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Re: [Amps] Triode Amps

To: <>
Subject: Re: [Amps] Triode Amps
From: "Jerry Booth" <>
Date: Mon, 6 Dec 2021 12:15:25 -0500
List-post: <>

I ran a pair of Eimac 3-500Z tubes in a homebrew amp for 27 years at 4.5 KV 
with RF switched bias and the tubes were good when I sold it to a friend. They 
are still working for him at full output today. 

           Jerry WØZD/3
   - 6 Decades of Ham Radio -

-----Original Message-----
From: Amps <> On Behalf Of Kim Elmore
Sent: Sunday, December 5, 2021 10:48 PM
To: jim.thom <>
Subject: Re: [Amps] Triode Amps

My guess is that the hardware to withstand higher B+ Is hard to come by. That 
and anything beyond 4 kV goes off the charts, so no one can say for sure what 
the characteristics are when run beyond 4 kV so it becomes an experiment that 
most aren’t truly ready to carry out. 

Kim N5OP

> On Dec 5, 2021, at 11:23 AM, jim.thom <> 
> wrote:
> Date: Fri, 3 Dec 2021 18:23:28 -0600
> From: Kim Elmore <>
> To:
> Subject: Re: [Amps] Triode Amps
> <I should have qualified my response: 1500 W is indeed possible out of 
> a <pair of 3-500Zs, but the cooling must be letter perfect: air 
> sockets, <proper chimneys and proper plenum pressure. I suppose you 
> could get 2+ <kW out of a pair of you ran them at a much higher B+ 
> (well over the 4 kV <max defined by Eimac), higher I_p and at the grid 
> dissipation limit, <i.e., and well out of their limits, but they won't 
> last very long that <before they fail. If cooling is sufficient, I 
> suspect you could run 1.5 <kW output for RTTY and FT8 (essentially 
> CCS), but there probably won't <be much head room.
> <Kim N5OP
> ##  Agreed.  Elevated  B+ is a non issue.  The tubes will hi-pot test  
> to
> 15 kv. To get 4 kv  loaded, would require 4.4 kv  un-loaded.  Been 
> there, done that.
> The cooling is straightforward.  Use chimneys that have the taper at 
> the top..and use anode connectors  with vertical fins.  And the 
> correct blower.  The  EBM papst blower from the AL-82 /  AL-1500 /  
> AL-1200  works fine.  Loads of other blowers that will provide even more 
> airflow.
> Use the correct amount of bias.  100 ma of idle current  is ample for a
> pair of tubes..and  2600 vdc.   And even less idle current  when  higher B+
> used.  IMD is superb.  40 ma of idle for a pair, when1900 vdc used 
> still results in good  imd.
> On  CW, idle current can be reduced  to just above zero 
> 10-20 ma.  You can't reduce the idle to zero on  CW, or u will then get key
> clix.   Same deal with data modes, idle current can be reduce to just
> 10-20 ma.
> Ideally, a  3 x   3-500Z amp, or even 4  tubes would be optimum, and leave
> some wiggle room.
> For data modes,  I would not run the existing amps at >  1 kw.   When
> switching from  SSB  to data modes, the average plate current doubles....
> and watts dissipated in the tank coils and band switch quadruples.  
> 1.5 KW on data modes is a bunch.  It's not just the tubes you have to 
> worry about, it's everything else, like tank coils, padders, 
> bandswitch, HV xfmr, etc, etc.
> Jim   VE7RF
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