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Re: [Amps] power limiting in 8410 amps?

To: <>
Subject: Re: [Amps] power limiting in 8410 amps?
From: "Leigh Turner" <>
Date: Thu, 8 Nov 2018 09:26:55 +1030
List-post: <>
Good point that you make here Glenn.


My comment about optimum screen voltage for unconstrained Po was predicated
on the assumption the design value of cathode NFB resistor(s) was still
correct and had not changed...its good later production 8410 amps were set
to the higher 225 V on the screen.


The use of stability and linearity enhancing NFB per se would not cause
significant gain compression; just linearly reduce the amplifier power gain?


In one of my 4CX800/GU-74B AB1 tetrode linear amps, there's a front panel
toggle switch selection of screen voltage for Hi and Lo power output, so
this has been a commonly applied technique in some designs.








From: GWP [] 
Sent: Thursday, 8 November 2018 2:51 AM
Subject: Re: [Amps] power limiting in 8410 amps?


Production 8410 amps have the screen voltage set to 225v.  Some very early
production amps were set to 200.

The tube operating bias is then adjusted for 200 ma idle Ip and an 8410 can
easily get 1500+ watts output.


As previously noted, if the cathode resistors have changed value and are
greater than 6 ohms that will increase negative feedback, limiting the gain
and maximum output.  Just cranking up the screen voltage is not the right
way to set up an amp.


Glenn AE0Q


On Wed, Nov 7, 2018 at 6:09 AM Leigh Turner <>

Yes I concur with this suggestion Steve.

With only 200 Volts on the screen in AB1 linear mode the 4CX1500B tubes are
likely being starved and held back like variously reported amongst 8410 amp
users. The screen voltage probably ought to be up around 275 to 300 Volts to
ensure unconstrained power output to 1500W+ without significant gain




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