Neutralizing Cap

[K9ACT]

Going to two tubes on my 8000 means that the neut cap has to be twice the value.

According to the handbook, I need a spacing of .250" for 1600 VDC AM.

Using the cap that I made for the single tube requires a spacing of about .1" to get to the needed value (about 17 pF).

This can be seen in the pic attached and to get there with this design, I have to make another one about 5" in diameter.  This would be a rather big klunky beast and it seems like there should be a better way.

The bias supply has been moved under the chasis and the new tube is where that was in the pic.

What do people use for N/C's for this application?

js


p.s.  I don't have much luck attaching pics to these messages so if it's not here, go to http://schmidling.com/rfd4.jpg

js

[KB2WIG]

some use a treaded rod placed  near the tube(s) to be nutered; use a ceramic feed through insulator and raise the rod up/down to change the capacitance....  or so i'm told...  Thats what i'm gunna do with my 813/810 maul...   klc

[K9ACT]

some use a treaded rod placed  near the tube(s) to be nutered; use a ceramic feed through insulator and raise the rod up/down to change the capacitance....  or so i'm told...  Thats what i'm gunna do with my 813/810 maul...   klc

Not sure I can picture that but it sounds more like a "gimmick" cap for a tetrode.  You need a serious cap for big triodes, especially in parallel.  In my case, about 16 puff.

js

[KB2WIG]

yes, ur right...........  I'M doing a PP 813 driven by PP 810 mudulators ... the brain (if its still in there) got it backwards. Anyway  good luck, gud night ............  klc

[WD8BIL]

Hi Jack....... are these tubes in PP or parallel ?

Tom's parallel 813 rig uses a nifty neuter setup.

Check it out here.

813s X 813s

[K9ACT]

Hi Jack....... are these tubes in PP or parallel ?

Tom's parallel 813 rig uses a nifty neuter setup.

Check it out here.

813s X 813s

Again, that is for tetrodes that are not supposed to need neutralizing in the first place.  I do not understand quite how it works anyway.

I solved the problem in a way that I do understand.

By coincidence, (Mack) I picked up two small NC's at the last hamfest, only because of all the work it took to make mine.  They are two small to do anything with but I was able to disassemble them and put it back together on one mount in parallel.  They are now about 6 pF with .25" spacing.  This in parallel with the original at .25" gives me just what I need.  A bit Rube'ish but it works.

Pic attached....

Gonna fire it up after my coffee.

js

[WA1HZK]

Big Triodes, Big cap. I ended up with around 10 pf. on a trimmer that had the same spacing as the final plate tuning cap in my 833 rig.
Keith
WA1HZK