[Johnson] My Viking II

[Gary Schafer]

Climber _ wrote:
> 
> Concerning your apparent low modulation levels: I am not sure what the 
> problem is in your specific rig, but be advised that the Viking IIs had 
> a serious shortcoming: There was just no way to 100 percent modulate the 
> things. This is because � even though Johnson CLAIMED the 807s 
> modulators were running in AB2, the fact is that if you ever tried to 
> drive them into grid current, the whole system would fall into 
> distortion. Hence, they are capable of AB1 only. I discovered this on my 
> own, but I later saw a copy of Electric Radio where the author had made 
> the same determination.
> 

One of the problems with the audio is the modulation transformer turns 
ratio. It does not load the 807s enough and they go into limiting before 
enough audio is developed. This was done intentionally to provide a 
degree of high level speech clipping. Also there are grid resistors that 
limit grid current. The grid supply is not regulated nor is the screen 
supply. However in the original transmitter, I do think that the tubes 
can be driven slightly into grid current on peaks. This works ok as 
there is enough capacitance in the grid circuit to keep the bias fairly 
constant. But it is not near good enough for serious AB2 operation.

807s need to run into AB2 to get enough audio out. Unlike some other 
type of tubes, like a 6146, that can be run in AB1 and get almost as 
much out as AB2 provides. There is a big difference in output of the 807 
between AB1 and AB2.

73
Gary  K4FMX