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THE AM BULLETIN BOARD => Technical Forum => Topic started by: W8ACR on February 07, 2018, 01:26:42 AM



Title: New home-brew 40 meter RF deck
Post by: W8ACR on February 07, 2018, 01:26:42 AM
I'm back. It's been a few years since I have posted anything in the technical forum, but I'm still loving AM, and I just finished a new home-brew 40 meter RF deck that I wanted to show off. This is a classic push pull triode design, 1940's technology. I'd appreciate any feedback/critique on my design or workmanship. The tubes are HK-54's (thank you Dennis Gilliam) and will be operated at about 1600V@250mA. I just finished it tonight, so I haven't tuned it up yet or had it on the air. That will be tomorrow, and I will let you know how it goes.

The grid circuit is conventional. The grid capacitor rotor is grounded for DC. There are home-brew parasitic suppressors in the grid leads. Both the grid and the plate leads are connected to the tube pins by Fahnstock clips.

The plate circuit is also conventional. The plate coil is home-brew with a fixed link. I didn't have a TVL 40 coil, so I had to roll my own. The primary winding is #12 THHN wire on a PVC coupling. The fixed secondary winding is vintage cloth covered #12 stranded hookup wire, and has 4 turns. I designed the plate circuit for a Q of somewhere between 6 and 10. The plate leads are silver plated copper strap. The plate capacitor rotor is grounded for RF, and is connected to the B+ line through a 27K, 1 watt carbon resistor.

Both plate and grid tuning knobs are on verniers and tune very smoothly. There is metering for plate voltage, plate current, grid voltage, grid current, and filament voltage. Meters are located on a separate deck.

I have some critique of my own work.

1. The neutralization caps do not match. This detracts from the cosmetic appearance, but should not affect the function.

2. The grid tuning capacitor is a bit small. Since I should only need about 30 watts of driving power, I think I will be OK.

3.The grid circuit is not physically symmetric. Since it is underneath the chassis, again, I think I will be OK

Pics are below. I'll get back with more info after I tune it up.


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 07, 2018, 01:47:22 AM
More photos


Title: Re: New home-brew 40 meter RF deck
Post by: N1BCG on February 07, 2018, 09:25:46 AM
Nice craftsmanship! It's good to see projects like this in 2018, particularly those that employ classic or unique components. Hope to hear it on the air!


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 07, 2018, 12:15:41 PM
LOL, I was just looking at the pictures again. I think I'd better connect the grid capacitor to the grid coil. :-[

Ron W8ACR


Title: Re: New home-brew 40 meter RF deck
Post by: WD5JKO on February 07, 2018, 01:14:20 PM

  Ron,

   Wow Ron, that could have been in a late 30'd radio handbook! Nice construction. Yes hook up that grid tuning cap! :-)

   You might find the fixed link on the output tank overly restrictive. A swinging link with a jack bar setup allows for loading to the same plate current for varying load impedances. Years ago I made a P-P AM kilowatt setup, and I had the swinging link along with a series adjustable C (large capacitance) to tune out the link reactance. The flexibility was really useful.

Jim
Wd5JKO


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 07, 2018, 01:51:43 PM
Hi Jim and N1BCG,

Thanks for the comments. Yes, a swinging link is always useful, but alas, I'm fresh out of jackbars and coils. I'm sure I could figure out how to homebrew one if my life depended on it, but this was easier, and I thought I'd give it a try. We'll see.

As I review the pictures, a couple of thoughts come to mind.

1. Maybe I should have mounted the tubes next to the tuning cap with the neutralizing caps on the other side. This would have allowed for shorter RF leads.

2. I could have mounted the grid coil on the top of the panel, in the horizontal midline behind a small metal shield. This would look nicer, and would allow for better symmetry, but the leads to the grid tuning cap would be much longer. I'm not sure if this might cause problems or not.

3. I wonder if it would be better to take the grid leads from the neut caps directly to the grid pins, rather than putting them on the other side of the parasitic suppressors?

I guess I'll find out soon enough if my design works. Just thinking out loud here. One nice thing about homebrewing - you know how it was put together, so you know how to change things if necessary. BTW Jim, could you draw a simple schematic of that series adjustable C? I assume it is in one leg of the output line.

Ron W8ACR


Title: Re: New home-brew 40 meter RF deck
Post by: KB2WIG on February 07, 2018, 06:10:50 PM


The cloth covered wire on your HB TVL 40 coil is strangely appealing, visually. I like the way you reduce strain on your plate caps -   wide conductor/braid....

