All,
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 -
jerryjtb@comcast.net
w0zd@w0zd.com
-----Original Message-----
From: Amps <amps-bounces@contesting.com> On Behalf Of Kim Elmore
Sent: Sunday, December 5, 2021 10:48 PM
To: jim.thom jim.thom@telus.net <jim.thom@telus.net>
Cc: amps@contesting.com
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 jim.thom@telus.net <jim.thom@telus.net>
> wrote:
>
> Date: Fri, 3 Dec 2021 18:23:28 -0600
> From: Kim Elmore <cw_de_n5op@sbcglobal.net>
> To: amps@contesting.com
> 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 ma...like
> 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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