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Author Topic: Homebrew transmitter progress updates  (Read 88207 times)
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W8ACR
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« Reply #50 on: April 13, 2013, 12:53:08 AM »

Update#10 - 4/12/13 - The bias deck was mostly built already, so I was able to finish it tonight. The bias and RF decks were then put into the cabinet and primary power applied. Here are some pics of the entire transmitter (minus the audio decks) lit up. The meter pic shows approx -200volts fixed bias grid voltage and 10VAC filament voltage. Ignore the meter label tags, they have to be changed. The 822 tube has a good filament at least. Tomorrow we'll see how the grid and plate handle RF.

Basic operation is as follows: Flip bottom center switch on control deck to apply primary power to all decks. This lights the top left green pilot on control deck. After 180 second warmup, middle green pilot light comes on indicating that warmup period is sufficient. I then flip top switch on control deck which enables the power supplies to be switched on and off. This also lights the third green pilot on the control deck. So when I see "three in the green" I know the transmitter is in the ready mode. To key the transmitter, I will first key the Viking 2 exciter, applying grid excitation, muting receiver, and enabling the foot switch. Then I will step on the foot switch to apply plate voltage to both RF final and modulators. Perhaps not the most eloquent keying solution, but it should work.


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« Reply #51 on: April 14, 2013, 03:27:18 AM »

Here is what the bias deck looks like out of the cabinet. I worked on it some more today, tidying up a few areas that were a bit sloppy. Looking from the rear, the transformers, left to right, are UTC CG-315 bias transformer, Stancor P-6461 10VAC@10A, Electro Engineering 5VAC@30A. The variac is to adjust the filament voltage. The diode rectifiers in the bias line are the only major departure from 1940's design in the entire rig, so I guess I'll apologize for their presence. After I get the entire rig up and running, I'll draw a schematic and post it. Hoping to apply RF to the 822 later today.

OK, now I'm gonna chill with Pink Floyd "Wish you were here" before I hit the sack.

Ron


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« Reply #52 on: April 14, 2013, 10:39:13 AM »

We are going to need an all up weight on this beast when its done Ron.....Great looking Buzzard Machine.....  Steve
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« Reply #53 on: April 15, 2013, 01:44:34 AM »

Update #11 - 4/14/13 - RF success! 500W inpoot, 380W outpoot! Cheesy She neutralized easily and, at least on 40 meters, no parasitics! Numbers right down the middle - Plate 2000V@250mA, Grid -230V@45mA.  I let her cook the dummy load for about 3 minutes  several times. No sparks, flashes, smells, smoke, or other nastiness, just the hum of transformers and relays. WHEW! Buzzard Baby has passed her first test with flying colors. Now, I gotta put together a speech amp and modulator. Getting close!

I'm gonna transmit several 5 second carriers on 7051 KHz tomorrow (Monday) at 7PM Central time. Hopefully somebody will be able to listen and give me a report.

Pictures tell the story. The plate current meter is showing plate+grid current.

I'm a happy camper Cool


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W3GMS
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« Reply #54 on: April 15, 2013, 07:25:42 AM »

Fantastic Ron!  Nothing like a plan when it comes together.  Glad the beast fired up without any problems.  I will listen tonight for your test signal.  If you can put a key in the Viking and key the entire rig, I could possible work you on CW using your latest creation.  At 8PM our time, propagation may allow me to hear you. 

Joe, W3GMS 
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« Reply #55 on: April 15, 2013, 08:26:10 AM »

Thanks Joe,

I might key for ident, but I'd better not try a QSO. The old Viker is prolly real chirpy, and the bias on the 822 is not correct for CW.

Ron
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« Reply #56 on: April 15, 2013, 10:05:02 AM »

Understand Ron.  I will listen around 8pm EDT and see if I can hear your newest creation.

Joe, W3GMS
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« Reply #57 on: April 15, 2013, 11:10:24 AM »

Steve,

I don't know how much she weighs, but I think she will qualify for the Heavy Metal Rally. Grin
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Rick & "Roosevelt"


« Reply #58 on: April 15, 2013, 11:14:49 AM »

...well, the "Rally's" old rules.

Very nice Ron.  Great stats.

Buzzard Baby's "Uncle Anon" is quite proud of your creation.
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RICK  *W3RSW*
Ralph W3GL
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« Reply #59 on: April 15, 2013, 08:15:16 PM »


You were heard with an RST 589 here in Delaware Just before and after
8:00PM eastern, Ron...  I called but you must not have had the antenna
on your receiving device   Shocked   I also noted your loading the antenna
exercise...

Should I send a SWL card for first signal report?