Nice work, you make me jealous.

klc


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 08, 2018, 08:49:48 AM
OK, so I applied power to the RF deck last night. Flipped the switch, no smoke after five minutes, good. Fixed bias showing -150VDC, good. No grid or plate current, good.

Applied RF at 7.250MHz, grid cap tuned for peak grid current, good. Adjusted the variable grid leak for 40mA to give -260VDC, good.

Applied Plate voltage, start with 500VDC, dip the current, good. Disconnect plate power supply and adjust the neut caps for minimum indication on the wave meter, good.

Increase plate voltage to 1000VDC, uh oh, plates are bright orange. With 1000V @ 250mA, I'm showing 125W output, so everything seems to be working except my fixed link plate tank coil not matching output impedance (my guess). Don't think it is a parasitic since plate current is not running away.

My solution - I'm building a home-brew swinging link output coil. Will update later.

73, Ron


Title: Re: New home-brew 40 meter RF deck
Post by: W2PFY on February 08, 2018, 03:58:47 PM
Quote
I'm building a home-brew swinging link output coil. Will update later.

Hi Ron, great to see you at it again! You really don't need a swinging link unless you want to maintain the old buzzard appearance. One one end of your link just install a variable capacitor in series to ground. That's what I have done in the past on my 610. I just purchased a PP T-55 triode amplifier on ePay and it has the swinging link and a series variable capacitor to ground. It looks like the cap on the T-55 rig is about 100 uuf. On the 610 I used a 500 uuf with a 1000 volt rating and it made it much much easier to adjust the proper tank plate current as compared to the variable link that is in 610 coils. So to recap, on the 610 I kept the link fully engaged all the time and made all my loading adjustments from the variable cap. On the T-55 I am going to use 610 coils and toss the swinging link.


Title: Re: New home-brew 40 meter RF deck
Post by: WD5JKO on February 08, 2018, 11:59:02 PM

Ron, 

   The setup shown by W2PFY has good illustrations. Here are some more that I found on the net:

http://www.antiqueradios.com/forums/viewtopic.php?f=15&t=228639

   I have a box of TVL & JVL coils. Most are in sad shape. I did pull out a salvageable 40m TVL, and the "T Jack Bar" with 5 banana jacks. I also have part of the swinging link which includes the bakelite piece with the side ways rod at the bottom. The "T Jack Bar" center banana if removed, and replaced with the link hardware (missing) would be close to W2PFY showed in his last post.

    You can have any of this for the cost of shipping. I attach a photo of what I described.

Jim
Wd5JKO


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 09, 2018, 01:15:58 AM
First of all, thank you to those who replied with offers of help or suggestions. Here is what I came up with. I managed to find a broken but salvageable TVH jack bar in the junk box. After a little epoxy glue, it looks half decent. I also found a swinging link arm and coil that is usable. I was able to home-brew a reasonable facsimile of a 40 TVL coil with PVC pipe and vintage wire. I had to move some things around, so now I have a few empty hole on the chassis, but all in all, I'm happy with how things came out. I do need to figure out how to move the swinging link from the front panel though. Another smoke test is in the offing for Friday.

Jim, thanks for the offer on the parts, but I'll hold off for now in the hopes that this setup will work.


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 09, 2018, 11:45:34 AM
Cool Old Buzzard set-up, Ron!

One suggestion:  That pi-wound-layered plate choke is prone to arc-overs in high voltage / high power service. They're OK at the 6146 level, but has way too much inductance and inter-turn exposure for high power 40M. Also, potential  inter-turn capacitance problems on 40 M. That choke is probably better for the BC band.  It may work, but is a weak link when the rig is stressed. Replace it with a conventional singe layer plate choke.

They look good, but in QRO, I have blown up every one of those big layered chokes over the years.  

T



Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 09, 2018, 12:09:45 PM
Hi Tom,

Thanks for the suggestion. Yes, I have also fried one or two of the pi wound chokes through the years, and I remember you mentioning this once before. Fortunately, I have two or three homebrew single layer plate chokes available from previous projects. Most likely, I'll pick the one that is self resonant at 40 meters :o

Ron


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 10, 2018, 01:31:00 AM
OK, a few more changes. I took K1JJ's suggestion and changed the pi wound RF choke for a single layer home-brew choke. I also added a new knob on the front panel that indicates the position of the swinging link coil. I did not try to put it on the air tonight. I'll try to tune it up tomorrow. Pics below.


