[Amps] need 1000pf blocking cap

[kk2ed at comcast.net]

Is the 15kv version rated for a higher current?  The RF Parts web sites says one thing (same as the 5 or 7.5kv versions), while my paper copy catalog from them says its higher (3x higher). Which is correct?

What about my concern of RF current?   Is it substantially higher than the DC current present at the plate choke?  How does one go about determining the level of RF current?  Can it be derived from Ohm's law - I=P/E  ?  Or does something else come into play?   If its simple Ohms law, then the cap is probably passing approx. 2a  (5kw/2700v), which SHOULD be ok for the HT-50's rating, but I thought I remembered reading that RF current is a little different.

Thanks for the tips on the soldering and vacuum relay. I check those first. All solder connections looked good and passed resistance tests. Same for the relay; I energized it without RF present, and it passed continuity tests in NO/NC positions.   The fact that the variable caps (LOAD, TUNE) had no effect seems to solidify my suspition of the doorknob cap.

Eric
KE2D



-------------- Original message -------------- 
From: "Carl" <km1h at jeremy.mv.com> 

> Its certainly at the extreme for that cap. 
> 
> The HT-57 comes in 7.5 and 15KV ratings. They are identical caps simply 
> marked different to reflect different safety margins. 
> 
> Ive seen many LK-800's melt the solder or crystalize it at the switch 
> contacts. Suck it all out and reflow new. Same at the relay and SO-239. Use 
> an ohmeter to verify the path. Ive also replaced several open vacuum relays 
> in those amps. 
> 
> Carl 
> KM1H 
> 
> ----- Original Message ----- 
> From: 
> To: 
> Sent: Friday, July 25, 2008 10:13 AM 
> Subject: [Amps] need 1000pf blocking cap 
> 
> 
> > Good Morning; 
> > 
> > I'm helping a friend repair an Amp Supply LK800 3x3CX800A7 amp which 
> > appears to have a bad plate blocking/coupling doorknop capacitor. It's an 
> > HEC HT-50 style 1000pf @ 5KV , X5V temp rating. 
> > 
> > The symptoms are as follows: Varying drive power varies plate and grid 
> > current, yet the load and plate tuning have no effect on readings, and 
> > there is no RF output at the antenna connection. The vacuum relay and all 
> > connections back to the blocking cap check good. I'm guessing the cap 
> > failed to an open condition. 
> > 
> > I have a box full of brand new HEC HT-50 style 5kv caps, from 50pf to 
> > 250pf, but nothing as large as 1000pf. There isn't enough room between the 
> > plate choke and tuning network, otherwise I'd fabricate four of the 250pf 
> > units together to reach 1000pf. 
> > 
> > Anyone here on the list have a 1000pf 5kv or greater cap for sale, or 
> > trade for my smaller value ones, before I place an order? 
> > 
> > One more question - he is pushing the hell out of this amp, to the point 
> > where he is easily reaching 2x plate dissipation. The amp had a larger 
> > Dahl transformer installed (3KV plate voltage), and plate current is 
> > around 2A+ when he pushes it hard, so I'm wondering if the cap blew from 
> > being stressed passed its limits.... According to the chart in the 
> > latest RF Parts catalog, the 5kv cap is rated at 5.6a @ 30MHz, but I 
> > recall reading in the past that the RF current is actually much higher 
> > than the calculated DC current, no? 
> > 
> > Should I just upgrade the cap to an HT-57 15kv 1000pf cap, which shows in 
> > the catalog as having a higher current rating? 
> > 
> > Also, he has the original stock LK800 transformer for sale if anyone is 
> > interested. If I recall correctly, after the diode and cap board, it was 
> > generating approx. 2700v no load, and 2400v loaded. If interested, contact 
> > me and I'll forward his contact info. 
> > 
> > Eric 
> > KE2D 
> > _______________________________________________ 
> > Amps mailing list 
> > Amps at contesting.com 
> > http://lists.contesting.com/mailman/listinfo/amps 
> > 
>