[Milsurplus] [MMRCG] Fixed Screen Voltage in Plate Mod. Transmitter PA?

[Ray Fantini]

C45 @0.001 is fairly dinky, would assume that the modification is at the junction of screen dropping resistor (R26) and the input side of the filter R35-C45 that’s on the plate of the clamp tube. Do you know what they increased it to? Figuring that the input to the screen grid is fairly high do not think it would take much. Maybe something like 1.0 or 0.33 @600 volts. Good place for an Orange Drop cap, they always make everything sound better.

Ray F/KA3EKH


From: arc5 at ix.netcom.com <arc5 at ix.netcom.com>
Sent: Thursday, October 8, 2020 11:38 AM
To: Ray Fantini <RAFANTINI at salisbury.edu>; MMRCG at groups.io; milsurplus at mailman <milsurplus at mailman.qth.net>
Subject: Re: [Milsurplus] [MMRCG] Fixed Screen Voltage in Plate Mod. Transmitter PA?

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One of the Johnson factory mods for the Viking II was to install a bypass cap in the Clamp circuit specifically to keep audio out.

------ Original message------
From: Ray Fantini
Date: Thu, Oct 8, 2020 8:51 AM
To: MMRCG at groups.io<mailto:MMRCG at groups.io>;milsurplus at mailman;
Cc:
Subject:Re: [Milsurplus] [MMRCG] Fixed Screen Voltage in Plate Mod. Transmitter PA?


The DX-100 and Viking 2 have a big difference in what they are doing with the operation of the screen clamp, the Viking is a much simpler system just using the 6AQ5 as a screen clamp to protect the PA tubes by sensing drive. If you wanted to experiment with effects of modulation on the screen maybe you can start by looking with a scope at the plate of the clamp, the junction of R35 and R26 and seeing how much audio is present? C45 is the second half of a RF bypass to decouple the screens of the PA from the clamp but you can try experimenting with a larger value there to remove the audio that may be present on the screen bus but be careful because too much capacity will result in keeping the screen high when conditions of no drive exist.

The real question is what's the effect of modulation on the screen of a 6146 in relationship to the output? Perhaps having modulation that in phase with the plate increases the efficiency of the tube?



Ray

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