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 10, 2018, 01:34:16 AM
Also, a few pics of the home-brew coil construction detail.


Title: Re: New home-brew 40 meter RF deck
Post by: KC2ZFA on February 10, 2018, 11:30:21 AM
in the pic of the jack bar there's a hole visible beneath the braid that makes the center of the coil. Put the plate choke there and move the hv feedthrough next to it. for the sake of symmetry :-) You may have to fashion a rotation stop so the link can't crash into the plate choke.

also, it may be good to begin testing with the tank coils closer to the link coil.


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 10, 2018, 01:25:05 PM
Hi Ron -

Good. That new choke looks to be about the right minimum  inductance and should work FB.  The chance of hitting a self-resonant point on 40M is very small. Odds are like throwing a dart and hitting the exact center... :-)


I didn't see the push-pull schematic - but should the bottom of the plate choke have a HV bypass cap placed there?  Maybe it's OK as-is with this old p-p design, but just want to be sure.

T


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 10, 2018, 04:58:09 PM
Peter, Thank you for the suggestions. I agree that the plate choke doesn't really look so great where it's at. It doesn't show it well in the pics, but the new plate choke won't fit into the space that you suggested. I could mount the plate choke under the chassis. That might look better. I also had the thought that the plate coils should be closer to the pickup link. I am in the process of redesigning the plate coils, and should have them done later today.

Tom, there is a 10kv, 500pf bypass cap under the chassis at the entry point for the B+ voltage. I don't have an inductance meter at the moment, but I think I once figured out the inductance of this plate choke at approximately 400-450uH. I hope that's enough. I do have one that's bigger, probably about 750uH, but it's really ugly. ;)

Thanks again, Ron


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 10, 2018, 05:32:11 PM

Tom, there is a 10kv, 500pf bypass cap under the chassis at the entry point for the B+ voltage. I don't have an inductance meter at the moment, but I think I once figured out the inductance of this plate choke at approximately 400-450uH. I hope that's enough. I do have one that's bigger, probably about 750uH, but it's really ugly. ;)

Thanks again, Ron

OK Ron -

You can keep the existing bypass cap where it is... but add another one right at the base of the RF choke. The choke circuit is an RF L/C filter that needs a cap bypass to work against that is right there... no long leads.   Look at pics of commercial amps and you will see plate choke bypass caps are all positioned right at the choke base.

Plate choke inductance:  This requirement will depend on the plate impedance when the rig is running.   The choke inductive reactance on 40 M >=  plate impedance X 10.

Just try it and if you get a sharp plate current dip with good output power - and the choke stays cool, then you are fine.


T




Title: Re: New home-brew 40 meter RF deck
Post by: w8khk on February 10, 2018, 05:46:23 PM
If the plate circuit was shunt-fed, as in a typical single-ended pi-net output, then the inductance of the plate choke, and avoiding resonances, is much more of a concern, because all the RF voltage is developed across the choke, and capacitively coupled to the pi tank circuit.

However, the choke characteristics are much less critical in a series-fed center-tapped push-pull plate circuit.  In this case, the high RF voltage appears at the ends of the center-tapped inductor (at the plates) and the RF voltage at the center tap is much lower.  In fact, it would be negligible if the plate circuit was perfectly balanced, yielding just the DC plate supply voltage.  (However, perfect balance is not easily achieved.)  Therefore, I do not believe you have to worry about damage to the plate choke with your push-pull configuration. 

Proper bypassing, with  capacitors, is prudent in both cases.

Your pictures show fine workmanship, and it is nice to see period-correct attention to detail.  Nice job!


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 10, 2018, 06:15:09 PM
Tom, Thank you. If I add another 500pf of bypass to the B+ line, will it start to bypass the audio frequencies? How much is too much? Perhaps it would be better to simply move the bypass cap that is under the chassis.

Rick, Thank you. I tried to make it look authentic as possible. I miscalculated on a few things though and now have about ten unused holes in the chassis :( Just wondering if you would add another bypass cap, or just move the one that is already in the circuit. (or neither)? Thanks again.

73, Ron


Title: Re: New home-brew 40 meter RF deck
Post by: w8khk on February 10, 2018, 07:18:12 PM
Just wondering if you would add another bypass cap, or just move the one that is already in the circuit. (or neither)? Thanks again.

