequencies below 30 MHz. Indeed, performance is
>good: -40 dB 3rd order IMD, for 2750 V DC plate voltage and 1100W power
>output. In this conditions, DC grid current is 0.95 mA for -34 V grid
>bias.
However, 0.95 mA of grid current doesn't seem to be much compared to the
25mA that was claimed for the (2) 4-250A amplifier.
- Eimac recommends 0.53mA of grid current for a 4CX1500B with an anode
supply of 2900V and a screen supply of 225V. However, the anode/plate
current is only 710mA instead of the rated 900mA. If 400V is used to
supply the screen of a 4CX1500B, the anode current would increase to
900mA, and even more RF output would be produced--with zero grid
current--with low IMD.
- The bottom-line is that you can run grid-current in a linear amplifier
which uses a specially-designed tetrode like the 4CX1500B or 4CX600J.
However, the permitted amounts of grid current are small. If more grid
current results in lower IMD---as Peter recommends---why not use 2mA of
grid current with the 4CX1500B to further reduce IMD?
> EIMAC says: "Because the 4CX1500B has very low grid interception
>it is possible to drive the grid positive without any adverse effects
>upon the distortion level or upon the driver. Class AB2 linear amplifier
>operation is therefore possible and recommended".
>
True, however, the vast majority of tetrodes are cleaner if they are
operated in Class AB1 instead of Class AB2.
Rich---
R. L. Measures, 805-386-3734, AG6K
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