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Author Topic: How big is your blocking capacitor?  (Read 27796 times)
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KM1H
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« Reply #25 on: May 23, 2011, 10:13:40 AM »

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The discussion has raised a few questions in my mind. 1. What is the minimal plate choke inductance recommended for 160 meters? My guess would be about 200uH.

Ameritron uses 225uH while most others are in the 100-150 range. Alpha copied the Ameritron in their current products.


 
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I know that a B&W 800 is not recommended for 160 and I think it is either 90 or 100uH.

The 800 is for 80-10. When the 160 rules for 1500W were allocated the 802 came along at 110uH and a higher current rating.

Quote
  2. For a medium powered RF deck running 300-500W with plate voltages of 1500-2000V, is a pie wound or single layer plate choke preferred, or is either OK?.

Both have series resonances that must be determined before usin. A piwound resonance is usually lower making them a poor choice above 40M. An exeption is the old National R-175 and 175A, the latter allowing 15M.


 
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3. I have some Sprague 500pf@20KV "doorknob" capacitors but they are not ceramic. They have what looks like a molded plastic shell, and do not look as high quality as true ceramic doorknobs. Are these caps ok to use as coupling/blocking caps? Furthermore, although these caps are all marked 500pf, they actually measure from about 350-450pf. Does this suggest a fatal problem, or are these most likely OK to use?
Thanks, Ron

Those are the classic TV doorknobs used in all sorts of ham equipment, tolerance is all over the place They worked fine at power levels up to 400-500W or so and could be paralleled for higher power but they are far from RF rated. Try one and if it doesnt overheat you are OK.

There are other doorknob type caps used in ultrasonics and other industrial applications. Heath used a 1000pf @6KV in their SB series of amps that have held up well over the decades but there are signs they are slowly failing.

Carl
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« Reply #26 on: May 23, 2011, 12:57:02 PM »

Carl,

It's interesting to see what the commercial linear amps use in the way of plate chokes on 160m, but surely they are working at a much lower plate impedance than Ron's class-C amplifier?

I make the optimum load for his PA something greater than 4.5kohm, whereas I'd have thought the Ameritron and Alpha linear amps would have been working around the 1.3kohm region. The inductance of his plate choke needs to be higher.

The RF component of the current through the plate choke is determined by the plate voltage swing and the reactance of the choke at the operating frequency. A useful reference is the inductance (Lc)that gives a peak RF current equal to the DC component. That's easy to work out and everything can be compared to that to judge performance fall-off.

For a class-C amplifier, Lc = 0.3 x Optimum Load/Fo, where Fo is the operating frequency. The Lc value for Ron's amp will be about 833uH. I try never to go below 0.33Lc, but 225uH would be 0.27Lc. It'll work, but the losses will be a bit higher than 0.33Lc. They may still be acceptable though.

By comparison, Lc for a linear like the Ameritron would be 185uH and so 225uH is 1.2Lc. Even 110uH would be about 0.6Lc.

In case you're interested in linear amplifiers, Lc = 0.25 x Optimum Load/Fo for Class AB or B.

I don't have any experience of American TV doorknob capacitors so my comments won't be very useful. I've had bad experiences with British ones, though, and stay away from them. Anyone wanting ceramic RF capacitors should look at qro-stuff on eBay. His prices are reasonable and although he's in the Ukraine, he has a contact in Texas for shipping within the United States. A friend of mine bought some 3300pF 10kV strontium titanate RF capacitors from him which had a current rating of 25A and they had an ESR of 0.094 ohms at 3.6MHz. Not bad for $10 each.
 
Dave.


   
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« Reply #27 on: May 23, 2011, 01:16:50 PM »

The common TV door knob caps:

Many of us have had poor experiences with them. They were designed for high voltage and low current, so will certainly blow up in loading/50 ohms C2  padding service. They also "talk back" when used as plate coupling caps under heavy plate modulation. And, NEVER use them in high power tank circuits where the Q*I circulating currents can make them go off like cherry bombs.

TV door knobs and similar caps should be avoided for use in higher power service.

However, there are plenty of good quality caps from the Russians on the web and other sources. Be sure they are rated for high current. It's not easy to tell from the outside, but physical size is a good indication.

