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Re: [Amps] Push pull amps

To: Carl <>, "" <>, "" <>
Subject: Re: [Amps] Push pull amps
From: "Fuqua, William" <>
Date: Mon, 3 Aug 2020 02:00:50 +0000
List-post: <>
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 

Bill wa4lav

From: Amps <> on behalf of Carl 
Sent: Sunday, August 2, 2020 4:51 PM
To: <>; 
Subject: Re: [Amps] Push pull amps

CAUTION: External Sender

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

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


----- Original Message -----
From: <>
To: <>
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
> ----Ursprungligt meddelande----
> Från :
> Datum : 2020-08-01 - 20:08 (CEST)
> Till :
> 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
> 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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