Kinda OT: Swan Mark I Linear expected output on 4-400A's in G-G?

[JBA]

Bob,
I can only guess that you have a shack full of 4-400A's at your place, and
that you don't mind showing your age! Hi!
While it is perfectly ok ( used to be common practice) to tie the screen and
control grid of a 4-400 tetrode, what you end up with may be, but is not
necessarily, a zero-bias triode. I think your grid current may be a little
high and your 90 volt zero-signal anode-current is lower than I like. I like
to run the zero-signal anode-current between 160 and 200ma to ensure good
linearity. Any less than 160ma and you run the risk of not being a good
neighbor, and any more than 200 and the amp becomes too hard to drive. True
zero-bias tubes are designed from scratch to be so, and must be operated
according to their designer's specs to work well with zero bias. So a
high-mu triode cloned by strapping together the multiple grids of a
screen-grid Tetrode or Pentode tube may or may not be operated efficiently
as a zero-bias tube, depending on its control characteristics and of course
how you power the tube.----BUT---- the 4-400A and 4-1000A Tetrodes have
always seemed to work well in GG.
What I'm trying to say is that according to the numbers you supplied in your
posting, your pair of 4-400A's are doing just fine. If you had a way to
supply the plates with a pretty stiff 4KV, they would really get up and trot
to around the legal limit. I would investigate the higher than usual grid
current ( I do remember that these tubes ran high grid currents in GG ), and
try to get that zero-signal anode-current up to at least 160ma.
I'm still at a loss as to why you don't just drop a pair of 3-400Z or 3-500Z
in the amp? NOS 3-400Z can be had off Ebay for $25 to $50 apiece. Of course
the Chinese 3-500Z will usually run you a couple of hundred bucks a pair.
Either will give you around 1300 watts output in the Mark l.
All this info and a buck will still get you a cup of coffee in some parts of
the country.
Cheers!
 de Benton wb5twc



----- Original Message -----
From: "Bob, WB2UJD" <BobbyTrip at AOL.COM>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Saturday, May 18, 2002 10:48 AM
Subject: Kinda OT: Swan Mark I Linear expected output on 4-400A's in G-G?


> OK you amplifier gurus,
>
> I have a Swan Mark I I have restored to mint condition and I am using 2
> 4-400A's in G-G in place of the 3-400Z's (3-500Z predecessor) it was
designed
> for.  It seems to work pretty well, but I am not sure it works as well as
it
> might.
>
> What might I expect as a reasonable power output from such a combination?
> Here are a few pertinent details:
>
> 3.75 MHz, just over 650W out with about 112W drive (not 100% sure of the
> HM-102 meter cal)
> The Mark I has an untuned cathode driven input.
> 2600V key down, 3300V idling, in operate (SSB) position
> 240 mA plate current
> 540 mA grid current (yes this is high, but tetrodes in G-G tend to do
this)
> 90 mA grid current idling (bias)
> Not 100% sure of metering cal either, but all metering resistors have been
> replaced and are 1% or better.
>
> DC plate input comes out as 624W.  Not sure how to calculate the grid
portion
> or if this contributes to total DC input, probably does in some fashion.
>
> In Tune/CW position:
> 3.75 MHz, about 450W out with about 112W drive
> 1600V key down, 1900V idling
> 300 mA plate current
> 500 mA grid current
> 55 mA grid current idling (bias)
>
> DC plate input comes out as 480W.
>
> I am looking for some idea of how the amp is doing and what might be
expected
> of such an amp (Or how likely I am to piss people off if I were to attempt
to
> use this amp on SSB!).  This is my first linear in close to 30 years as a
ham!
>
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