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New home-brew 40 meter RF deck




 
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Author Topic: New home-brew 40 meter RF deck  (Read 7762 times)
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W8ACR
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Penta 254W


« Reply #50 on: February 17, 2018, 01:22:14 AM »

Thank you Peter, very helpful

Ron W8ACR
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« Reply #51 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
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« Reply #52 on: February 20, 2018, 11:49:26 PM »

OK, so here is the final finished version of the RF deck. I have made about ten contacts with it so far and it's performance has been very good. I am taking it to the Bismarck hamfest this weekend to enter it in a home-brew contest. I have settled on running it at 2000VDC and 150mA. I get about 220 watts of carrier at this setting. This is well within the rated plate dissipation of the HK-54's, and the plates don't glow so bright at this setting. One thing that seems to have helped is that I have applied epoxy to the coil windings. This holds them stable and in a tight close wound configuration. This is visible in the pics.

I have one additional question for the group. Right now, I have about -140VDC of fixed bias on the grids. I leave the plate voltage on continuously, and this fixed bias keeps the tubes from conducting during receive. When RF drive is applied, the grid leak takes over and the bias voltage is lowered to -270 or so. I do not have an RF choke or a bypass cap on the bias line aside from the electrolytic filter caps. Does anyone think I should add these components? I understand that these inductances and capacitances can contribute to the generation of parasitic oscillations. I intentionally left them out, and it seems to work OK without them.

The last modification that I am going to make is to wind new plate coils with some #12 enameled soft copper that I have. I will try to space the turns out a bit, but keep roughly the same inductance.


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« Reply #53 on: February 20, 2018, 11:51:31 PM »

More pics


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« Reply #54 on: February 21, 2018, 02:29:54 AM »

Hi Ron
       So thats the rig I worked you on!   fantastic signal on 40, band held up for a long time.
I love it when 40 hangs in there, and the vertical works.

dean (kleenex Smiley
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« Reply #55 on: February 22, 2018, 12:50:07 PM »

Hi Dean,

Well, that is the RF deck part. The rest is in two six foot racks. I don't know how to build things in a compact manner. So good to hear you again after many years. Yes, very good conditions on 40 meters that night. Hope to hear you again soon.

Ron
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« Reply #56 on: February 22, 2018, 02:33:30 PM »

I would lose the PVC in favor of polystyrene or G10.

Matt
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« Reply #57 on: February 23, 2018, 03:40:20 AM »

Ron
     who cares how big if you can fit it in your shack!  As I mentioned I got a bauer 701b. one big box!  4x400's x 4-400s
Lots to do to get this going. but I plan to use 4-250's and eventually 3-500z's for modulators, plate volts will go down
and hope to do 500w input nicely. a pic of the mod iron (nice iron!) below.   do you think you'll build another final for another
band?

Dean



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« Reply #58 on: February 23, 2018, 10:33:21 PM »

Hi Dean,

I have two other RF decks at the moment. I have push pull 254W's on 20 meters and I have a band switching 160/75/40 meter deck which runs a single Taylor 822, but I'm in the process of redoing that deck because my one and only 10V filament transformer went PFZZZT! a few weeks ago. I'll probably convert it to a single 254W, which is a 5V filament tube, but it needs a much smaller neutralizing cap, and I'm also fresh out of disk neutralizing caps. I think I'll probably build a bunch of single band RF decks, all push pull. I have some 808's that I'd like to use, they look kind of cool, but they have a 7.5V filament. I have about 6 or 7 254W's, so I could use those, I also have an RCA 8000 tube, but again, a 10V filament.

I know you guys are probably getting tired of my pictures, but I did manage to home-brew some new plate tank coils with my #12 enameled copper. So here is the rig with the new coils. I miscalculated on the number of windings, so the inductance is lower than before, so I had to add a padding cap across the plate tuning variable cap. I suspect the Q of the tank circuit is higher than optimal, but I made one contact in the new configuration, and got a good report. I also seem to have a bit higher efficiency with the higher Q. With 300W input, I now get about 235 watts output.



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« Reply #59 on: February 23, 2018, 11:25:51 PM »

Ron
     who cares how big if you can fit it in your shack!  As I mentioned I got a bauer 701b. one big box!  4x400's x 4-400s
Lots to do to get this going. but I plan to use 4-250's and eventually 3-500z's for modulators, plate volts will go down
and hope to do 500w input nicely. a pic of the mod iron (nice iron!) below.   do you think you'll build another final for another
band?

