[AMPS] Splatter

[Harv Shore]

4CX1000 grids are "zero" dissipation --  common information.
For anyone who does not believe this
well
It helps the economy to buy more

One of the first modifications Collins suggested for the 30S1 was a diode
clamp to avoid grid current.

Used in that way the 30S1 I had never needed a tube replacement in well
over 40 years of heavy duty operation!

----------
> From: Ian White, G3SEK <G3SEK@ifwtech.demon.co.uk>
> To: amps@contesting.com
> Subject: Re: [AMPS] Splatter
> Date: Sunday, June 08, 1997 12:54 AM
> 
> Andrew Williamson wrote:
> >I don't know much about amps or tubes but I'm learning a LOT from the
> >discussions on this reflector.  That's why I subscribed.  So, here's a
> >question which I hope isn't too far off topic.
> >
> It's absolutely spot-on topic.
> 
> 
> >As a user of an ETO91b which uses a pair of 4CX800A/GU-74b's can anyone
> >tell me what the grid current is when the red 'grid max' light
> >illuminates.  Is it in the 3-10mA range that is 'nutsville'.  I'd hate
> >to damage the tubes as I believe they are quite delicate in the grid
> >current area, even if they are cheaper than 3CX800's.
> 
> It depends what you call "delicate". The 4CX800 has a rated maximum grid
> dissipation of 2W, which means that at the normal bias voltage of -60V
> you could run as much as 30mA grid current without damage. 
> 
> You wouldn't want to do that, for linearity reasons. The tubes are meant
> to operate in class AB1, which is why the ALC operates at the first
> trace of grid current and the amplifier trips to standby at 3mA. Also
> the 91b has a low-impedance grid bias supply, so small amounts of grid
> current do not spell IMD "nutsville".
> 
> What about the 4CX350A, 4CX1000 and other tubes with a rated grid
> dissipation of "zero"? Think about that: it cannot be literally true.
> The grid must have some power dissipation capability. 1mA of grid
> current adds only 60 milliwatts of heat load to the grid (which is right
> up close to a cathode that dissipates 1000 times more heat energy). I'm
> not saying that anyone should aim to run grid current, but the "Absolute
> Maximum" power dissipation rating of "zero" should not be taken
> literally.
> 
> It's also notable that "zero" is only seen in data sheets that are 30
> years old now. It was probably meant to educate people about those new
> tubes that were not intended to run grid current, but it was over-
> dramatic and created a "disaster" mentality among users. With later
> tetrodes intended to be operated in class AB1, Eimac and Svetlana have
> started to give more realistic numbers.
> 
> 
> 73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
>                           'In Practice' columnist for RadCom (RSGB)
> 
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