I would also take into consideration the output capacitance of the tube as
well as the stray capacitance of the socket, chimney(if metallic) and C1
mounting hardware. I would not use the Pi-L numbers in the ARRRRRRGGGL
handbook as they have been wrong for years,IMHO.
Bob W6TR
----- Original Message -----
From: <zl1aih@ihug.co.nz>
To: <amps@contesting.com>
Sent: Saturday, September 02, 2006 5:14 AM
Subject: Re: [Amps] Pi-L values
> Hi All,
> I'm working on a Sunair GSL-1900A amplifier - 4CX1500B, passive
> grid AB1, 1.6 to 30mHz auto-tune. Anode 2500v at 0.650 amps,
> requires 40 watts drive for 1kW out.
> Getting close to a kW into a dummy load (a Heathkit 'Cantenna', so
> the tests are brief) on 160 and 80. About 800w on 40, dropping to
> 600w on 20, 500w on 15 and almost nothing on 10 metres.
>
> In the Pi-L network, C1 and C2 (vacuum variables) are coupled by a
> toothed belt - motor driven by a servo amp. The manual describes
> C1 and C2 maintaining a capacitance ratio of 2.5:1, L1 has taps
> selected by the bandswitch while L2 is 17uH variable to some
> minimum inductance - the continuously variable part of L2 is a
> 'pancake' inductor, with minimal lead length. L2 is motor driven by
> another servo amp.
>
>>From Bill Orr's handbook (23rd Edition), the Pi-L table C1 and C2
> values show a ratio of between 4.5 and 5:1 for an anode impedance
> of 2000 ohms - close enough for 2500v @ 0.65A.
>
> L1 and part of L2 have factory-fixed taps - only the 'pancake section
> of L2 is continuously variable. C1 and C2 are variable but their
> capacitance ratio is fixed.
>
> Test conditions :-
> Amplifier running - no HV. 1900 ohm resistor from anode to ground.
> MFJ analyser connected to output side of L2.
> C1/C2 and L2 adjusted manually band by band for minimum SWR
> reading - all bands less than 1.2:1 except for 10 metres. The highest
> frequency that will yield less than 1.5:1 is 25.5 mHz.
>
> Question - Did anyone on the the list ever use the GSL-1900A amp on
> 10 metres?
> Also, I may have missed something along the way, but why do the
> amp manufacturers use 2.5:1 ratio when Bill Orr's (Eimac's) tables
> show an average of 4.75:1?
>
> Question - Observing the serious decrease in power output with
> increase in frequency, should I be looking at the C1/C2 ratio as a
> likely problem?
> Disregarding the difference in recommended ratios, it's possible that
> some previous owner has "played around" with the toothed belt.
> The manual calls for a Vector Impedance Meter to set the capacitors -
> I don't have access to one. Maybe there's a work-around?
>
> Your advice is welcomed.
> 73, Ken ZL1AIH
>
>
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