Dean



Dean,

This may be useful for you if you want to transformer-couple to the 3-500Z grids. Finding no 'old type' driver transformer to match their grids, I did a lot of study and came up with a range of Edcor speaker-line transformers. It included looking at almost all of them so there is a decently comprehensive table. The 3-500 take very large grid current peak >1A so there was no good match from 'old iron' drivers.
I guess solid state or cathode followers might have done but the original transmitter used a transformer, wanted to honor that.

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« Reply #60 on: February 23, 2018, 11:31:00 PM »

More pics

I really like how you've built that. Those tubes run some color?
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« Reply #61 on: February 24, 2018, 03:22:01 AM »

Pat
       Interesting, I downloaded that xmfr list.  I have usually used a reverse connected tube output xfmr
like a hammond.  On my old 304tl x 833's I used a to-330 acrosound and drove it with a SS amp.  the 701b
uses SS direct drive to the 4-400 modulators. I'll see how that works as-is.  I just like triodes.

Ron,
       All HB finals I love it. I've been collecting parts for over a year, and finally realized I have nothing other
then a hand drill here.  I'd be 90 or dead before I get something HB'd, so the bauer's built.  I just have to convert &
get it running.  I see 10v xmfrs show up for sale, I'll watch out for one.  I've used old variac cores of suitable
power, take out the armature and wind my own filament windings on them!  just a few turns and your back
in business.  btw I like the pics!

Dean
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« Reply #62 on: February 25, 2018, 01:26:48 PM »

OK, here is some weirdness for you.

I picked up three 100TH's at the hamfest yesterday. On the way home, it occurred to me that I could use them in the 40 meter RF deck. They are the same tube base and filament voltage as the HK-54's. All I would have to do is change the grid bias and redo the neutralization. So I plug them in when I get home and two of the three light up. OK, so one bad filament, but I'm still in business. I make the adjustments, and apply about 500V to the plates, and so far so good, about 75W of carrier showing on the meter. turn the B+ slowly up to 2000, and now the tubes are doing their thing. 2000V@250mA and now I have 400W of carrier output. Make three contacts. Good reports. Had the rig on the air for over an hour with no problems.

This morning, I go down to the shack and turn on the rig. Tubes do not light. Jiggle them in their sockets a bit, nothing. Pull them out and measure the resistance of the filaments. BOTH filaments are open.  Huh I guess that after sitting idle in a box for 25 or 30 years, the filaments just couldn't handle the sudden application of power. Should I have let the filaments cook at a lower voltage for a while, or was this just bad luck?
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« Reply #63 on: February 25, 2018, 01:36:19 PM »

Oh Patrick, I forgot to mention, yes, they run a medium red to dull orange at 2000V and 150mA. The glow varies in intensity with modulation, a pretty neat effect.

Ron
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« Reply #64 on: February 26, 2018, 01:58:13 PM »

Ron, take the tubes and turn them with the pins pointing up and re-heat and re-solder the pins. I have saved may 100TH and 250TH by doing that. The original solder may have just soldered the tips of the wires in the base and by re-soldering them, you may bring them back to life? Be sure to flow a goodly amount of solder into the pins. If you have any solder slop on the pins, carve off the excess with a pocket knife. If the filament are indeed broken, you may see parts of them inside the tube. They use a spiral filament and they usually don't break. You should not use a filament transformer that is for example 20 amps where only ten amps is needed. It could result in a thermal shock to the filaments. Most old buzzards would use a rheostat over a variac to control the filament transformer primary to prevent inrush currents.

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« Reply #65 on: February 26, 2018, 04:04:21 PM »

Thank you for the hint Terry. I did look carefully at all three tubes and I could not see any fragments of broken filaments. I was a bit puzzled by that. Now I have some hope that I might be able to bring them back to life. I'll let you know how it goes.

Ron W8ACR
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« Reply #66 on: February 26, 2018, 08:28:23 PM »

Oh Patrick, I forgot to mention, yes, they run a medium red to dull orange at 2000V and 150mA. The glow varies in intensity with modulation, a pretty neat effect.

Ron

It's a beauty of those thin type plates vs graphites as in Chinese 3-500Zs. They can gain or lose color much faster, very pretty!
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« Reply #67 on: February 26, 2018, 09:24:52 PM »

Pat
       Interesting, I downloaded that xmfr list.  I have usually used a reverse connected tube output xfmr
like a hammond.  On my old 304tl x 833's I used a to-330 acrosound and drove it with a SS amp.  the 701b
uses SS direct drive to the 4-400 modulators. I'll see how that works as-is.  I just like triodes.