For plate chokes, the pi-wound chokes should be used only for lower power rigs, like <150 watts plate modulated, in my opinion. Thye will implode due to the close proximety of the turns to each other when high RF voltage/current is present. I like single layer wound plate chokes best.  We have had threads about winding your own using #24 or #26 insulated wire here.

T
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« Reply #28 on: May 23, 2011, 01:41:56 PM »

Tom,

I'm amused by your idea of low power. We have a power limit of 100W of carrier on 80m through to 10m. 150W is high power for us. On 160m our limit is 8W of carrier above 1.85MHz where most AMers hang out. You guys just don't know how lucky you are.

In the west we've always had a poor opinion of Russian components and their reliability, but some of the RF stuff coming out of Russia and its former satellites these days is excellent. I've been quite impressed by it.

Yep, there's no reason why you shouldn't run with a single-layer plate choke down at 0.25Lc or so. It just means you need to use very low loss capacitors with a suitable current rating in both the coupling and HV decoupling positions to avoid increased RF losses compared with higher values of plate choke. It ain't impossible though, it just needs very good caps.

Dave. 
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« Reply #29 on: May 23, 2011, 01:59:02 PM »

Dave,

Heheheh.... yep, it's all relative and we are fortunate to have reasonable AM rules here.  Though, listen to some of the UK stations down in the ssb 75M DX window and some of the contesters and draw your own conclusions...

Pictured below you will see a sample of the plate chokes I wind for my big rigs. This happenes to be an older 4-1000A modulated by a pair of 833A's. I build them with Plexiglass for viewing pleasure.  

I just wind a choke and try it out on the bands 160-20M. If it doesn't blow up, there are no resonances. If it does, I add or subtract some turns and try again. It's actually difficult to randomly wind a choke that falls into a small ham segment by luck.

Yes, the Russians have gotten their act together. They certainly are talented engrs when they want to be.

T


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« Reply #30 on: May 23, 2011, 02:27:42 PM »

Hi Tom,

Well, there's ham radio and then there's DXing and contesting. If someone has to lie and cheat because he absolutely has to be the strongest signal on the band, then that's up to him. I'm saddened by the way "sport radio" has come to dominate amateur radio, but its certainly not progress in my book.

The amp looks impressive. I have some 4-400 tubes somewhere, but not being that interested in SSB any more I haven't done anything with them. One would loaf along at our SSB power limit.

I can appreciate that some people prefer the empirical approach. It's just that I like to get to the bottom of everything and understand it, so I develop little theories for this and that to help me find the optimum values for a given design. In the case of the plate choke, of course, there really is no single optimum value and a whole range of values can give quite good results. They all have different resonances, of course, and that tends to determine the "best" value for multi-band use in the end, so my little theories probably aren't much help to many folks apart from me.

I was writing to Ray, the editor of ER, when an email came in from the forum about your post, so I'd better get back to that.

73,

Dave.
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« Reply #31 on: May 23, 2011, 03:45:49 PM »

Id make a 2000V@ .25A a 4K Rp using your countrymans spread sheet which has been my go-to source for a few decades Dave.
http://www.ifwtech.co.uk/g3sek/

Be it C or linear the typical 1200-1500W amp has a Rp of ~2K or lower.

Im busy taking a gas tank out of my sons car today so my comments here are hit and run until after dark.

Carl
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« Reply #32 on: May 23, 2011, 04:17:44 PM »

OK Dave, Carl, Tom, Thanks for the comments. My current coupling/blocking cap is a large, black cylindrical Sangamo cap that came out of a 5K Gates broadcast transmitter. It must weigh 2lb or more. The end terminals are 3/8 inch machine screws. I'm not home right now or I'd put up a picture of it. It is rated 500pf@10KV, 10Amp RF current@1000kc, so I'm sure it's up to the modest task currently assigned to it. I was concerned that 500pf might be a bit meager for 1.9mHz, but from the comments, I think it should be usable.

Regarding the plate choke, I am currently using a high quality 5 pie National unit that was designed for amateur band use. I don't know the model number, it's not a 175 though. By it's physical appearance, I would guess it to be at least 1-2mH and 500mA or more. I do have a B&W 800, and another similar single layer plate choke. I suppose I could connect these in series - that works - right? If I do this, I should put a bypass cap between them, as well as one on the power supply side of the first choke in the supply line - right?

