Not wanting to get involved with cooling problems while adjusting the amp
under full steam, I opted to redo the tuned input using 69 ohms from the
cathode to ground. I had to shuffle the fixed silver micas around on 160
but in the end got all the bands down to 1.2:1 with the resistor and the
result was all bands came in at 1.2 to 1.3:1 under full power into a dummy
load. I am a happy camper. Now that I have it working on all bands 160
thru 20, it's for sale on QTH.com.
73 Bob W6TR
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "Bob Maser" <bmaser@tampabay.rr.com>; "AMPS" <amps@contesting.com>
Sent: Thursday, October 26, 2006 12:34 PM
Subject: Re: [Amps] 3CX3000 Tuned Input Calibration
>
>>I have completed the alignment of the tuned input of my
>>3CX3000A7 using a 50
>> ohm non-inductive resistor connected between the cathode
>> and ground. I then
>> drove the input with my FT1000 and was able to adjust all
>> bands to 1.1:1.
>> When I put it all back together and drove the amp on 20M
>> lo and behold, the
>> SWR is 1.6:1. I believe that the reason for the
>> discrepancy is that the
>> input impedance for a 3CX3000 is not 50 ohms but probably
>> lower.
>
> Don't let anyone tell you slapping a resistor from the
> output of the network to ground simulates the load by the
> tube. It doesn't.
>
> You initially got lucky Bob. It fell apart on 160 because of
> the filament choke. It will fall apart on the highest bands
> because of stray capacitance, inductance, and transmission
> line length to the tube that is not terminated perfectly in
> 50 ohms. In my experience the impedance is slightly higher
> than 50 ohms, close to 60.
>
> Part of the input impedance of the tube is modified by the
> feedthrough capacitance. This means some phase shift is
> involved in actual operation. Using the 50 ohm resistor will
> only get you close on the higher bands, although there are
> cases where dumb luck takes over. Once close you will just
> have to trim it in.
>
> 73 Tom
>
>
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