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[AMPS] 4-1000A Project

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Subject: [AMPS] 4-1000A Project
From: (Jon Ogden)
Date: Wed, 04 Apr 2001 07:35:29 -0500
on 4/4/01 12:51 AM, Bob Marston at wrote:

>> STAY AWAY from the B&W 801 switch assembly.  Lots of guys and old assembly
>> diagrams say it is for the 4-1000.  It's not  I used it and wish I didn't.
> Hmmmmmmm  the B&W 800 and 802 are plate chokes  The tank coil assemblies
> that include a band switch that were manufactured by B&W are the 850 and
> 852.

The part numbers I am speaking of are the 850 and 850A and 851.  I could not
remember the numbers, and at the time I did not have time to dig out my data
sheet on them.  It's been a while.  The 850 has the shaft at DC potential
requiring insulated coupling.  The 850A and 851 have the shaft at ground

The 850 and 850A are designed for 2KW PEP CW and SSB.  The 851 for 500

> Seems to me I recall a couple of years ago that someone queried the
> list this matter and I pointed out the 850 was designed to be used for a
> plate impedance one would expect to find with approximately 6kv and the 852
> was set up for approx 3 kv. Then you jumped on the list in your normal half
> cocked fashion crowing about how YOU got the 850 to work at the lower voltage.

Huh?  I don't think so.  I have NEVER done anything at 3KV, Bob.  That was
not me.  My amplifier runs at 6KV and it always has.
> Now you say that you wished you had not used it. WELL which is it Jon ????
> Were you wrong then or are you wrong now ?

No, Bob.  You are wrong in asserting that I crowed about getting the 850 to
work at a lower voltage.

I used the 850A because that is what I had and I made it work.  However, if
you calculate the inductance values needed for a good Q, they don't work or
at least they don't work well.

Since you know so much about my application, Bob, let's talk specifically
about it!

My 4-1000A runs at an idle plate voltage of about 6000 volts.  Under load,
this voltage is around 5750 Volts with around 600 mA.  So that makes the RF
Plate Load Resistance equal to about 5324 Ohms.

Plugging this number into the equations for a Pi-Net we get the following
values for a Q=12 (12 was picked because the numbers for a Q of 10 were not
formatting correctly on the spreadsheet and I don't feel like screwing with
it) for the tank circuit:

R1 = 5324
R2 = 50

3.5 MHz  L=20.35
7.0 MHz  L=10.67
14.0 MHz L=5.39
21.0 MHz L=3.6
28.0 MHz L=2.7

All values of L are in uH.

Now the B&W 850A, has the following inductance values:

3.5 MHz L= 13.6 uH
7.0 MHz L= 6.5 uH
14.0 MHz L=1.75 uH
21.0 MHz L=1.0 uH
28.0 MHz L=0.8 uH


Now the B&W spec sheet specifically says that the impedance range it is
designed for is 2000 to 4000 Ohms.

Let's look at what our L values are for 3000 Ohms plate load and a Q=12:

3.5 MHz= 12.56
7.0 MHz= 6.59
14.0 MHz = 3.33
21.0 MHz = 2.22
28.0 MHz = 1.66

So even in its "proper" impedance range, the 850A still does not have the
best values on the higher bands.  With the 850A's inductance values, the Q
on 20 M will end up being about 24 to 25 with the 850A (1.75 uH).  On 15M,
the Q ends up being about 28 to 29 (1.0 uH).  And on 10M, the Q will end up
around 26 to 27 (0.8 uH).  And that's with the impedance being in the
"proper" range!  So the 850/850A were not good even for the lower impedance

In my case the Q on 10M would have come out to be a whopping 45!  That is
NOT good for proper tank circuit efficiency.

Now, Bob what I did that you may have gotten the idea of something "lower"
from was to lower my load impedance through the use of a step down
transformer.  It is an inductance placed in series with the DC blocking cap
before the tune cap.  By doing series to parallel conversions, you can
calculate the math on this.  This inductance acts as a simple step down
transformer and brings the load impedance of 5324 down to a more manageable
level for more reasonable Q.

By adding 1.1 uH in series as I describe, you end up transforming the plate
load impedance to about 1287 Ohms at 10m, 2721 Ohms at 15m, 4072 Ohms at
20m, 5014 Ohms at 40m and 5265 Ohms at 80m.

It's much easier to match the 1287 Ohms to 50 Ohms at 28 MHz.  With the 0.8
uH that the B&W has, we now have a reasonable Q of about 12 to 13.

Now, my coil that I added may or may not have an inductance of 1.1 uH.  I
had no real good way to measure it.  I had to end up removing the 10m coil
on the 850A and replacing it with just a wide copper strap.  So I suspect
that my inductance value may be a little different.

Thanks for bringing this up.  I'd wanted to post my design about the step
down transformer for these new guys and I'd lost all previous copies of
stuff I sent out about it.

I can't take credit for this, since someone else on the list suggested it.

One additional nice thing about this transformer was that adding it shifted
the VHF resonances in the plate circuit WAY away from the 80 MHz or so that
you get with a 4-1000A.  Using a grid dip meter prior to adding the coil,
the plate circuit had a sharp resonant dip right around 80 MHz.  After
adding the coil, the dip was gone and I could not find it anywhere from 50
to 120 MHz.  My amplifier is stable and the only suppressor network I have
is a single turn coil and NO suppressor resistor.  I took it out as it was
continually burning up due to the circulating currents at 10m.  Some other
old timers suggested that the 4-1000A could be made stable without it.
Indeed they were right!

I have mistuned my amplifier into bad loads, operated it on the wrong band
and regularly run full legal limit on all bands and have had no problems
with it in over 2 years other than having one of my tubes go gassy.

So Bob, before you go off half-cocked why don't you get your facts straight.



Jon Ogden
NA9D (ex: KE9NA)


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

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