73, Ron

The bypass capacitor is intended to prevent RF radiation from the B+ lead from the modulator to the RF stage.  For 40 meters, it should not matter whether it is under the chassis where the B+ enters, or at the bottom of the RF choke on top of the chassis.  The wire at toe top of the RF choke and at the bottom simply adds a bit more inductance to the RF choke overall.  Either way should be fine.   At higher frequencies, more attention to detail, and shorter wires, are important.  I would leave it alone and see how it plays.

By the way, I have never had a pie wound or cylinder wound RF choke fail with series feed, as you are doing.  I also think the plate inductor halves (in your latest photos) are close enough to the link.  If it dips correctly and loads satisfactorily, then you have a keeper.


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 10, 2018, 08:44:06 PM
Tom, Thank you. If I add another 500pf of bypass to the B+ line, will it start to bypass the audio frequencies? How much is too much? Perhaps it would be better to simply move the bypass cap that is under the chassis.

73, Ron

Good point. The Tron recommends 500 pF as the maximum plate bypass to allow the highest audio frequencies to pass unimpaired.  So best to use just one 500 pF placed at the RF choke input as you sugggest.  I use about 500 pF, even on 160M.   My usual tank is a pi-network, so I have to contend with even more capacitance thru the plate coupling cap and C1/C2 to ground.

Rick - I still think a bypass cap right at the plate choke is best. Lets take it to an extreme... how long can we make that lead from the plate choke to the modulator - three feet, ten feet, before a bypass cap?  I don't think it serves any purpose to put the cap any farther away than it has to be, to keep with the practice of short leads and stability on 40M.

T





Title: Re: New home-brew 40 meter RF deck
Post by: KC2ZFA on February 10, 2018, 09:01:18 PM
Ron, looks GREAT.

I think it's a bad idea to move the rfc below deck. Instead move the jackbar a bit to make
space.

As to below deck, I think you should move the tuning cap so that's directly below the holes going up to the grids and move the input coil right next to it. The grid cap's shaft could be
connected to its panel control via a flexible shaft.


Title: Re: New home-brew 40 meter RF deck
Post by: w8khk on February 10, 2018, 09:53:47 PM

The Tron recommends 500 pF as the maximum plate bypass to allow the highest audio frequencies to pass unimpaired.  So best to use just one 500 pF placed at the RF choke input as you sugggest.  I use about 500 pF, even on 160M.   My usual tank is a pi-network, so I have to contend with even more capacitance thru the plate coupling cap and C1/C2 to ground.

Rick - I still think a bypass cap right at the plate choke is best. Lets take it to an extreme... how long can we make that lead from the plate choke to the modulator - three feet, ten feet, before a bypass cap?  I don't think it serves any purpose to put the cap any farther away than it has to be, to keep with the practice of short leads and stability on 40M.


The total bypass capacitor's capacitive reactance at the highest audio frequency should be at least ten times the modulating impedance.  

In the first post of this thread, it is indicated the amplifier will run 1600 volts on the plates at 250 milliamperes.  This yields a modulating impedance of 6400 ohms.   The capacitive reactance of a 500 pf capacitor at 5000 Hz is 63,662 ohms, about ten times the modulating impedance, so we are in the ballpark here.  For modulated RF amplifiers running at higher voltage and lower current, the modulating impedance is larger, so a smaller total bypass capacitance is needed.  Lower voltage, higher current amplifiers could tolerate somewhat larger bypass capacitance.

I certainly agree that it is best to have the bypass capacitor as close to the cold (RF, not DC) end of the RF choke.  So above chassis would be best.  My point was that the inch or two of wire from the bottom of the choke to the capacitor under the chassis probably does not warrant drilling more holes to move it.  As far as stability is concerned,  the issue here is to avoid feedback from un-bypassed plate circuit wiring in the grid circuit under the chassis.  It is also for this reason that I would recommend AGAINST moving the plate choke under the chassis in proximity to the grid coil, as mentioned in the previous post.


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 10, 2018, 10:11:19 PM
I would say we are in agreement on all points, Rick.

