The AM Forum
March 28, 2024, 01:12:16 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Will this blocking cap hold?  (Read 10505 times)
0 Members and 1 Guest are viewing this topic.
K3YA
Contributing
Member
*
Offline Offline

Posts: 134



« on: September 23, 2012, 12:43:45 PM »

I'm going through my available choices for a plate blocking capacitor for a new transmitter.  In the past I've had a couple pretty spectacular failures of this component and want to avoid another.  An internet search shows that most of the practical discussion on the issue has taken place right here on AMfone.  So, looking for suggestions.

I've used one of these as a HV bypass at 2500V plate modulated without issue.  And it doesn't talk back much. But blocking capacitors are more stressed.  This will be a 3.3KV plate modulated 75M monoband final. It has a big modulator and I may play with the 3 diode circuit, so the voltages on positive peaks will be up there.  These are about 1 1/4 inch diameter and are a lot bigger then the little TV doorknobs. I'm thinking I can get away with one on 75 meters.
 

I have something huskier, 20KV .001, but it presents some mounting and size issues.


Also have smaller 10KV 250pF mica with the cast end bells, but think that may be a little marginal on the voltage rating.  Anybody with experience in such matters? 

Thanks
Charlie
Logged
W7TFO
WTF-OVER in 7 land Dennis
Contributing
Member
*
Offline Offline

Posts: 2521


IN A TRIODE NO ONE CAN HEAR YOUR SCREEN


WWW
« Reply #1 on: September 23, 2012, 01:15:39 PM »

What you have there look like 15 KHz filter caps from old TV sets.  OK for that job, but I have seen them grenade on RF at 1 MHz.

These are really tough caps:

eBay 320457929812

I have dealt with the seller Vlad before and always had good luck.

These mount with metric threaded bosses on either side.

73DG
Logged

Just pacing the Farady cage...
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #2 on: September 23, 2012, 02:44:12 PM »

3.3 kv X 4 for plate modulated Class C amplifier is 13.2KV peak voltage at 100% modulation. 

Those 30KV caps should EASILY take even big spikes from the rig.


C
Logged
K6IC
Contributing
Member
*
Offline Offline

Posts: 745


« Reply #3 on: September 23, 2012, 04:22:04 PM »

Charlie,

Personally,  I'd call the 500 mmf 30 KV caps,  TV HV Caps,  and would not use them on a big rig.  They might be OK on lower bands ...   But would not bother with them.

The Russian caps like/similar to those that Dernnis referred to seem to be very bulletproof.  QRO Parts sells some of differing values,  depending on what your circuit needs.  RF current can toast caps that are not really designed for RF.

Whatta I know ??!?!.  Good Luck,  Vic
Logged
WQ9E
Member

Offline Offline

Posts: 3282



« Reply #4 on: September 23, 2012, 04:56:29 PM »

RF current and the resultant heating, not peak voltage capability,  is the usual problem with these smaller doorknob caps.  I would go with the suggested Russian caps.
Logged

Rodger WQ9E
kb3ouk
Member

Offline Offline

Posts: 1640

The Voice of Fulton County


« Reply #5 on: September 23, 2012, 06:52:17 PM »

I've heard of people using these with good results.
http://www.ebay.com.my/itm/x1-Russian-Vacuum-Capacitor-VV-200pF-25kV-NIB-/190667134505?pt=LH_DefaultDomain_0&hash=item2c64a59629
Logged

Clarke's Second Law: The only way of discovering the limits of the possible is by venturing a little past them into the impossible
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #6 on: September 23, 2012, 09:01:21 PM »

Using those 50's TV caps at that level is a huge mistake. Clark can use them in his new J-500. Tongue

A 1000pf 857 style ceramic doorknob at 15KV is another good choice, they have a big safety factor and are RF rated. Ive run them at 5KW plus with no problem in an industrial amp.

Quote
3.3 kv X 4 for plate modulated Class C amplifier is 13.2KV peak voltage at 100% modulation. 


2X voltage and 4X PEP power at 100% modulation.

Carl
Logged
kb3ouk
Member

Offline Offline

Posts: 1640

The Voice of Fulton County


« Reply #7 on: September 23, 2012, 10:12:18 PM »

7-8 kv would be the minimum you would need, so 10 kv caps are good. I'd be a little more concerned with the current handling abilities of the cap more than the voltage handling.
Logged

Clarke's Second Law: The only way of discovering the limits of the possible is by venturing a little past them into the impossible
Bill, KD0HG
Moderator
Member

Offline Offline

Posts: 2563

304-TH - Workin' it


« Reply #8 on: September 23, 2012, 10:12:39 PM »

Yup, don't use the old timey TV caps.

