[Amps] Push pull amps

Sun Aug 2 22:00:50 EDT 2020

An interesting note. Split capacitor Plate Neutralization was very common for triode tubes even if they were single tube amplifiers.
A few did use " grid neutralization", a good example was the BC-375 which used taps on the grid tank coil. But E.F. Johnson also used grid neutralization with two triodes in parallel and a PI output network. It was the Courier Amplifier with 2 811A tubes. One problem with neutralized triode single ended tube amplifiers it that generally they had to be tuned with the plate or grid tuning capacitor in nearly the same position on each band. Unless, a small capacitor was added to one side of the split tank capacitor to account for the tube's interelectrode capacitance on the other end. Otherwise tank circuit would not be balanced throughout the tuning range. Note the small 10pF capacitor on the grid tank circuit in the Courier. Most triodes have very little Plate Cathode capacitance with few exceptions such as the 833. So is not a problem for most plate neutralized triode amplifiers.
  A little off subject but, I once had a BC 653 transmitter with two 814 tubes in the final. The 814 was either sort of like 2 807's or half a 813. It had only two components in the plate tank circuit. A capacitor to ground (plate tuning cap) and a large adjustable multi-tap coil. The coil was in series from the Plate tuning cap to the antenna terminal.
   It was sort of a L network to match a whip antenna which was electriclly short thus Capacitive, That antenna capacitive reactance completed the PI network.

73
Bill wa4lav

________________________________
From: Amps <amps-bounces at contesting.com> on behalf of Carl <km1h at jeremy.qozzy.com>
Sent: Sunday, August 2, 2020 4:51 PM
To: sm0aom at telia.com <sm0aom at telia.com>; amps at contesting.com <amps at contesting.com>
Subject: Re: [Amps] Push pull amps

CAUTION: External Sender

Cross netralization is often a single band afair with plug in coils and home
construction.

Parallel is far easier since the input can be swamped to burn off exciter RF
as well as eliminate the need to neutralize.

Perhaps the primary reason for parallel is the ease of bandswitching, the pi
network, and matching directly into that new fangled coax

Carl

----- Original Message -----
From: <sm0aom at telia.com>
To: <amps at contesting.com>
Sent: Saturday, August 01, 2020 5:34 PM
Subject: Re: [Amps] Push pull amps

> Easier cross-neutralisation has been mentioned, but another major argument
> for push-pull,
> especially at higher frequencies, is that it becomes easier to get the
> proper L/C ratio
> as the circuit tuning and stray capacitances essentially are in series.
>
> 73/
> Karl-Arne
> SM0AOM
>
> ----Ursprungligt meddelande----
> Från : k2vco.vic at gmail.com
> Datum : 2020-08-01 - 20:08 (CEST)
> Till : amps at contesting.com
> Ämne : Re: [Amps] Push pull amps
>
> I think the main reason they liked putting triode amplifiers in
> push-pull was to simplify the neutralization. Of course it is possible
> to neutralize single-ended amps with a bridge circuit or to use
> inductive neutralization, but I think it's easier to obtain complete
> neutralization with a push-pull circuit.
>
> The change came about with the popularity of tetrodes which are easy to
> neutralize in single-ended configuration, the popularity of pi-network
> output circuits, and the desire to get away from plug-in coils (think
> about bandswitching a balanced tank circuit). Also, TVI shielding was
> difficult with plug-in coils.
>
> I think probably the change happened after WWII, when surplus coax
> became available. Shielded, single-ended circuits with pi-networks
> running into coax-fed antennas became the hot setup.
>
> 73,
> Victor, 4X6GP
> Rehovot, Israel
> Formerly K2VCO
> CWops no. 5
> https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.qsl.net%2Fk2vco%2F&data=02%7C01%7Cwlfuqu00%40uky.edu%7C854abadd91d14c33108b08d83726d9e8%7C2b30530b69b64457b818481cb53d42ae%7C0%7C0%7C637319987314958366&sdata=W686CVduqXYlZpiMrhlFR7fPhzGAOzoDioF7%2F3vsRSM%3D&reserved=0
> On 01/08/2020 20:02, Tom Osborne Sr. wrote:
>> I recently purchased some QST's from '47, 49, and 50.  I notice a lot of
>> transmitters and amps back then used 2 tubes in push-pull circuits,
>> instead
>> of parallel, like we mostly do now.
>>
>> What was the idea behind push pull compared to parallel, and when did the
>> change take place?  Thanks and 73
>> Tom W7WHY
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