BTW, you make a good point about the choke having less stress when in the series-fed  (push-pull) configuration.  I can see how that would be the case.  All of my pi-wound explosions have occurred using standard pi-network rigs.   Though, there may be a time when Ron makes a mistake and things get unbalanced, a tube shorts, the rig takes off with a parasitic, etc. The single layer choke would have a better sense of humor when kicked in the BAs... :-)

T


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 10, 2018, 10:16:43 PM
So here's the outcome of the smoke test. It actually seems to work! At 1500V and 200mA, I get 210 or so watts out according to my Drake W4 meter. So that is about 70% efficiency which I think is OK for an old buzzard push pull circuit. I was going to run it at 250mA, but the plates glow pretty bright even at 200mA, so I don't want to push them too hard. The data sheet on the HK54 says that the plate should be a dull red at full output. I would say that this is more like a medium orange (see pics). Anyway, it's pretty, and it seems to be doing what it's supposed to do. I did load it up to 1600V@250mA and got about 250 watts output, but the plates looked pretty hot, so I backed off to 1500V and 200mA. That should be enough power for most casual AM operating.

Tom, I get a nice dip at resonance, so I think the plate choke is probably adequate.

Now I need a few spare HK-54's or TG-35's which are basically equivalent. Anybody got a few spares?

The first pic below shows the meter readings at rest. It also shows the position of the RF deck in the cabinet. A 20 meter RF deck goes just above the new 40 meter deck. The 20 meter deck has push pull 254W's and is capable of 500W carrier output. It was my first push pull project and isn't so pretty, but it works great!

The second pic shows the tube filaments glowing at rest, and the last pic shows the plates glowing red/orange at 300W input

I'm happy tonight.  :)

73, Ron



Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 10, 2018, 10:27:47 PM
What a thrill, huh?  Especially when it works right away like that. Congrats, OM.

Will it be plate modulated using a common plate modulator for all the RF decks?  Plan on some audio negative FB?


Did the RF deck neutralize without much trouble?

T



Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 11, 2018, 03:32:47 PM
Yes Tom, quite a thrill to use a transmitter on the air that is truly unique to your station. I wish I understood modern electronic technology better, I'd build something a bit more up to date, but I also like the nostalgia of using a transmitter from the 1930's or 40's.

The new RF deck neutralized quite easily.

I have one modulator for the various RF decks, it uses 811A's driven by a pair of 6A3's. I use a modulation reactor and a UTC S-22 mod transformer. The driver transformer is a UTC S-9. The speech amp uses a 6SJ7 followed by a 6N7 phase inverter which drives the 6A3's. There is no negative feedback. All the audio is home-brew except that I power the speech amp with a 0-300VDC Lambda regulated power supply. Lots of old glass envelope 6L6's in that power supply, four or five I think. The 811A's run at  ~1300VDC and have -2.4 volts of bias. Idling current is about 40mA. The 6A3's use cathode bias. I use a Heil classic mic through a W2IHY mic amp. Nothing fancy at all, but I get good reports.

My third RF deck is a 160-80-40 meter deal with an old Taylor 822 triode in the final position. It has a pi network tank with grid neutralization and works FB. I can put two RF decks into the cabinet at any given time, and it just takes a few minutes to switch the necessary cables from one to the other. As you can tell, I like triodes. I don't know if K9ACT is still around, but he once said "triodes are nice", and I agree.

Murphy paid a visit to my shack last night. I was testing the new RF deck into a dummy load, and I wanted to recheck the neutralization one last time. I tuned up the RF deck so that it was tuned to resonance, and then I disconnected the B+ cable from the back of the chassis (B+ is off of course). I did this from the front of the cabinet by reaching through, and after the cable was disconnected, I let it drop so that it would just dangle. I turned away but then heard a "pop". The circuit breaker at the house breaker box also snapped off. This was not a loud crack or sizzle, and there was no smoke or smell. I immediately thought that the B+ cable had hit against something that had caused a short of some kind, but when I went behind the cabinet to investigate, I could not see anything that it would have hit, and I saw no damage of any kind. All the various decks of the transmitter are fused, but none of those fuses blew. I reset the breaker and turned on the power to the RF deck, and only one of the final tubes was lit. I jiggled it around in it's socket, but no joy. Being an Eagle Scout, I was prepared. I just happen to have a couple of spare HK54's. So i get one, plug it in, let it warm up a bit, and hit it with about 500V on the plates. Man, you've never seen such wild blue and purple colors. It looked like an 866/3B28 cross. LOTS of gas in that tube! OK, so I grab another one, and plug it in. This time it looks OK, but when I apply RF drive, the plate goes to white hot (exaggeration) quickly while the other plate has no color at all. I check grid bias voltages, all OK. I switch the tubes in their respective sockets, and the same tube goes nuclear while the other tube does nothing. I get my last spare, and the same thing happens. The original tube does nothing, while the "new" spare gets hot very quickly. OK, one last combination to try. I take out the original tube, and put the two spares into the sockets. This time it seems to tune up normally again.

