A question was ask on how I determined the value of the resistor to use.
I took the tube manufacture data sheet used the resonant load impedance
and driving impedance values. Simply took that value in a carbon
composition
resistor and attached them to the circuit in there prospective location,
I gave the MFJ what it wanted to see in order to achieve a resonant circuit.
I hope that made sense.
Chris
----- Original Message -----
From: G SEVEN <superiorwavelength@prodigy.net>
To: AMPS <amps@contesting.com>
Sent: Monday, March 12, 2001 10:42 AM
Subject: [AMPS] 3CX3000A7
>
> Mucho thanks to everyone who responded to my post, I will attempt to
answer
> everyone all at once here. Originally this was a Henry 3000D rf deck. It
> had been rebuilt into a multi band HF amp by Jim Day in Bakersfield. I
had
> purchased the amp from a Amateur operator named David Shoen who lived up
in
> 5000 foot elevation of Labec California. He was in the process of moving
to
> Europe at the time. When I went to look at the amp at his shack it was the
> last piece of equipment in the shack so I was unable to test it. So not
> being able to test it and seeming like a upstanding individual of the
> Amateur society and not being able to test it, I bought the unit with a
> decent discount
>
> Once I delivered it home and fired her up low and behold nada output. So
> after going through the components and testing them I found a bad tube,
bad
> vacuum tuner amongst a few other little things. So not feeling comfortable
> working behind the person who built it I
> gutted her out and rebuilt her from near scratch. Here is a excerpt of
> specifications I was given on the amp prior to it going out of service,
>
>
> SNIP............ "answers:it is an original design,not a henry remake.
> The transformer is a Peter Dahl 4800v continuous at 2 amps.
> full bridge power supply.rf section has no roller inductor,it is
> band switched.yes to vacuum caps and vacuum relays.voltage sag
> is 5% on ssb,and 7-8% on cw,excellent regulation.it puts out
> 7000 watts with 300w drive,and 8500w with 400w of drive on 160
> thru 20 meters.idle current is 400 mils.When I drive it with my
> ft-1000d,it runs abt.5kw.and with my old icom 730,abt.3kw.
> I hope this answers your questions---oh,yes,it is shippable by
> truck.
> Thanks,
> David" SNIP..........................
>
> To answer some of the questions. Yes I am aware that the plate voltage is
> higher then specifications, I know of many people using higher plate
voltage
> with out saveer detriment to the tube. Filament voltage is slightly above
> spec, approximately 8.5 at idle and when in transmit drop a little under
8V.
> After I get it functioning I had planned on using a variac on the primary
of
> the filiment xformer to dial it in. For testing purposes I wanted to use
as
> minimum circuits needed to see it run so that in the event there is a
> problem I have less to go back and troubleshoot. I will add bells ad
> whistles later.
>
> How I did my tank circuit and achieved Q is with a MFJ Analyzer, I have
> used the analyzer on tuning solid state amp combining networks and has
> worked out great for that. A friend of mine has used it to tune the tank
on
> 3cx3000a7, pair of 3cx3000a7, and 4x8877 and it worked out great. I am
not
> a mathematician nor do I understand scientific notations so I rely on
simple
> ways of achieving the same end result in the simplest way I can.
>
> Now with that said let me explain what I did. I plugged in the MFJ to the
> antenna connector and jumped out the antenna relay. I installed a
> approximately 1425 ohm resistor from the plate to ground, I then played
with
> my band coil and vacuum variables until I was able to tune into a receiver
> at the operating frequency on the MFJ. Then I read the MFJ readout to see
> what the display gave me. I kept changing the coil until it canceled out
> all the X (reactance) and was left with a purely resistive load of 50 ohms
> at 1.1 SWR on the MFJ readout.
>
> I did the same with the input Toll. The input consist of a air tuner
going
> through a approximately 1" dia. copper coil, and then through a 100pf door
> knob to the filament connection on the bottom of the socket. When tuning
> with the MFJ I had to tap the copper coil at the right spot on the wind to
> get a good match.
>
> So that's it in a nutshell, I hope I was able to clearly communicate
myself.
> If there is another way to cross reference my tank Q to see if its right
> please share, I would like to try it.
>
> thanks again for the help,
> KRS
>
>
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>
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