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New home-brew 40 meter RF deck




 
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W8ACR
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Penta 254W


« 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.


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The AM voice of Knox, North Dakota
W8ACR
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Penta 254W


« Reply #1 on: February 07, 2018, 01:47:22 AM »

More photos


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The AM voice of Knox, North Dakota
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« Reply #2 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!
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W8ACR
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Penta 254W


« Reply #3 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. Embarrassed

Ron W8ACR
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« Reply #4 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
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« Reply #5 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
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The AM voice of Knox, North Dakota
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« Reply #6 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
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W8ACR
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Penta 254W


« Reply #7 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
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« Reply #8 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.


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« Reply #9 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
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W8ACR
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Penta 254W


« Reply #10 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.


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« Reply #11 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



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« Reply #12 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 Shocked

Ron
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« Reply #13 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.


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« Reply #14 on: February 10, 2018, 01:34:16 AM »

Also, a few pics of the home-brew coil construction detail.


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« Reply #15 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.
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« Reply #16 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


* Choke2.JPG (2114.52 KB, 4032x3024 - viewed 62 times.)
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #17 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. Wink

Thanks again, Ron
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« Reply #18 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. Wink

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


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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

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« Reply #19 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!
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« Reply #20 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 Sad 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
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« Reply #21 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.
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« Reply #22 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



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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #23 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.
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« Reply #24 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.
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