I'm still not sure what happened, but the end result was one dead tube from a broken filament, and one tube that lights up OK, but apparently does not conduct. The icing on the cake was that one of my spares is also no good from gas in the envelope, sure looks cool though. More testing today.


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 11, 2018, 08:43:37 PM
Ron -

That sounds like a "catastrophic crap-out."  You probably have it fixed by now.

That's the problem with old tubes - the gas.  I have a batch of about (20)  25-G old power tubes. Maybe four were good and the rest arced over.  The seals are good for just so long.

So you're an Eagle Scout! Fantastic.  Many people have no idea how hard it is to become an Eagle and what it says about the young man who attains it. I would hire an Eagle Scout in a heartbeat.  I am a Life Scout... have 27 merit badges and was ready for the Eagle board of review. Sad to say that I was 13 at the time and the troop committee thought I had advanced too fast and insisted I wait. Ham Radio came along and that was it for scouting.   I should have told that committee to F themselves and finished. [sigh]

I'll bet there are some more Eagle Scouts on this BB.

T


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 11, 2018, 10:02:29 PM
Yeah, I slipped in under the wire. I got my eagle scout on December 20, 1975 and turned 18 on Jan 7, 1976. Ham radio almost derailed me too. I got my novice license in August 1975, and spent a lot of time on the radio that fall. My middle son also got his eagle a few years ago. He and I are a lot alike, but I can't get any of the boys to get interested in ham radio. Computers are where it's at these days. Boy Scouts was a big part of my teenage years. I went to Philmont scout ranch at age 16, and that sealed the deal for me. I was definitely going to live in the American West. I grew up in Ohio, but once I visited New Mexico, I knew I would not stay in Ohio for long. Since 1997, I have either lived in North Dakota or Montana. I have a cabin in Montana out in the middle of nowhere, and once I am retired, that is where I'm headed.

The 40 Meter RF deck is now working adequately, but I can't seem to get it to work as efficiently as the handbooks say I should. According to the handbooks, a single HK-54 should be capable of 200 watts output with 250 watts input. I have two of them in push pull, and at 300 watts input, I get about 200 watts output which is right at rated plate dissipation. The numbers I get from my meters look good however, and the waveforms on the O'scope look good, so I'm not gonna complain. I did try to put it on the air tonight, but no response to my CQ. Will keep trying.

Here are some pics of the transmitter after I added the new RF deck. The top deck is the 20 meter 254W push pull RF deck. The next panel is just a plexiglas window, and then the new 40 meter RF deck is just below the window. Below that is the meter deck, followed by the Bias deck and the control deck. To the right are the audio decks and the power supply decks. As you can see, I added an "RCA" logo to the new RF deck. Don't know why........I had one and thought it might look nice. ;)



Title: Re: New home-brew 40 meter RF deck
Post by: w4bfs on February 12, 2018, 10:49:05 AM

....   I should have told that committee to F themselves and finished. [sigh] .....

T

Can you get a merit badge for that ??  ???


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 12, 2018, 11:55:06 AM
OK Ron -


An 17 year old Eagle is a dedicated one... :-)  


For more power out:

When fine tuning the rig for max output, I usually play around with the inductance/ Q ratio just to be sure it is reasonably close to the desired plans. There is usually a sweet spot where you can squeeze out a few more watts, despite careful design calculations beforehand.

Be sure the high voltage is at maximum or something that can support the desire for more wattage. I've found that getting near the max rated voltage is best overall.

Be sure nothing is getting warm, like the coils, chokes, etc.   Swap the tubes around to be sure they are good.

Use a lot of bias and keep drive / grid current up to maximum or slightly higher than rated. I've found maximum drive is best for max audio peaks too.

I've rarely built a rig that puts out full power until I do some diddling around, especially with a more complex rig - unlike a simpler linear amplifier.

Make sure the grid  bias is not creeping up excessively over time.   RF rectifying into the fixed bias filter capacitor can do this.  (if you use a combo of fixed bias / grid leak, that is)

Be sure your power measurement is accurate and can be verified by measuring the RF voltage across a 50 ohm non-inductive resistor.

Check the filament voltage under full rig power. Be sure the AC  120/240 input is not sagging to affect the fil xfmr.

But in summary, my guess is if you experiment / tap the coil in a few different spots, you may find a better transfer match and more output power.

There's more, but run with this for now....

