The AM Forum

OK, I'm getting down to the gritty-nitty.  Got this loading cap today.  The gap looks to be .031 and it's about 1100 pf.  The rotor is really free-wheeling.  It's tight front-to-back along the axis, I don't want to mess with its tension so I'll need a vernier.  That was in the plan anyway.  With 2KV on the plate of the single 813, my Tune cap is double-ganged... 150 x 2 @ 4500 volts.  I'll include a HV switch to short the 2 gangs together.  Tank is Air Dux  So far so good.

Question:  I'll need to switch in some fixed padding caps across the loading variable.  I calculate a max of 2500 pf would do the job.  To be safe, what voltage would you recommend for the extra 1500 pf and should they be mica?

With 400W into 50 Ohms the peak voltage is 200V. With 150 Ohms it would be 350V. So I think a 500V Silva Mica would work great. Available from Mouser for about $4.

- Paul

You might find this document helpful:  Not real easy to read, but lots of info on the 195-1 and 195-2.

http://bama.edebris.com/manuals/illumitroniceng/airdux

OK.  I'm not great with math.  I wanted to be an astrophysicist at one time.  Fail. I found a source for some 560 PF micas at 1KV.  I can get 8 of them so I can come up with a combination to hit the 2500 pf in steps with a SP5T military ceramic switch with room and poles to spare.  I'm sure this is a 195-1 and should be good for 400 watts input.  The #2 is silver-plated and good for up to a KW.  SHOULD THE FIXED CAPS BE MICA?

This is what we use in our legal limit finals.  You won't anything else

https://www.fairradio.com/catalog.php?mode=viewitem&item=969 (https://www.fairradio.com/catalog.php?mode=viewitem&item=969)

This one is over 2500 pf and it was my original choice.  I'm just not sure if the plate spacing is enough.

One millimeter is 1kV for a spark gap, normally a capacitor from an BC radio is good enough for 500V of Voltage.
This means that under carrier conditions (e.g. 1/4 of power) 250V over 50 ohms is (sqrt)2.250= 177.5V (eff)2/50= 630Watts.
For higher voltages means more spacing.

73'Martin

It's working for all the class E guys out here that has to handle over 1500 PEP.  Now if you are not going to nominalize to ~ a 50 ohm Z and you are at a voltage max on your VSWR there might be a problem.  But the OWL guys normally use link coupled tuners and the coax guys use "T" matching tuners.  That cap should not be a problem especially using a single 813.

I have not had a single arc with my 1500 PEP AM station.

From practical experience, I've found those multi-section 365 caps to be good for about 2500 watts pep (extreme limit) on 75M with a matched 50 ohm antenna load before starting to have occasional arc-overs on voice peaks.  Voltage arc overs will depend on the humidity, antenna impedance and the frequency, with lower ratings as frequency increases and as matched impedance increases.

* This equates to about a 500 watt AM carrier output into 50 ohms.

In general, they are a good all-around 1500 w pep condenser. Some have wider spaced plates than others, so get the widest available. I used to use one on the output of my 8877 linear. But since then I use vacuum variables and turns counters for all KW class stuff.

T

Yes, 1500 or 2000 watts pep is no issue with those loading caps.

The absolute max input I'll be running is 400 watts...probably closer to 350 AM.

That's what I mean about 600w carrier is approximately 2400w PEP.

For 400 watts input, assuming 75% less output = 300 watts out.  See attached calculation for PEP output.

Maybe I'll get one of those Fair Radio jobs.  As long as they are proven and cheap, why not?  As I said in other threads, I'm building this on the KISS principle and the "S"er,the better!