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AM TX/Amp Build Log




 
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AI7CH
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« on: April 03, 2022, 12:47:03 PM »

Hi all,
A few months ago when I asked about the AM transmitter/amp project from an early '60's Handbook, someone suggested that if I actually built it, I should do a build log. So I'm building, and here's the first couple of steps.

I knew I wanted to use circuit boards for some of it, so the first one is the rectifier/HV power supply. Pretty basic, and I'm limiting the HV to ~750V, so no great shakes. The transformer center tap is pulled off to supply 375V and 200V (Zener regulated) for the modulator.

The second is the WA1QIX modulator. I know Steve designed it as a mod to the DX-100, but I just decided to use it with a few mods for the voltage rails I have available. It doesn't seem to make too much difference. I talked to Steve about it, and it's a very flexible and forgiving design, so it should work well. The pcb is still missing the tube sockets. It's designed to mount inside the chassis with the tubes extending through the top.

Also is a pic of a little project I did a couple years ago for TX/RX timing. The input is the PTT switch, with drives for up to three relays - Bias switching, RX mute and Antenna switching. Each delay on both TX and RX is individually adjustable through software. I originally came up with it for a linear amp project, but it makes sense here too. Only drawback is it needs a separate 12VDC supply for it and the relays.

Bill - AI7CH


* HV.jpg (74.48 KB, 640x480 - viewed 114 times.)

* Modulator.jpg (61.47 KB, 640x409 - viewed 129 times.)

* TimingMicro.jpg (67.39 KB, 640x446 - viewed 120 times.)
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AI7CH
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« Reply #1 on: April 03, 2022, 07:37:19 PM »

Started cutting the chassis today. Its hardly ever 'done' until the end, but you have to start somewhere. I'm sure there will be a lot of added mounting holes for terminal strips and ground lugs as I progress.

Also drilled the chassis for front panel and did a test fit. So far so good.


* Chassis1.jpg (56.85 KB, 640x480 - viewed 109 times.)

* Chassis2.jpg (57.64 KB, 640x480 - viewed 101 times.)

* FP_Mounted.jpg (51.5 KB, 640x480 - viewed 137 times.)
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AI7CH
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« Reply #2 on: April 05, 2022, 07:26:12 PM »

Started mounting some parts to get a better idea of the fit and where to put terminal strips. Probably be having to remove them in the future (I wonder how many times...?) . Roll Eyes


* Parts.jpg (53.23 KB, 640x480 - viewed 126 times.)
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K1JJ
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« Reply #3 on: April 05, 2022, 07:47:09 PM »

Hi Bill,

Very nice workmanship and a great start to the project!

I like the engraved panel.

What is the final amplifier and is this going to be screen modulated using the QIX DX-60 mod?

I'll be closely following your project.  Be sure to post the measured results.

Tom, K1JJ
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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.

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AI7CH
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« Reply #4 on: April 07, 2022, 03:38:09 PM »

Hi Tom,
The amp will be pretty much like the original article - 2x 6146B's. I'm going to use 6883B's since they are cheaper and NOS is far more available. The filaments are rated down to 10V, so using the 6.3V and 5V windings in series should work.

The original article was pushing the 6146's way too hard at 1000V. That might be ok back when they were fairly cheap, but I'm going to turn things down a notch and run them 750-800V. I thought long and hard about the plate choke issue too. I'm using one from a TS-830S and hopefully it'll work ok on the WARC bands - it should anyway. I measured it on the bench and it seems to have a self-resonance right at 24MHz, but once everything is built up around it, I'll measure it again. Hopefully it'll move a tad.

I kept the Linear Amp mode, but it's main use will be AM. I made a pcb for the QIX modulator (one of the pix). Went over a couple of minor changes with Steve, too, and he thought it'd work fine.

The front panel is from FrontPanelExpress.com. They make one-off front panels and aren't as expensive as you might think. Pricing is built into the software they provide, so you can make it as simple or fancy as your pocketbook dictates.

Bill AI7CH
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Patrick J. / KD5OEI
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« Reply #5 on: April 09, 2022, 12:59:40 AM »

I like what you are doing there, and the work from front panel express. I want to use them but have some vintage odd shaped meters and am not confident that I have measured correctly. I found the software really easy to use.
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AI7CH
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« Reply #6 on: April 09, 2022, 08:28:03 PM »

Hi Patrick,
Odd shapes are a challenge with the software. If you can come up with a DFX file for the shape, you can import it directly. If you can trace the meter shape and screw pattern, I may be able to help with creating a DFX file, depending on the complexity.