Ron
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« Reply #33 on: May 23, 2011, 04:32:27 PM »

OK Carl, I wondered where you'd got to today. I'm not going to quibble about 500 ohms but the plate load depends greatly on conduction angle in class C and between 150 and 90 degrees there can be more variation than that. I don't know what G3SEK's spreadsheet is based on but I prefer to trust Terman's "Radio Engineering."

Ron, sounds like you've got the National R-154 or R-154U. Either way, that's a 1mH choke and should do fine on 160-40m. That's 1.2Lc!!!

Good luck with the project,

Dave.
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« Reply #34 on: May 23, 2011, 04:43:41 PM »

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My current coupling/blocking cap is a large, black cylindrical Sangamo cap that came out of a 5K Gates broadcast transmitter. It must weigh 2lb or more. The end terminals are 3/8 inch machine screws. I'm not home right now or I'd put up a picture of it. It is rated 500pf@10KV, 10Amp RF current@1000kc, so I'm sure it's up to the modest task currently assigned to it.

Those caps are FB, Ron. I have used them (and still do) in many big rigs as both plate bypass and coupling caps. Never had a problem. They are overkill for your intended use.

The current rating goes up somewhat, to a point, as the frequency is increased above its 1 mhz rating.

BTW, a friend who knows RF said that those particular type caps start to look inductive above 40M or so. They were designed for use in BC rigs, so you can understand. I have used them on 20M, but have always used more RF-friendly ceramic caps above 20M. Just something to consider if you have problems later on the higher bands.

Yes, you can always use chokes in series, though remember that the more complex the choke, the more chances for possible combinations of resonances on the higher bands.  I would opt to use a smaller choke at the plate circuit  and then a larger one for the  lower bands behind it, if you need to series them. It's almost like the technique of winding smaller turns groups at the top of a choke, etc.

Adding a bypass cap between two chokes is OK, as  long as the first choke has enuff inductance for the lowest band. Worst case example wud be to have a 20uh choke first and a bypass cap to ground, then the bigger choke. It may be OK on 10M, but on 160M it wud be an RF short to ground. (Or, as Carl suggests, use a relay to switch the bigger choke in for the lower bands)
T
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« Reply #35 on: May 23, 2011, 08:46:18 PM »

Dave, I agree about it not being worth a quibble, a pi-net set up either way will tune the other with no apparent difference.

Here is a nice little rig using a R-154.

Carl
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« Reply #36 on: May 24, 2011, 04:40:21 AM »

Ron,

Looks like you're all sorted out and ready for 160m now. That Sangamo cap from the Gates rig sounds like a monster. The 500pF capacitance will only drop your efficiency on 160m by 2% compared with my recommended value of 735pF for the coupling cap, so your power output will probably only drop by 8W or so compared with 75m. With a 1mH plate choke, I make the effective rms current through the coupling cap less than 0.5A, so it is going to be overkill by some margin but if you haven't got a better use for it .........

I agree with Tom that you have to be careful about the inductance of internal capacitor connections and the external leads you use. You need to try and choose suitable values of capacitor that series resonate with the leads at the very top end of the frequency range you want to cover. That way they never become too ineffective as capacitors.

Tom's already commented on the use of an extra choke in the HV lead, so there's nothing more for me to add.

Dave.
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« Reply #37 on: May 24, 2011, 09:47:31 AM »

I forgot the link yesterday!
http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2003%20No%205.pdf

If using 2 chokes run them at right angles, most use a vacuum relay to switch in the lower one as its nearly impossible to cover 1.8-30MHz without a nasty resonance at least for a QRO amp. Henry does this with their "big" amps.

Others have found with homebrew using tubes with handles that forgetting to switch in the lower choke for 160/80 the amp still worked OK as long as a big bypass was in the circuit to keep RF out of the electrolytics. The power loss wasnt enough to smoke the 40-10M choke.

Carl
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« Reply #38 on: May 24, 2011, 11:26:55 AM »

Good idea about using the relay to switch in the lower band choke and possibly a bigger bypass cap, Carl.  That way we can optimize parts for the  6M-20M zone , if desired, and still have large values available for the 160-40M areas.

As you said, getting a fixed-value QRO amp to function from 160-6M is quite a task, though it can be done. 

Interesting on the 8 watt difference, Dave. Looks like 500pf is surely the lower limit in Ron's case.


Well, I've reached the limits of my knowledge on this subject, so we be black out... Wink

T
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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.
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