I think the plate impedance is higher for push-pull than parallel operation
with the same voltage and average plate current (no time now to look it up),
because the tubes conduct over different parts of the RF cycle. Think about
class B -- at any time one tube is completely cut off and the other is
conducting in a push-pull circuit. So the impedance is twice what it would be
for a parallel circuit.
Vic K2VCO /4X6GP
> On Jan 27, 2015, at 12:21 AM, Carl <km1h@jeremy.qozzy.com> wrote:
>
> In a series fed PP circuit the Tune cap has to stand off the full DC voltage
> plus the Peak RF. On AM that is 4X the DC voltage for spacing. Altho cap
> values in PP are lower they are dual sections with very large spacing at
> times and quite long. The Tune cap in my first PP 250TH rig in 57 was about
> 12-14" long so I converted to swamped grid parallel with a PiNet with much
> smaller physical caps with the one I built about 15 years ago. With the pair
> of 6C21's on AM I went with vacuums.
>
> The plate choke can be whatever you want that has enough XL to block any
> stray RF from a not perfectly balanced series fed tank circuit.
>
> In a shunt fed circuit the voltage rating is the same as in a PiNet but now
> choke resonances apply.
>
> Plug in grid and plate coils are still readily available on Fleabay and
> elsewhere for multiband operation. Just remember to turn the voltages off,
> even a few hundred volts from the bias supply can wake you up.
>
> Carl
> KM1H
>
>
>
> --------------------------------------------------
> From: "KA4INM" <ka4inm@gmail.com>
> Sent: Monday, January 26, 2015 11:02 AM
> To: "amps" <amps@contesting.com>
> Subject: Re: [Amps] push pull or parallel ?
>
>>> On 01/26/2015 10:41 AM, Carel, pc5m wrote:
>>>
>>> 160mtrs contest past and we need more power... Plan is to add 3 dB by
>>> throwing in another tube (2.5kw anode dissipation tetrode, grid driven).
>>> I could go for push-pull of just parallel the tube's. I think push-pull
>>> would be nice to get less even harmonics, but seems to be a big hassle and
>>> just put the tube's in parallel is much easier. Or am I missing something ?
>>
>> In my opinion push-pull class "B" amplifiers are the the most overall
>> efficient and stable type of amplifier. The low idling current and
>> easy cross neutralization make common cathode operation very favorable.
>> The disadvantage is the double tuned plate circuit, but the need for
>> the familiar plate RF choke is optional. Band switching a P-P
>> amplifier is daunting, but for single band operation it is sweet.
>> Using a loop of coax as the output coupling (in the center of the
>> center tapped plate coil reduces the harmonic emissions even further.
>> --
>> Ron KA4INM - Youvan's corollary:
>> Every action results in unwanted side effects.
>>
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