One thing I didn't do..... I forgot about the draft angle on the meter housing. I measured the meter diameter at the back of the housing - the hole I spec'd was way too small for the meter to actually seat on the panel. A lot of drum sanding later......

Here's my progress for the last couple days - installed parts to start wiring, and started working on the tank circuit and trying to figure out how to install the taps to the coil (not much room...). Also waiting for more high voltage hook-up wire for the plate circuit.

Bill AI7CH


* 04092022.jpg (65.32 KB, 640x480 - viewed 130 times.)
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AI7CH
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« Reply #7 on: April 18, 2022, 09:52:43 AM »

A little more progress this weekend. I finally got my station back on the air, so spent a bit of time actually operating...


* IMG_0179.jpg (152.52 KB, 640x853 - viewed 139 times.)
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K1JJ
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« Reply #8 on: April 18, 2022, 12:03:41 PM »

Hi Bill,

The project is looking very FB!   Your building techniques show that you have been at it for a while.  The use of fasteners, routing, layout, panel work and general construction is much better than average.

I'm curious what technical background or current career you have and what rigs you've built in the past?  

Some guys seem to pick up professional techniques just by studying a lot of commercial rigs, while others still have coarse habits (like me at times) and improvise a lot.... Grin

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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« Reply #9 on: April 18, 2022, 05:51:05 PM »

I forget the ham who looks a little like Abe Lincoln..........not being nasty........but he was a master at cutting metal for big old monster projects
Fred
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Fred KC4MOP
AI7CH
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« Reply #10 on: April 18, 2022, 07:07:43 PM »

I guess it's ok to admit that I'm a (close to) retired EE. Been designing, prototyping and building stuff for 35+ years. But nothing having to do with RF or tubes. I've home-built some iffy stuff too, but at one point I just decided that if I was going to spend the time, money and effort to build something for myself, from scratch, it should look good too. I just spend a lot of time thinking things through, planning and....  still end up having to re-do stuff and improvise Roll Eyes. It's just part and parcel to home-brewing.

Heck, that beautiful front panel almost ended up in the scrap bin. I placed the Tune and Load caps a wee bit too close together and the Load cap ran into the Tune cap. Luckily I could save it by cutting off a little mounting tab on the Tune cap (see pic). I should have made a prototype panel before spending $ on the final one.

I spent most of my career designing microprocessor and dsp-based instrumentation, with a little software thrown in. So a long way from RF and radios. I got my Novice in 1978, got on the air at the school club (exactly twice) and promptly let my license expire. School, life and family got in the way until 2014 when I got my General and my Extra in 2020 during the Covid lock-down. I just love building stuff and old hybrid radios. My modern radio is a TS-440S, along with a TS-830S and an FT-901DM, both in various stages of restoration. And hopefully soon to be joined by this one.....

Bill AI7CH


* Oops.jpg (42.53 KB, 640x411 - viewed 104 times.)
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K1JJ
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« Reply #11 on: April 19, 2022, 12:19:47 PM »

Hi Bill -

Quite the picture...  Grin    Yep, those "oops" mistakes can take on many forms. Some we save the day and others have no way out.  I've done my share...   last year I mistakenly reversed the pin socket rotation on a homebrew 813 rig. Like flipping a socket upside down. It took me a while to figure that one out.

Then there's the mechanical problems when we decide to replace a part and/or modify a rig and realize there's no more room.  

FB on the EE background.  Congrats.  Excellent degree to have for living life .

That screen modulated rig will serve you well with perfect audio, no audio transformers, as long as you load it heavily.  That's one of the drawbacks to screen modulation; needs critical adjustments, low power or they can sound "fuzzy."  But when tuned right they are broadcash quality.

Keep up the good work and we'll look forward to seeing the scope patterns and maybe hearing it via .wav file off a hi-fi SDR recording.

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.
AI7CH
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« Reply #12 on: April 19, 2022, 09:11:18 PM »

Thanks, Tom. 

When I get there, you're going to have to help me figure out how to tune this thing up. Even though I've got 2 hybrids, I just go by the book when tuning them up, so I'm not sure what 'load it heavily' means. Since my plan is to use a DDS (from Hagerty with an adjustable output buffer) as the exciter, getting the drive level right should be fairly easy. Other than that, I'm just out in the ether.

