It may well be that adding parallel coax cables is giving more capacitance
nearer the cathode. Short coax length at HF really doesn't act as a coax
cable because it is a very small portion of a wave length. It looks more
like just a capacitor.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim W7RY
> Sent: Sunday, December 23, 2012 12:21 AM
> To: amps@contesting.com; Paul Baldock
> Subject: Re: [Amps] GS-35B Efficiency Varies with frequency?
>
> 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
> >>
> >>_______________________________________________
> >>Amps mailing list
> >>Amps@contesting.com
> >>http://lists.contesting.com/mailman/listinfo/amps
> >>
> >>
> >>-----
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> 12/12/12
>
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