[Amps] drive power

[Karl-Arne Markström]

The drive power ratings for different circuits (grid-driven and cathode-driven) may look
confusing.

In the grid-driven AB1 circuit, you only need drive voltage which corresponds to sufficient power to overcome the grid circuit losses and the electron transit time losses. On the lower bands these sum up to a fraction of a watt. 
It is however considered good engineering practice to limit the stage power gain to 100 or 20 dB, because a higher gain can be prone to stability problems.

The cathode-driven or "grounded-grid" circuit uses up a fraction of the driving power in the grid circuit, and the remaining drive power is fed-through to the plate circuit and appears there as useful output power. A "normal" grounded-grid circuit is limited to a stage gain of about 10 - 30 due to the inherent degenerative feedback.

There have been many designs of power triodes that use this gain range to their advantage, so a 100W class radio conveniently can drive a 1000 - 1500 W output amplifier.

In the cathode-driven triode-connected 813 case the drive power quoted is in the 10 - 15 W per tube range 
depending on the plate voltage, which would correspond to a required drive of 20 - 30 W for a pair.

Nothing prevents an user to use a "pad" or attenuator before the amplifier input, or to "swamp" the input circuit
(grid or cathode) to absorb excess drive power. This would be in line with a design practice sometimes used with 
grid-driven amplifiers using very high gm tetrodes or pentodes where the grid circuit consists of a 50 - 100 ohm resistor.

73/

Karl-Arne
SM0AOM






----- Original Message ----- 
From: "Steve Flood" <flood at ixi.net>
To: <amps at contesting.com>
Sent: Saturday, March 17, 2007 3:00 PM
Subject: [Amps] drive power


> As I muddle through the understanding of amp design, there is a remaining topic that is confusing me - in regard to matching drive power to the exciter (in this case a modern 100w transceiver).
> 
> I understand that my 100-w tranceiver should be run at maximum output to the amp input.  Apparently, this requires a grounded-grid setup to accomodate the relatively high drive level.   Yet there seems to be many amps that are grid-driven that require very small amounts of drive power.  Are these amps not designed to be used modern transceivers?  Are they commonly driven by homebrew low-power exciters?
> 
> In my case I am trying to design a g-g 813 amp.  The only g-g information I have found is in the Orr handbook which says 25w drive - which I asume is pert tube.  Do I next assume that I need 4  813's match my transceivers 100w max ouput level?  With what have folks been driving all those homebrew 2x813 amps over the years?
> 
> I've been told I'm wasting a good pentode by using it in grounded-grid.  Yet the 813 specs say something like 0.2 watts of grid-drive power, which is obviously (?) not meant for modern transceivers. 
> 
> Steve, KK7UV  
> 
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