Very interesting Paul.
The SB-220 tuned input is about 8 cable inches from the tubes. I may try to
use 3 pieces of RG174. RG58 is a bit large and would not fit through the
grommet in the chassis... If you've ever seen the tuned input for a
Ameritron AL82, or AL1200 (I think the AL1500 is this way too) there are 3
pieces of Teflon (rg174 size) cables between the power supply area and the
tube sockets. And I think there are 2 pieces in parallel between the power
supply area and the tuned input network that is behind the front panel and
the band switch.
Did you try 2 pieces of RG58 in parallel? What made you decide on 3 pieces?
Thanks!
73
Jim W7RY
-----Original Message-----
From: Paul Baldock
Sent: Thursday, December 13, 2012 12:43 PM
To: Carl ; amps@contesting.com
Subject: Re: [Amps] GS-35B Efficiency Varies with frequency?
Problem is solved:
Originally I had about 8" of RG-58 between the PI input and the
Cathode. I thought this would be short enough.
After reading info from W8JI's web site I now have 3 pieces of RG-58
in parallel from the PI input to the Cathode. I also moved the 10M
cap and put it right on the Cathode, and compensated by reducing the
cap for all other bands.
Efficiency is now better than 60% on all bands and about1300W PEP
output on all bands with 70W (of dits) drive and around 3.1KV on the plate
For those that asked, the reason the plate Q is high on the WARC
bands is that I am using the next higher standard band for double
duty so I have less inductance than would be ideal. There is not
enough position on my switch to do it correctly
Thanks for all the suggestions.
- Paul KW7Y
At 08:02 AM 12/13/2012, Carl wrote:
That is characteristic of a tired tube and the poor PS regulation doesnt
help.
Carl
KM1H
----- Original Message ----- From: "Paul Baldock" <pbaldock@frontier.com>
To: <amps@contesting.com>
Sent: Monday, December 10, 2012 7:50 PM
Subject: [Amps] GS-35B Efficiency Varies with frequency?
I am building a GS-35B amplifier. The efficiency drops off a lot between
80M and 10M. Although the gain is quite constant. Does any body have
suggestions as to why this is or whether this is typical.
The input PI network has a Q of around 2. The PI output has a Q between 10
and 20
Here's some results with 70W Drive:
3.80MHz. Q=10, Output Power 1.1KW, Vp 3000V, Ip .54A, Ig .26A, Eff 68%
7.10MHz. Q=13, Output Power 1.1KW, Vp 3000V, Ip .60A, Ig .28A, Eff 61%
10.1MHz, Q=20, Output Power 1.0KW, Vp 2900V, Ip .68A, Ig .30A, Eff 51%
14.2MHz, Q=13, Output Power 1.1KW, Vp 2900V, Ip .70A, Ig .29A, Eff 54%
18.1MHz, Q=16, Output Power 1.0KW, Vp 2850V, Ip .78A, Ig .31A, Eff 45%
21.2MHz, Q=13, Output Power 1.0KW, Vp 2800V, Ip .82A, Ig .37A, Eff 43%
24.9MHz, Q=19, Output Power 1.0KW, Vp 2850V, Ip .79A, Ig .35A, Eff 44%
28.5MHz, Q=16, Output Power 1.1KW, Vp 2750V, Ip .85A, Ig .38A, Eff 47%
You will note as efficiency goes down the Grid current goes up (in
general) for the same drive. Is this normal?
Things I have tried:
1. Checking that the bias voltage is not affected by RF. No signal
Vp=3600V, Ip=140mA, Vb=34.5V.
2. Rebuilding 10M PI Output in a different configuration. (It is actually
an L-PI)
3. Checking that the metering is not affected by RF
Any Suggestions?
What is a safe Max Ig for the GS-35B?
Thanks
- Paul KW7Y
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