[ARC5] ARC-5 on 20 meters

[Fuqua, Bill L]

  A better way to make the output a doubler is to loose the neutralization capacitor and operate the grids of the 1625's in push pull but retain
the parallel plates. You then have a class-c push-push doubler. The fundamental and third harmonic will cancel out and you will also have
better efficiency.
  73 Bill wa4lav
________________________________________
From: ARC5 [arc5-bounces at mailman.qth.net] on behalf of Dave Merrill [r390a.urr at gmail.com]
Sent: Thursday, December 29, 2016 2:55 PM
Cc: ARC-5
Subject: Re: [ARC5] ARC-5 on 20 meters

On Thu, Dec 29, 2016 at 12:49 PM MICHAEL ST ANGELO <mstangelo at comcast.net<mailto:mstangelo at comcast.net>> wrote:

I searched for ARC-5 in the QST archive and found an article in the June 1948 issues of QST "ARC-5 Transmitter Modifications".


Even though the title says ARC-5 the article discusses two ways to modify the BC-459-A for 14 MHZ (or as they said back then 14-Mc.).


Both methods involve modifying the output to a PA doubler.


The first  mod is simple but you lose the PA and Oscillator condenser (capacitor) tracking. remove the C67 padding cap and C65 PA tuning cap from the circuit and replacing with a 75 uufd (pF) variable.


The second mod preserves tracking and improves doubling efficiency.


- Remove the C67 padding condenser and replace with a 35uufd variable.

- Remove 10 rotor plates from the PA tuning condenser.

- The plate coil is pruned down to 5 1/2 turns.


Plate-circuit tracking is adjusted with the new variable and by positioning the top turn on the coil.


To increase the doubling efficiency


- The tap of the grid leak R74 and grid leak bypass condenser C58 is moved  to the bottom of the grid coil.

- The  value of the grid leak bypass condenser is changed from 0.05 ufd to 0.00025 ufd.

- Remove the fixed neutralizing condenser C62.


Mike N2MS

The 1949 edition of Hints & Kinks has three articles on modifying SCR-274N transmitters for 20 meters.  The first, written by William Orr W6SAI, uses a 6AG7 doubler feeding a single 1625 configured as a doubler on 20 or an amplifier on 40.  He starts with either a BC-457-A or a BC-696-A, very extensively modified.  This article is too big to attach due to reflector size restrictions.

The other two articles by W8ZGO and W1DX are attached.  They both modify a BC-459-A.

73 Dave N9ZC