Bill AI7CH
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AI7CH
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« Reply #13 on: April 30, 2022, 11:51:23 PM »

Finished the under-chassis wiring today. Now to tackle the remainder of the tank circuit, RF cage and T/R stuff. Have the sheet metal on order from OnlineMetals... so just waiting.
Bill AI7CH


* IMG_0187.jpg (172.21 KB, 640x853 - viewed 101 times.)
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F6BGV
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« Reply #14 on: May 01, 2022, 11:23:38 AM »

Hello,

Here are 2 achievements :

- BGV-2014 (2 x 813 HF and 2 x 811A AF)
- BGV-2018 (1 x 813 HF screen modulation)

All my frames are laser cut.

Best 73 !
Jean-Pierre



* BGV-2018 (2).JPG (1614.77 KB, 3264x2448 - viewed 67 times.)

* BGV-2014 (1).JPG (1685.21 KB, 3264x2448 - viewed 94 times.)

* BGV-2014 (2).JPG (1673.23 KB, 3264x2448 - viewed 92 times.)
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« Reply #15 on: May 02, 2022, 02:20:16 AM »

Looks pretty nice!
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F6BGV
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« Reply #16 on: May 02, 2022, 11:25:47 AM »

Yet, another TX from BGV !

BGV-2002 (initially 3 x 6146B Clamp modulated, then 2 x 6146B, plate + screen modulated by 2 x 6146A).

Best 73
Jean-Pierre


* BGV-2002 (3).JPG (3219.67 KB, 3264x2448 - viewed 71 times.)

* BGV-2002 (1).JPG (1615.08 KB, 2448x3264 - viewed 76 times.)

* BGV-2002 (2).JPG (2753.75 KB, 3264x2448 - viewed 89 times.)
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Radio, radio, quand tu nous tiens !
K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« Reply #17 on: May 02, 2022, 11:37:04 AM »

Bill:  Good job on the wiring and parts layout!  Looks very compact. You have a little bit of room remaining on the right side of the chassis wall, but that's about it... :-)

A professional job.  

When you get to the testing point, post some tone test scope pics with audio triangle and sine waves  from 30 Hz to 10 KHz and we can go from there. Tune the parameters (especially the loading) to get the highest and cleanest audio peaks waveforms.   With screen modulation, you will find that the best waveforms are when the final is running very inefficient, low loading capacitance, C2 unmeshed.   In contrast, max carrier power out (max C2) produces a stunted, non-linear look and eventual flat topping.  That transfomer-less, direct screen modulated rig will probably surprise you and pass a decent squarewave at 10 Hz.  I have a series modulated 813 rig that will.


Jean-Pierre:   A unique and neat build!  It's interesting how we can see the difference in European parts vs: American.  Two 813s make a robust final. I have an 813s X 813s rig here too.  Your 811As will be working hard to keep those 813s happy... :-)

* Update:  Your 6146 rigs are beauties too!  That is a good combination of power levels.. 813s / 6146s.


Tom, K1JJ


Below:  Part of my pair of 813s X  813s plate modulated rig - it's a real workhorse at 2800V :  


* DSCF0021.JPG (323.68 KB, 1280x960 - viewed 92 times.)
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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.
F6BGV
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« Reply #18 on: May 02, 2022, 01:18:28 PM »

Thank you Tom, your work is very professional !

My 811A's and my 813's (600W input) are under 1500V and here is the trapezoidal picture : cool !

Best 73
Jean-Pierre


* BGV-2014 (06-18).JPG (1556.74 KB, 3264x2448 - viewed 71 times.)
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AI7CH
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« Reply #19 on: May 06, 2022, 07:13:43 PM »

Finished most of the RF cage today. Just waiting for some 4-40 L-brackets to attach the perforated top sheet.

The top ended up pretty close to the 15M-10M section of the tank coil, so I may have to tweek the tap points once the cover is in place.

Does anybody have anything to say about adding a small fan to the back of the cage? 6146B's typically don't need forced air, but the top chassis cover will be about 1/2" from the perforated top of the cage, so I don't know how much natural circulation there will be to the outside world.

73's
Bill AI7CH


* IMG_0190.jpg (68.11 KB, 640x627 - viewed 73 times.)

* IMG_0191.jpg (78.56 KB, 640x756 - viewed 73 times.)
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