The AM Forum

Can unused socket terminals 2 and 6 be safely used as tie-points?

Specifically, socket pin 2 would be handy for connecting a bypass cap and RFC on the screen circuit as shown.

Bonus screen question:    I have notes regarding the screen circuit bypass caps.  One note says .001 6kv, and another note referenced to K1JJ's 'Hollywood' rig - to use 500pF here.   Is 6kv really needed?  I can more easily find 3kv ceramic discs.

Steve KK7UV

Hi Steve,

I think a 3KV rating is fine for the screen bypass of a plate modulated 813.  The choke will cause swings,  but I have never popped a 2KV cap there yet.  I HAVE popped a 1 KV disc there, however.   I use 500 uF as bypass values for both screen and plate DC circuits (coupling and bypass) because I have seen audio shunting effects above  ~5KHz or so with .001 uF or bigger.   You will need to sweep the audio ranges when testing and be aware that these caps can have an effect, based on various parameters specific to the rig.  I have even had to resort to a 300 pF bypass in the screen to optimize a rig for extreme high audio frequency headroom..    If the rig remains stable for RF, then you can find a good compromise.  Some rigs will be fine with .001 uF.

As for the tube unused pins, I would not trust any of these unused pins for hanging unrelated parts. You can eyeball it and probably see the clearances, but it can be a crapshoot. There may also be unwanted coupling. I would go the extra mile and install some ceramic standoffs in the best spots for the job and then forget about it... :-)

Good luck with the rig!

T

Thanks Tom.  I'll experiment with the cap value when I get to the audio testing step.

The rig build is coming along nicely.  The HV supply has been done for a while now, the PA/Mod section is under construction right now.  The output tank and meter panels are 95% done.

If the pins do not connect to any elements inside the tube, then yes they can be used as tie points. Octal Tube power supplies have done this for years. I do it on my 813 screen modulated transmitter with no problems.

3kV to 3.5kV caps on the screen bypass is  sufficient.


Phil - AC0OB

Very nice clean looking rig build there!

                      _-_-bear

PS what did you use for mod iron?

Thanks Bear - - iron is Thordarson T11M77.

This is definitely a great looking rig, with controls in orderly positions, and 100% proof that panels to not have all be the same color and finish for a masterly and confident appearance.

Thanks Opcom, 

The white panel is the PA/modulator section and is not finished yet.  It will have several more holes under the meters for various controls, jacks, etc.  It has a white synthetic protective film on it, but will eventually be painted the same gray as the other panels.