[AMPS] re:4-250A (AB2 in grid-driven amplifiers)

[Rich Measures]

>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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