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73,  Ralph  W3GL 

"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow
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« Reply #60 on: April 15, 2013, 09:09:56 PM »

OK Ralph,

Thank you for the report. You are right, I didn't have the receiver hooked up to the antenna. I was listening to myself on the 75A-4. Apparently the CW note must have been OK. It actually sounded OK here as well. But what you hear in your own receiver doesn't always correlate well with what others are hearing. When I was first tuning up, I had an arc on the RF deck and blew the AC line fuse. Later I found the problem - there was an arc across the neutralizing cap. I'm gonna have to put a bigger one in there before I try to modulate it. Yeah, a card would be great way to commemorate the first RF emission from Buzzard Baby.

Thanks, Ron
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« Reply #61 on: April 15, 2013, 10:47:31 PM »

OK Ralph,

Thank you for the report. You are right, I didn't have the receiver hooked up to the antenna. I was listening to myself on the 75A-4. Apparently the CW note must have been OK. It actually sounded OK here as well. But what you hear in your own receiver doesn't always correlate well with what others are hearing. When I was first tuning up, I had an arc on the RF deck and blew the AC line fuse. Later I found the problem - there was an arc across the neutralizing cap. I'm gonna have to put a bigger one in there before I try to modulate it. Yeah, a card would be great way to commemorate the first RF emission from Buzzard Baby.

Thanks, Ron

Sorry Ron, but I was running the 160M Antique Wireless Net tonight at 8PM.  Our main NCS was not feeling well and he asked me to take the net.  If Ralph heard you, I am sure I would have heard you even better up here Wink  Hi Ralphie!!

Joe, W3GMS   
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« Reply #62 on: April 16, 2013, 08:01:24 PM »

Update#12 - 4/16/13 - After the neutralizing cap arc last night, I replaced the neut cap with a larger one and it worked FB. I tuned up the RF deck on 75 and 160 also. At first when I tuned up on 75 meters, I couldn't get it to tune properly. Tried different taps on the coil to no avail. Couldn't figure out what might be wrong. Checked the coax connections to make sure they were tight, and...............found out I was still attached to my 40 meter single band vertical. I had forgotten to switch over to the dummy load. Embarrassed I'm sure that putting a 350 watt carrier into an SWR of infinity is not good for your final, your coax, or your impedance matching balun at the base of your vertical antenna. Undecided Hopefully nothing got cooked into molten goo.

Connected the transmitter to the B&W 374 DL and numbers again right on target. 370W output at full load on both 75 and 160. No tuning issues at all that I could tell. I must be getting the hang of this homebrew thing. Now if I can just figure out a system to keep track of what I'm transmitting into. Tongue

Speech amp construction to commence tonight.

Ron
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« Reply #63 on: April 16, 2013, 09:02:09 PM »

I like the old fuses....Part of my plan. Nice wrk es tnx for sharing
Carl
/KPD


Where can you get those old fuses?  I absolutely agree.  If you pop one of those putties they shouldn't shatter like the 3AG types

Al

PS: I answered my own question.  They are called "P" type plug fuses
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It's alive. IT'S ALIVE!!!


« Reply #64 on: April 16, 2013, 11:53:25 PM »

Wow you work fast!
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I'm outta control, plain and simple. Now I have a broadcast transmitter.
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« Reply #65 on: April 18, 2013, 12:40:48 AM »

Update#13 - 4/17/13 - Started working on speech amp last night, drilled and punched holes and did some spray painting of front panels. I now only have one more front panel to paint black - the big one with the variacs - it might just stay gray for a while. Wink The first picture below shows what the speech amp deck looked like when I started tonight, and the other picture shows my progress for today. All the major parts are now mounted on the deck. I'll start wiring tomorrow. The driver transformer is a UTC S-9. The other transformer is a 6.3 volt filament transformer for the 6A3 drivers. The tube lineup will be as follows: 6SJ7 mic amp, 6N7 phase inverter, 6A3 drivers transformer coupled to the zero bias 811A's. Mod transformer will be a UTC S-22. A 50H Dahl mod reactor will be used coupled with a 1uf@5000V oil cap. I previously used this system with good results. I am wondering what the consensus is regarding the return path for the 1uf capacitor. Previously, I put the cap from the mod transformer to ground. I am thinking about returning it to the upstream side of the mod reactor. Waddyall think?

If my current rate of progress continues, it's feasible that I might be on the air this weekend. I do not have antennas for 75 or 160 at present, so I'll be restricted to 40 meters. OTOH, I might just rig up a loading coil for the 40 meter vertical and try to get on 75. We'll see.


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* update 4-17-13 004.jpg (122.56 KB, 640x480 - viewed 607 times.)
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« Reply #66 on: April 18, 2013, 12:22:49 PM »

Hi Ron,

I just got a chance to go thru the latest posts. Man, you do work fast and put out some nice work.

