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[AMPS] 4-1000 is this a good tube?

To: <amps@contesting.com>
Subject: [AMPS] 4-1000 is this a good tube?
From: jono@enteract.com (Jon Ogden)
Date: Thu, 18 May 2000 09:30:47 -0500
on 5/17/00 10:11 PM, Bobo at kb8gnf@orwell.net wrote:

> 
> I have a chance to buy a homebrew 4-1000 amp. I'm not familiar with this
> tube. Is this tube easy to drive? Forgiving? What type of out put can be
> expected for a single tube with a serious power supply?

Bob,

I forgot to mention yet another thing to consider when using a 4-1K.

When running at high anode voltages, the output imedance of the tube begins
to get rather large.  If memory serves me it's on the order of 5 KOhms or
so.  The tube also has an output capacitance around 7 to 9 pF.  Now at low
frequencies, building a tank circuit that will effectively transform 5 KOHms
to 50 Ohms isn't a big deal.

However, on 10 meters or so it becomes a little trickier.  The output
capacitance of the tube begins to be a larger and larger factor as a
component of your tune capacitor.  When placed in the socket and put into
the chassis, myself and others have estimated the anode to ground
capacitance at something on the order of 15 to 20 pF.  When you work out the
design equations for a pi-network of reasonable Q (10 to 15) matching a 50
KOHm/20 pF load to a 50 Ohm load, you'll find that the tune capacitor value
ends up being on the order of around 25 to 30 pF or so (this is all from
memory.  I have it written down somewhere but don't have time to find it so
my numbers may not be exact).  Anyhow, my point is that by the time you
factor in the output capacitance of the tube (15 to 20 pF) that leaves you
with very small tune C that is needed (10 to 15 pF).  A tune C that small
can be difficult to find and realize.

So what others have suggested and what I have done is add an inductor of a
micro henry or so in series with the blocking cap in the anode output line.
This inductor acts as a step down transformer and makes the impedances
easier for the pi-network to match.

Also because the output impedance of the tube is a large portion of the tank
circuit effective tune C, you will get a lot of circulating current in the
tube.  Make sure your blocking cap and supressor resistors (if you use them.
My topology didn't need them) are capable of withstanding the current and
associated heating that will accompany them (component ratings lower as
thermal conditions increase).

If anyone wants a more detailed write-up of what I did, I have one that I
can e-mail.  Also, I have an Excel 97 spreadsheet that can be used to do
calculate the pi-network and step down inductor (series to parallel
transformation routines).  I think the spreadsheet is on my webpage.  If
not, I'll e-mail it to anyone who wants it.

Have fun!

73,

Jon
NA9D

-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Member:  ARRL, AMSAT, DXCC, NRA

http://www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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