RF Parts has a great selection of good blocking caps..Check out their USA Sprague 650 pf @ 18 KV for $10 bucks a pop.

http://www.rfparts.com/caps_ceramicdoor.html#spraguespecial

Bill
Logged
K3YA
Contributing
Member
*
Offline Offline

Posts: 134



« Reply #9 on: September 23, 2012, 10:38:29 PM »

Thanks for the comments.  One of the members of this board is sending me a couple of more suitable capacitors so I think my problem is solved.  

The Russian vacuum capacitor looks quite interesting, but is nearly as large as the big mica cap that I have.  I bet it's a bunch lighter however.  Also it should be quiet.  Some ceramic capacitors can really talk back.

Back in the day, TV doorknob caps were commonly used as plate blocking capacitors and considered at the time to be pretty decent parts.  I've used quite a few successfully over the years, but mostly on 75 or 160 meters.  I still have a parallel pair in my 3CX3000F7 80M amp.  I also have a 20kv TV doorknob that's been doing bypass duty on the cold side of the plate choke of AM 4-1000 for the past 30 years and I think they are fine for this purpose.  In fact I've never had one fail in any of my rigs.  But I repeatedly hear of other people having trouble with them so I would prefer to use something more reliable.  The brown TV doorknobs go back on the shelf.  Wish the mica caps weren't so darn big and heavy.

Charlie
Logged
The Slab Bacon
Member

Offline Offline

Posts: 3934



« Reply #10 on: September 24, 2012, 09:29:56 AM »

Just FWIW, I blew a pair of transmitting doorknobs to smitherines on the initial test firing of my 4X1 rig. They went off like a hand grenade! Out of desperation, to finish up my initial testing, I tried a pair of "TV doorknobs". They are still in there almost 13 years later...............

(YMMV)  Shocked
Logged

"No is not an answer and failure is not an option!"
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #11 on: September 24, 2012, 01:01:39 PM »

My old transmitter had those exact door knobs that are in the photo. Those are not little TV doorknobs and he even posted that they are NOT the little TV jobs.

They worked perfectly and lasted for years.  The new deck that Robert built, used a black square looking cap. Probably from a BC rig. 

I have a box of 570PF 40KV Sprauges here.  Red in color with Yellow writing.  I would not hesitate to use them.  Charlie, If you want a couple let me know.

The one thing I was thinking about when I read his original post was that he had 3300 volts, He did not say anything about a 4-1000 or how much current.  My thought was 500 watt AM transmitter but maybe I was wrong on that. 

Carl, I am not sure what you meant by 2x and 4 times pep.  But my handbook taught you me that you take the plate voltage, Then times it by 4 to get peak voltage. The book suggests 50% increase over peak which would be 19.8KV in his case.  He will go over 100% so 30 to 40K would be overkill.

C
Logged
The Slab Bacon
Member

Offline Offline

Posts: 3934



« Reply #12 on: September 24, 2012, 01:15:38 PM »

Clark,
        Those are still what we refer to as "TV Doorknobs" 500pF @ 30kV was a real commom size used on older TVs. They were made by various manufacturers but were all the same value. I have accumulated a goodly box of them over the years. I've had good luck with them, but others haven't.

I blew a pair of Centralab 500pF's to bits while dummy load testing the 4X1 rig!!
They literally sounded like a blockbuster going off and all that was left of them was the mounting studs and a few crumbs in the bottom of the PA deck! Remembering what the Bird 43 was showing, after I caught my breath, and saw the tube was still lit, I said "this is a good thing! ! !"  Grin  Grin 
Logged

"No is not an answer and failure is not an option!"
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #13 on: September 24, 2012, 01:35:32 PM »

wow.  Some serious smoke frank! I dont run high power here so I guess for me, those would work fine and have in the past.  I never run over 1500 and most of the time, The even the big transmitter never goes over 1000 pep.  But if you are going to blow smoke like that, Maybe get some better caps  Grin, The one in the ebay link does not have enough voltage so I would skip those!

I have the Sprague 715C caps here.  40KV  I have lots of 570PF and a few 960PF.  They look just like this one here:

http://www.mouser.com/ProductDetail/Vishay-Sprague/715C40DKT50/?qs=sGAEpiMZZMtJOOgnx%2fXJmUPXy%2fgnLeV3

Mouser wants $52 each!   If everyone agrees that will work, I will send some off to him for free.  I have 20 of them or more. They came from a Power supply network for a phone system I think... I dont remember exactly.

C
Logged
K3YA
Contributing
Member
*
Offline Offline

Posts: 134



« Reply #14 on: September 24, 2012, 02:27:11 PM »

I have a 40KV hockey puck ceramic blocking cap in my 4X1/833A transmitter.  Not sure of the manufacturer.  I put it in after I blew up a transmitting mica and wanted something heavy duty.  It works fine and stays cool, but talks back like crazy. Like little pizo tweeter!   Replacing it is somewhere on my list of things to do.  Because of this I've shied away from epoxy encased ceramic capacitors on plate modulated transmitters.  But if your Sprague's don't talk back I bet they would do a nice job. 