Dean

Dean

    The good quality TO-330 transformer probably has too much secondary resistance, probably cause a large power loss if trying it with 3-500Zs. I've done a lot of experiments having changed out 304TLs for 3-500Zs. The driver transformer that was in the TX here is a purpose made 40 Watt CG-512, but it is not suitable either for 3-500Zs, which have about 1/4 the driving impedance of 304s and 833s and very hungry for grid current since they are zero bias. I lost about 20W in the CG-512 transformer and suffered distortion and a lot of asymmetrical compression in the wrong direction.

I should clarify, the 1A grid current I stated on 3-500Zs is at the highest peaks measured, not average but it's the peaks that suffered with the old transformer. That's what sent me on my quest!

Looking at the Bauer schematic, the audio driver is a high impedance capacitor coupled circuit for AB1 and not suitable for supplying grid current.

Plenty of drive voltage though, maybe too much for 3-500s. The driver could drive push pull MOSFET source follower gates and thereby get the grid current. A little add-on and you'd need possibly a 200V power supply for them, maybe 300mA CCS and 1A peak. I used 6080 tubes in my testing, but needed more like 300V and two 6080s for each grid, all sections in parallel to handle the peak current. Was a crazy hookup - a place for solid state definitely, or the right transformer which was the simplest thing.

Patrick
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« Reply #68 on: February 26, 2018, 10:55:56 PM »

Hi Terry,
I tried your suggestion, but to no avail. I guess they are kaput. Sad
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« Reply #69 on: February 27, 2018, 11:07:11 AM »

Ron,
Tubes on their way
bob ka7woc
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« Reply #70 on: February 27, 2018, 11:24:32 AM »

OK Bob, Thanks a bunch.
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« Reply #71 on: February 27, 2018, 01:28:43 PM »

I had a pair of 3-500Z that I KNEW had good filaments, but I couldn't get any continuity through the pins even after resoldering them several times. What I had to do was completely clean all of the old solder out and start over with a good amount of heat and flux. They've been fine ever since. It might be worth trying.
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« Reply #72 on: February 27, 2018, 02:40:23 PM »

That's too bad Ron! Sounds like Criss has found  a cure for that filament problem and it may be worth another try? It depends how much an effort you want to put into it. The tubes are getting rare and generally when found on eBay, they are as usual big bucks. In amature service assuming you never have a plate current runaway, they should last 100 years. If you were to put a hour meter on the filaments and the plate transformer you would find that your yearly usage would be very low.

I am going to take my one dead 100TH and saw the base off with a dremel blade and see what is really going on? I'll start that project next week so Ron, don't throw them out just yet. I have over the years, given dead large tubes to audiophiles. They love to have them for display! I had another 100TH that became gassy and a friend made a regenerative radio out of it. He commented that it was a bit ridiculous to be burning that much filament current for such a project but despite the negatives, he said it works great!
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« Reply #73 on: February 27, 2018, 03:15:58 PM »

OK Chris,

Thanks for that info. I'll give it another try. They sure look like good tubes, and I cannot see any evidence of filament fragments in any of the tubes. In my experience, a broken filament usually leads to at least a few small fragments on filament material rolling around inside the glass envelope. I have some good silver solder and flux that I'll use.

Thanks again, Ron
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« Reply #74 on: February 27, 2018, 05:35:42 PM »

You should not use a filament transformer that is for example 20 amps where only tem amps is needed. It could result in a thermal shock to the filaments.


Hi Terry,

I've probably  heard that suggestion before, but it never dawned on me why... but it makes sense. The initial low resistance surge would indeed be greater with a filament transformer using larger diameter wire and a larger core.  I've always used Variacs on my fil xfmrs without exception. In addition, the Variac allows precise fil voltage settings which will also extend tube life and performance.

Yes, tubes that are supposed to have forced air or at least adequate ventilation are prone to overheated filaments and solder drip. I had two 3-500Z amplifiers  (both commercial without chimneys) that used fans. They both dripped solder and the fils became intermittent.  Once I added real chimneys and blowers, the repaired filaments never failed again.

Try touching the filament pins of a poorly cooled "forced air" tube and you will burn your hand. My acid test is that once I turn the rig completely off after a transmission, filaments off  - and wait one minute with the blower on - then ALL parts of the tubes - glass, seals and pins should be only warm to the touch. Then you know the tube will have maximum life and will probably never fail due to seals and other common cooling/heat related problems.


BTW, a dummy something I learned the hard way:  Never suck air from the top down thru the tube bottom and out. I once had a blower on the outside of the house that pulled air out. It was a big (and expensive) external anus 3CX tube. It was quiet and efficient. But the tube failed quickly because the hot air thru the plate pushed hot air onto the filament seals and the fil seals failed. Always push air in from the bottom thru the top as most smart people do.. :-)  The filaments deserve the cool air first since they are smaller and have less effect on the plate cooling - compared to the opposite direction of air flow.

T


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