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Re: [Amps] Why does power output increase?

To: <>, "'Victor Rosenthal 4X6GP'" <>, <>
Subject: Re: [Amps] Why does power output increase?
From: "Paul Christensen" <>
Date: Sun, 5 Jun 2022 08:56:35 -0400
List-post: <>
>"I still wonder if the filament voltage should be measured while transmitting 
>or on standby..."


This may be unrelated to your amp issue, but I nearly ruined a FU728F tetrode 
tube because of filament voltage starvation.  That tube uses a 9.0 VAC 
filament.  The amp is an OM Power 2000A+.  The installation section of the 
manual recommends setting the transformer tap to match the AC lines voltage - 
in my case that's 250VAC.  

Unlike your problem, output power would drop to 50-70% of the initial keyed 
power.  Filament voltage dropped to under 8V when trying to make 1500W.  That's 
a big problem with indirectly-heated cathodes.  Changing to the 240V 
transformer tap fixed the problem.  Filament voltage at full power is 9.0V and 
9.4V during key-up.  The moral of that story is to ignore the installation 
instruction and choose the tap setting that results in 9.0V filament voltage at 

So, to answer your question, I believe filament voltage should normally be set 
at full output power.  OTOH, if there's AC line sag due to a long cable run, or 
a long undersized power cord, then that should be corrected in order to 
minimize the filament voltage differential between key up and key down at full 
output power.  Otherwise, it's possible to end up with dangerously high key-up 
filament voltage when filament voltage is set for full output power.  

By the way, your 20m sigs have been very loud into N FL for the last two weeks 
-- sometimes the strongest DX on the band.  

Paul, W9AC 

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