T


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 12, 2018, 12:08:28 PM
OK Tom,

Lots of good suggestions. Will play around with various settings and see what I come up with.

Ron


Title: Re: New home-brew 40 meter RF deck
Post by: WD5JKO on February 12, 2018, 02:22:28 PM


Ron,

   Another concept from fellow member Giorgio in3iex is shown here:

http://amfone.net/Amforum/index.php?topic=29030.0

    This could be as simple as adding a cap across some of the output coil turns on each side which are tuned to the third harmonic. That should get you to 90% efficiency.  :P

Jim
Wd5JKO


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 12, 2018, 03:12:28 PM
Very interesting, Jim.

Most comments in that post said there was a big improvement except one.  Bill/HG  said it gave just 2-3% more eff but then his rig was already driven very hard.  This shows the value of driving a conventional class C grid hard even if there is no 3rd harmonic resonator.  I run my 4-1000A grids at about 50 mA each - about 10 mA higher than recommended and get great eff, high audio peaks, etc.


Bill said:

"Years ago, I tried that 3rd harmonic trick on my 304-TL rig.
Driving the snot out of the tube in a conventional pi-net gave me around 85% measured efficiency, the 3rd harmonic trap added, at best, two or three percent.

The thing is, when you drive a triode hard, the plate waveform does approach a square wave, being switched hard between "on" and "off".

I suspect this 3rd harmonic wave shaping scheme works the best when drive to the Class C stage is fairly low."


T



Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 12, 2018, 04:05:48 PM
OK, so the tube data sheet says that maximum grid current is 30mA, and a typical value in plate modulated service is 20mA. I've been driving the two tubes at 40-45mA. How high do you think I can safely go without putting the tubes into danger of failure?

I've got lots of leeway in the grid voltage department. I think the maximum grid voltage is -600, I've been using -250 to -300.

I've got plenty of grid drive capability, up to 150 watts or so, and I can vary the grid leak from 0-10K ohms

Ron


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 12, 2018, 05:29:17 PM
Ron,

I have zero experience using HK-54s, but based on other homebrew class C tube rig experience, I would start experimenting with parameters.

Try increasing the grid current / drive to the maximum rating and then even go past it a little to see if it makes an improvement. At the same time vary the grid leak resistor settings, drive power and change the loading and tank Q.  Take careful in and out power measurements and you will find the sweet spots for max power out and eff.  Later, do the same thing using an audio sine wave and see how high the modulation audio peaks will go cleanly.  (careful with mod xfmr)

See if there is a saturation point when it comes to grid current and drive. You are already near the grid ratings.  Many of my rigs will keep modulating higher and higher % as I increase grid current and then reach diminishing returns. The best spot is to get everything within tube ratings, though the grid sometimes needs slightly more than recommended current. At least this is the case with my 813 and 4-1000A tetrode rigs. I understand that triodes have a bigger appetite for grid current performance.

I looked at the HK-54 specs and see the max voltage as 2KV in plate mod service.  Maybe you can increase your voltage to 1800V if the mod iron can take it.

It is amazing how high the efficiency can get when a tube is driven very hard with heavy bias, almost approaching class D.  Maximum HV and you have a real man's rig.. :-)

You already know to be careful and methodical so not to blow out a tube or mod transformer when testing.

I'd say you are probably already close to a good operating rig which simply needs adjustments.

T


Title: Re: New home-brew 40 meter RF deck
Post by: KD6VXI on February 12, 2018, 06:08:25 PM
The 3rd harmonic filter is fairly well known, even being mentioned by Eimac in the 3-500Z datasheet (the one which has the switching / pwm data on it).

I've heard it called taloe and Taylor circuits.

Also called class F.

--Shane
KD6VXI


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 14, 2018, 08:39:45 PM
Success! I made my first contact tonight on 7285 KHz using my new RF deck. North Dakota to Ohio. Got a good report. I ended up running the RF deck at 1700VDC@150 mA for 255 Watts input and about 200 watts output. Will probably push it a little harder next time, but very gratifying to get that first QSO under my belt.


Title: Re: New home-brew 40 meter RF deck
Post by: KA7WOC on February 14, 2018, 09:40:14 PM
Congrats
I rooted around the shack and found some tubes.  I don't know if they are good or not.  I will get them shipped out soon (next week) and you can test them.  2@HK54 and 2@Fisher 822
Bob ka7woc





Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 14, 2018, 10:37:41 PM
OK Bob,  Thanks, Hopefully we can get a SKED set up soon.

