Re: [Amps] 3.5 kV 2A REGULATED Power Supply: Schematic ?

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

GGLL wrote:
> I've been following this nice discussion, and want to ask something 
> about Ian's post, ¿what's bad with running exciter at a lower level and 
> to have the amplifier "seriously under-driven"? (it seems to be not 
> only a lower power output matter).
> My modest guess is something related with operating points due to 
> distortion considerations, is it?.
By "under-driven" I simply mean that you're not utilising the full power 
output capability of the amplifier.

With a good tetrode amplifier - the right tube, the right power supply 
and the right operating conditions -  you can obtain low distortion 
right up to the point of grid current, and even beyond that into class 
AB2 if the grid dissipation allows it.

Rich's line of argument is that you under-drive the amplifier in order 
to ensure that it can never be driven into grid current. But he is 
forced into that position because he advocates a high-impedance bias 
supply that cannot handle grid current without changing voltage.

Darn right I'm being stubborn about rejecting that! Not only will such a 
bias supply *cause* IMD if there's any trace of grid current, but also 
it is contrary to tube manufacturers' recommendations. For example, 
Svetlana's recommendations for the 4CX800A are that "for stability, the 
source impedance [of the grid bias supply] should not exceed 1K ohms."

Simply design the bias supply to have a low impedance, so that it *can* 
handle grid current (in either direction) if necessary. If the bias 
voltage is held constant, there will be no sudden jump in IMD if the 
tube is ever driven into grid current.

Almost all "class AB1" amplifiers actually do nudge into a small amount 
of grid current in order to develop an ALC signal. The tube data sheets 
allow this.


There is still a need to protect the grid against excessive power 
dissipation due to overdriving. A high-impedance bias supply will 
protect the grid, but it also creates IMD with any trace of grid 
current. We should *all* reject that "solution" because it's unethical - 
you're solving your own problem by dumping IMD onto everyone else.

The proper solutions to grid protection are ALC combined with a 
low-impedance bias supply... or else turning the drive power way down 
and under-utilising your amplifier.


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