Glad it tested out with no instabilities.  That rig will last the rest of your life and beyond. Better find a good heir for Buzzard Baby... Wink


Will you be running any THD tests when the speech amp is finished?  Does the overall modulator have audio negative feedback?  I'd be curious.   I have NFB in mine and expect to run some THD and IMD tests within a coupla weeks, assuming it all goes smoothly when fired up.


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.

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« Reply #67 on: April 19, 2013, 01:08:07 AM »

Tom,

No negative feedback. I didn't think it was necessary with push pull low mu triodes as drivers. I don't have the equipment to do THD tests, but I'll probably put an audio oscillator at the mike input and see how it looks on an o'scope at the drivers. See schematics of speech amp and modulator below.

Ron


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* audio decks1.jpg (116.76 KB, 2320x1700 - viewed 642 times.)
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« Reply #68 on: April 19, 2013, 11:31:56 AM »

Ron...I like the speech amp...There is a bass and treble boost circuit in the Radiotron Designer Handbook that will plug right on to that 6SJ7 mic amp tube that you are using..Simple ...2 pots and 4 caps and a couple resistors......I have built several of these using that boost circuit and have been pleased with the results...your project is looking good....Steve
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« Reply #69 on: April 19, 2013, 12:06:28 PM »

Tom,

No negative feedback. I didn't think it was necessary with push pull low mu triodes as drivers. I don't have the equipment to do THD tests, but I'll probably put an audio oscillator at the mike input and see how it looks on an o'scope at the drivers. See schematics of speech amp and modulator below.

Ron

Looks like a straight-forward audio driver.  If you include the 811As in the loop, NFB can improve your overall modulation system and make it even cleaner with better THD results. Maybe later on when you get settled..

It's good you have only one audio transformer in there before the 811As. Makes NFB more stable... two transformers in the loop would make it harder to do and less stable.  You could tap off one of the 811A plates and run some NFB back to an early preamp stage that is 180 degrees out of phase.  

When you get to this point, I can suggest some values to try. It's very easy to do and the results can be stunning.

You will need some extra audio gain to work with since every one dB of NFB reduces the overall system gain by one dB.  Six dB or so of NFB might do it for you.   I usually use as much as I can get away with.

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 #70 on: April 19, 2013, 12:50:49 PM »

Steve, I have one of those RCA Radiotron handbooks, I'll take a look at the circuits that you mentioned.

Tom, I have seen several types of NFB circuits, so I'm generally familiar with them. I have seen them come off the plate of the driver, and I have seen them come off the plate of the modulator. I'd be interested in seeing the NFB circuit you have in mind. How do I know which 811 to use to be 180 out of phase? There will probably be about 1000-1100VDC on the plates of the 811's. I should have plenty of spare audio unless I run the final at full bore. Even then I should have some spare audio.

Ron
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« Reply #71 on: April 19, 2013, 09:56:38 PM »

Ron,

Where is the mod reactor??  With the way you have the mod xfmr connected you're going to need one.

With 1850 volts on the final you'll need to increase the plate voltage on the 811s or you may fall short of 100% modulation.  I run 1500 volts on my 811s with about 4 volts of bias.  I use a 4.2v zener on the center tap lead of the filament xfmr.

Fred
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« Reply #72 on: April 19, 2013, 11:02:38 PM »

Fred, Mod reactor is there, just not shown in this part of the schematic. It is physically located on the power supply deck, so I didn't draw it on the modulator schematic. With 1000V on the modulator plates, 811's should be capable of 250 Watts output, and my modulator plates will be a bit higher than that, probably 1150V or so. I will be running the RF final at about 400-450 watts input, so it should be capable of 100% or at least very close.
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« Reply #73 on: April 19, 2013, 11:29:11 PM »

Ron,

OK FB on the reactor not shown in the schematic.  As for making 100%, you need to have the right turns ratio on the mod xfmr.  You need a step-up ratio, mod to final.  A lot of mod xfmrs are just the opposite step-down, mod to final.

What mod xfmr do you have and what's the turn ratio?

Fred
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« Reply #74 on: April 20, 2013, 02:01:53 AM »

OK Fred,

Well, I was planning on a secondary impedance of about 8K ohms (1800V@225mA). At 1100V, the 811's should also be wanting an optimum plate to plate impedance of about 8K ohms. My mod transformer is a UTC S-22 multi tap unit, capable of many different ratios. I'll pick taps that give me a 1:1 ratio at first to see how that does. Since I have a little spare audio power, I might try a small step down ratio which would effectively raise the plate to plate impedance to 10K ohms or so. As I understand it, this improves frequency response. I think this is feasible as long as the modulator has power to spare.
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