Probably a good idea to test any ceramic cap for talk-back by rigging it  across the modulated B+ of a transmitter and see if it's noisy before installing it.

I have one of those brown 30kv TV caps as a bypass in the modulated B+ of a transmitter and it's quiet as a church mouse.  I have had some smaller TV caps that were noisy however.
Logged
The Slab Bacon
Member

Offline Offline

Posts: 3934



« Reply #15 on: September 24, 2012, 03:13:05 PM »

They were holding ok with just the dead carrier, but once I socked an atomic YAELO into the mike, all hell broke loose! ! ! ! !  Shocked  Shocked  Grin
Logged

"No is not an answer and failure is not an option!"
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #16 on: September 25, 2012, 03:50:06 PM »

Those TV doorknobs increased in voltages as did the CRT size in those old sets. Once they went to color 40+ KV was used.

I blew a few in the "old" days before I got clued in. The dielectric is High K and rapidly heats and drifts with RF current......a lot more than generated by a 6BG6 at 15kc.

The WW2 type transmitting micas often are moisture contanimated and leaky, I quit using those on the 1 and 2 holer 4X1 amps in the 80's when they started popping and using 4 of the Centralab style RF rated in parallel. Also blew them as padders on the 160M shunt fed vertical. You really have to double up or even go to 4 at 80/160 as the current rating is low down there if running serious QRO PEP. I think Chuck finally went with 4 of the 857 size when I was giving that 4x 8877 amp a lookover and that was SSB. Still plenty of sun and with the 3 el yagi up on that ridge he was causing bedlam in EU.
Logged
K3YA
Contributing
Member
*
Offline Offline

Posts: 134



« Reply #17 on: September 25, 2012, 04:01:28 PM »

I would think that there is very little current passing through the blocking capacitor on low frequencies.  Wouldn't it be the sum of the average plate current plus a very small percentage of the tank circuit circulating current?
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8893


"Let's go kayaking, Tommy!" - Yaz


« Reply #18 on: September 25, 2012, 05:55:22 PM »

Charlie,

I'm a little late to the party here and see you have some good caps coming your way.

But here's my input based on actual experiences:

Those big white mica caps usually work beautifully as coupling caps and plate bypasses, especially on 160-40M. Plus they are plentiful and inexpensive - found at flea mkts.  Several of my KW big rigs and linears have run them over the years without a single failure. Your 10KV unit may be light, but would probably survive due to its quality.

HOWEVER, I am told by a guru friend that they tend to get a little inductive above 40M. I've run them on 20M, but never higher. A simple measurement would show the truth, but I'll bet they are fairly decent.   They were designed for broadcash use, so this is probably why. I have also used them for limited C2 padding, even as much as 1000 pf on 160M. Never had one heat up or blow up. They can handle some serious current and have the mass.

Carl, I see you've had leakage problems with them?  Interesting - none here so far.


As for the TV classic 30KV 500pf small door knobs...  They might survive in a 2000v  813 rig, but when we get to a 4-400 or 4-1000A class plate modulated rig it's very risky.  I've had a few blow apart over the years. They also like to sing under plate modulation if pushed hard. They were designed more for high voltage, thus the current handling capability is low.

Another place to be careful using doorknobs is as padding for C2 loading. Circulating L/C current can heat them up and then pow. I've seen this done quite a bit in marginal amps sitting at flea markets.

Funny story: I have a ham buddy who built a 4-1000A linear with about 3KV.  He didn't have a plate tuning capacitor, so figgered he's put in a stack of door knobs with a rotary bandswitch. Can you imagine the circulating current between these doorkobs and the plate inductor during high power resonance?    I warned him not to do it, but he wouldn't listen.  The explosion was deafening.  Shocked  It took him a day to clean all the crap out of the insides of his amp. He cracked the glass of the tube's chimney but the 4X1 survived.  

Of course it wasn't the TV door knob's fault - they were not designed to handle big circulating current...


BTW, there ARE some caps that look similar to door knobs that are of high mica quality and work pretty well for RF. But they are usually rated at 1-5KV and have a good current capability for their small size.  It's easy to recognize these better caps vs: 30Kv TV door knobs at the flea mkts.

T
Logged

Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #19 on: September 25, 2012, 06:58:25 PM »

Quote
Carl, I see you've had leakage problems with them?  Interesting - none here so far.


Tom, I meant the 5KV size brown ones that everybody was using back then. Dont have any handy to take pix. Never tried any of the big BC ones but might have a few somewhere.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.048 seconds with 18 queries.