Thanks again, Ron


Title: Re: New home-brew 40 meter RF deck
Post by: K1JJ on February 14, 2018, 11:49:26 PM
Success! I made my first contact tonight on 7285 KHz using my new RF deck. North Dakota to Ohio. Got a good report. I ended up running the RF deck at 1700VDC@150 mA for 255 Watts input and about 200 watts output. Will probably push it a little harder next time, but very gratifying to get that first QSO under my belt.

Ron,

78% efficiency is quite acceptable.

So what did you do besides raise the plate voltage to 1700V ?   How hard are you driving the grids now?  

You will probably find that the rig runs the cleanest audio when the final is loaded heavily, but then the power output is reduced as the tradeoff.   It's now a matter of tweaking the parameters.   Those old triodes should be good for 84% eff if driven real hard. You're almost there.

T


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 15, 2018, 12:01:46 AM
Hi Tom,

Well, The grid current was 45 mA and the grid voltage was about -280. I just played around with the different parameters and this is what I came up with as far as keeping the plate dissipation within acceptable limits. The plates of the tubes were still a dull orange, but I believe that I was within acceptable limits. After my QSO I loaded it up to 200mA and I think I'm still OK, so I'll try that setting tomorrow. Anyway, I know that it works, and that it sounds OK on the air. I'm happy again tonight.  :) 73, Ron


Title: Re: New home-brew 40 meter RF deck
Post by: KL7OF on February 15, 2018, 09:06:45 AM
Ron...congrats...looks very nice...I hope to work you on 40....
  A couple things I want to comment on....
I like the PP output coils you wound ..If they were closer to the swinging link, wouldn't you get more output?
I notice you have the link "fully engaged".  If the coils were closer to the swinger, you could always back the swinger out a little....if it loads too much...
That gassy tube you have may still be usuable...One of the 100th tubes you sent me a few years ago was gassy and purple and even had a small purple torroid shaped doughnut that crawled up and down the long plate stem when in use....I used that tube along with a non gassy one in my PP 100th rig for about a year before I got a replacement..  I am on travel right now but I will be in the shack in a week or two....Steve


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 15, 2018, 10:42:39 AM
Hi Steve,

Good to hear from you. It's been a while, hasn't it? I'm not sure which picture of the coils that you saw, but I did rewind them onto different forms and now are much closer to the swinging link. At 200mA plate current, the swinger is about 80% engaged. I can get quite a bit more plate current, but the plates get uncomfortably bright, so I keep the plate current low and don't push the tubes too hard. I ran it last night at 150mA, but I think up to 200mA is probably safe and that will also get the plate impedance down. I think I have the mod transformer set for about 6400ohms plate impedance.

I understand you are down south right now. Hopefully we can have a QSO when you get back.

73, Ron


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 16, 2018, 11:08:01 AM
Could somebody please count the turns and also measure the spacing between the turns of a commercially produced 500W 40 meter plate coil (eg. TVL40 or equivalent). I have acquired some #12 enameled soft copper wire, and am going to try and wind a better plate coil.

Thanks, Ron


Title: Re: New home-brew 40 meter RF deck
Post by: KL7OF on February 16, 2018, 08:04:53 PM
picture of 40 meter coil in my 250 th rig.. It happens to be on my computer..


Title: Re: New home-brew 40 meter RF deck
Post by: KB2WIG on February 16, 2018, 08:34:56 PM


  that's some girl.


klc


Title: Re: New home-brew 40 meter RF deck
Post by: KC2ZFA on February 16, 2018, 09:12:24 PM
Hi Ron,

I attach the official HDVL and TVL coil info. The TVL coil table columns are labelled as for the HDVL coils.

The starred footnote says "total effective capacity required to effect resonance on low frequency end of specified band."

Each half coil has 11 turns occupying (start of wind to end of wind) 1 11/16 inches. Separation between coils (where the link goes in) 13/16 inches.

Also note, TVL's are 250W  coils.

Peter


Title: Re: New home-brew 40 meter RF deck
Post by: W8ACR on February 17, 2018, 01:22:14 AM
Thank you Peter, very helpful

Ron W8ACR


Title: Re: New home-brew 40 meter RF deck
Post by: WD5JKO on February 17, 2018, 08:04:13 AM


Ron,
   
   Remember on page 1 of this thread is a photo of my TVL 40m coil. It is original other than the coil holding strips have disintegrated and fallen off.

Jim
Wd5JKO
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands