The AM Forum

I hope to get my Desk KW going this summer. I have managed to find three PL-175A pentodes. I wonder if they would work better in the transmitter than the 4-400A tubes?

Is the PL-175A a pentode or a tetrode?

http://www.nj7p.org/Tube4.php?tube=PL-175A

Picture (http://home.comcast.net/~n6jv/pl175a.html)

This source says it's a pentode.

http://home.comcast.net/~n6jv/pl175a.html (http://home.comcast.net/~n6jv/pl175a.html)

It has five pins just like a tetrode. You would think it would require one more pin?

IIRC the supressor is tied to the filament inside of the tube so they could be a drop-in replacement for 4-400s in AB1 or AB2 grid driven service.

Just FWIW, I do get a little more power out of my Invader 2000 with them vs using 4-400s.

I think Frank is correct about the suppressor tied to the filament but none of the Johnson (or other schematics) I am aware of show this feature.  The tube was also used in the later Thunderbolt amplifier and is a drop in for the 4-400A.

I wonder however about using it in a plate modulated amp if there are any differences in operational performance?

With the Desk KW, it pays to check/clean the intake fan screen (on the RF deck grid compartment fan) yearly as dust will buildup and reduce airflow.  After one of the fans died in the RF deck I used one of the pedestal fan motors for replacement and then replaced both pedestal fans with external mount units and the bottom (intake) fan has a filter to reduce dust buildup in the Desk KW.

I also added an external screen grid meter which allows you to monitor screen current and also calculate actual input since the current meter in the Desk KW reads total cathode current.