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Re: [Amps] SB220 running plate current when no drive; Problem

Subject: Re: [Amps] SB220 running plate current when no drive; Problem
From: gudguyham--- via Amps <>
Date: Thu, 8 Oct 2020 16:01:27 +0000 (UTC)
List-post: <>
3-500 tubes with directly heated cathodes, unlike tubes like 8877 and 3cx800’s 
etc do not have fragile grids nor does reasonable peak grid current have 
cumulative degrading effects on the grid.  Because the grid is robust a ICAS 
rating can rise to damn close to 1kw pep output.  The same goes for most all 
types of similar tubes.  Hams have to realize that we are not AM broadcasters 
that are on the peddle 24/7.  Amps like the SB220 run out of soup at around 
1200-1300pep and in no way does an sb220 ever stress a good Eimac 3-500.  Many 
Henry amps ran these tubes to 2kw pep on SSB.  One might want to be 
conservative these days due to the increasing cost of tubes.  

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 On Thursday, October 8, 2020, David Gow <> wrote:

For actual Eimac tubes the Eima DATA sheet shows much greater
efficiency and usable power with 4000 volts on the plates.  Henry made
several amps with a 3200 or 4000- volt switch and  the 3KA  I had easily
made the legal limit  at 4000 volts with the original tubes after 21 years
withHenry spec. grid and plate current being carefully maintained.  He
wanted to buy new tubes and I discouraged him,  When I last heard from him
four years after the sale he was still running the Eimacs. with a new pair
of chinese tubes on the shelf gathering dust losing vacuum.
Dave W7VM

On Thu, Oct 8, 2020 at 6:27 AM Joe <> wrote:

> What is the current (pun intended) max grid current for a pair of
> 3-500Z's now days?
> And does it matter what the Plate voltage is?
> Joe WB9SBD
> On 10/8/2020 6:39 AM, Rob Atkinson wrote:
> > Never heard of an ICAS rating for the 3-500.  I have a dual 3-500
> > amplifier with a pair of 20 year old RF Parts 3-500s and they're still
> > at full output, but they have been run conservatively especially with
> > regard to grid current.
> >
> > 73
> > Rob
> > K5UJ
> > _______________________________________________
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> >
> >
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