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[Amps] some more tetrode questions

Subject: [Amps] some more tetrode questions
From: "John Lyles" <>
Date: Wed, 14 Sep 2011 08:36:35 -0600
List-post: <">>
I agree with Carl (and Ed) about raising the grids DC off ground and still 
running GG for RF. Raising the plate V on a 4-1000 with zero bias will raise 
zero signal idling current and grid current. 
That is exactly what I am doing with my TH628 amplifier at workplace. Both 
control grid and screen grid are actively biased with DC but bypassed to ground 
in cavity for RF, and it is cathode driven. 
Plenty of gain, 15 dB. I pulse the control grid into cutoff (-600) when I don't 
need RF power (and remove driver RF), leaving plate and screen high. 

I use a comparator to measure the sampled plate voltage, and if it drops below 
10 kV, I switch off the screen (1500 VDC). Otherwise, destroyed screen. 


> ----- Original Message ----- 
> From: "Ed Heimbach" <>
> To: <>
> Sent: Tuesday, September 13, 2011 11:37 PM
> Subject: [Amps] some more tetrode questions
> A few weeks back I asked about reconnecting a 4-1000....
> Currently the amp is connected GG, both the screen and the grid strapped 
> directly to the chassis.
> Many differing viewpoints were offered on how I may increase the output of 
> the amp, easiest being to raise the plate voltage.
> However, I have no budget for a new power supply.
> I believe I shall try this.
> Disconnect screen and grid straps,then place a 650 pF, 18kv doorknob cap 
> between tube pin and ground.( why 650pF? because I have a bunch of them).
> Next is to connect bias and screen voltage at the tube socket.
> I will use a regulated bench supply for the bias, and a spare Hi volt supply 
> (completely separate from the plate supply) for the screen.
> Plate supply is variable, up to 4.5kv.@3A (big iron from old A.M. station)
> Cathode drive will be retained.
> Any comments? Any obvious faults to my logic?
> 73, AB3HT
> Yes! If you cant get well over1500W with that supply either the tube is 
> weak, the input network design is poor or simply not used, or both.
> Carl
> KM1H

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