[AMPS] Re: Variable Screen Voltage!

[Phil Clements]

Been there, did that! It works very nicely.
I built mine from the article in the January 1974 issue of
Ham Radio, by W4OQ.
(((73)))
Phil, K5PC

> Now, let us complicate things.  Assume a ham has a 3-1000 amplifier with a
> defective tube - which runs 3,000 volts on the plate.  It is a conventional
> grounded grid configuration.  He looks for a replacement tube but finds they
> are $800 new and $300 used.  Suppose he has a box of 4-1000 tubes in his
> cluttered hamshack and thinks he will try one of those in grounded grid.
> However, at 3,000 volts his 100 watt driver will only produce about 850
> watts.  Modifying the power supply is difficult.  So, he comes up with the
> idea of raising the screen voltage above ground to increase the sensitivity
> to allow the 4-1000 to work at 3,000 volts.  Now, assume he installs a
> variable but regulated screen voltage on the screen grid and adjusts the
> screen voltage so that the resting current of the 4-1000 is the same as the
> 3-1000.  Let us say the regulated screen voltage is 185 volts.  So, he
> builds a small  regulated supply to fit into the original cabinet that
> produces 185 volts.  Now, let us assume that the amplifier now drives with
> 100 watts to a full 1,500 watts output.
>
> Any comments about whether this would work.  Since the manufacturer does not
> provide data for this operation, should it not be done.  Is this the 300 mpg
> carb that Rich says is impossible?  Since the screen was varied at one
> point, is it incapable of working?  Since adjusting the screen voltage while
> leaving the grid at zero voltage was suggested by G2DAF in his original
> article, does that mean it cannot work?
>
> Any constructive thoughts?




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