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Old High School Broadcast Transmitter on 40?




 
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Author Topic: Old High School Broadcast Transmitter on 40?  (Read 9102 times)
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VE3LYX
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« on: December 24, 2014, 05:14:04 PM »

This afternoon I was working on my shack and reading the mail on 40M on or about 7920. There was a discussion going on about the new rigs and out of the blue the subject turned to one of the fellows playing with  his old high school broadcast transmitter and perhaps putting it on the air on  40 sometime. He described it as a 50c5 final modulated by another 50c5 and if I heard correctly a 6aq5 speech amp. I immediately put the heaters on on my 45 transmitter but before I could join the conversation company arrived for a visit and I was done. However I have a 50L6 transmitter that I cathode modulate on 80M plus another 50L6 screen mod)unit that does both bands. Also a 6l6 AM only rig. I would love to hear that 50C5 rig on air but I didn't write any calls down . All I remember is one earlier on had call letters ending in BBC and there was a reference to that made. Someone will read this who knows. They were discussing 940 rigs just prior to this. I would like to hear the on air test and would be up for a QSO with one of my PW rigs if possible. (50c5 to 50l6 perhaps)Funny thing was I was at the time beginning construction of a 117p7 tubed Am rig. I sure hope someone will see this and connect the dots. Nothing more fun then low power
,  a bit of luck and a good QSO. The BK deal last week proved that for sure on Cw , now it is AM time.  Squeak up!
don
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Don VE3LYX<br />Eng, DE & petite Francais
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« Reply #1 on: December 24, 2014, 05:31:59 PM »

Running low power on 75M daytime and even 40M daytime should be a lot of fun. 40 gets picky sometimes and doesn't want to cooperate.

Fred
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Fred KC4MOP
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« Reply #2 on: December 24, 2014, 09:54:22 PM »

Funny, we were talking about the same thing around here, Clark (N1BCG) and I were talking about gifts we want under the tree, a Heathkit, or Knight kit, or maybe an Eico to play with and build on Christmas day.
Well, maybe mom and dad could swing a HW7...

Then we got onto those old AM transmitter kits they used to sell.
I built one in high school, I think it was a 12AX7 and a 6bq5, but it might have been a 50c5.
We both remember them working quite well for what they were.
Last night, I was looking through my old QST's and found a 13 watt rig, a 6aq5 plate modulated by a 12ax7, a fun rig no doubt, but hardly something to clear a frequency on 80 meters at night!
(QST August 1953)
People hear me running 5 watts carrier on 40, but working someone running 5 watts is rough most times.
Its very easy on the person running 5 watts, not so easy on everyone else.
Plus, if running low power, you must make really long transmissions, its a rule.

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VE3LYX
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« Reply #3 on: December 25, 2014, 10:48:12 AM »

Oh I didn't know about that rule. That article you have sounds very interesting. My experience with low power has been if folks know you are coming some will listen for you which makes a huge difference between success and failure. Others aren't interested and that is OK too. In every instance but one when I have made a sked or been asked to try it has been successful. I try to get in and out quickly figuring if they were kind enough to help me try I shouldn't hog their time. A few times my reports have only been - 2 db from my big rig on same freq at the same time. Anyway I am hoping too hear that 50c5 rig they were talking about. Some one was just running a PW rig a few minutes ago but didn't stay at it. I could copy him.
don
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Don VE3LYX<br />Eng, DE & petite Francais
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« Reply #4 on: December 25, 2014, 12:37:48 PM »

It works well sometimes, other times its not so great.
If you have 4 guys running 100 to 300 watts and a guy running 5 or 10 watts comes on and makes a long transmission,
half the guys can not hear him, other people think there is no one on frequency and start calling CQ, etc.

I love the idea of QRP, I used to do it all the time on CW, my first real rig was an HW7, but AM does not work well for rag chewing at 5 watts most times.

I will likely build some sort of low power rig using a 2e26 or something like that, 10 watts carrier would be nice.
I already do the old buzzard transmissions, so I will be all set for 80 at night with my 10 watts.
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VE3LYX
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« Reply #5 on: December 25, 2014, 12:54:58 PM »

Of course if a group of like minded folks got together somewhere on a band for the purpose of low power AM that could work very well. Some can't hear ,some don't want to I fear. However if on 40M we ever get away from using only the area used by the foreign broadcasters and go to the unused portions which are many then anything is possible. Old buzzard for me is hard. Don't have that much to say. So fire up that 10 watter and give it a shot.
don
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Don VE3LYX<br />Eng, DE & petite Francais
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« Reply #6 on: December 25, 2014, 01:45:17 PM »

You have to carefully pick your times for QRP. Forty meters in the daytime can work well if the band is not crowded. Back in the 80s and 90s, I had many QSOs with Mike - N3HJQ. I don't think he ever ran more than 10 Watts. On 75/80 meters, afternoons and late nights are best. Back in the days when I spent a good amount of time on the air after midnight on weekends, we would often run low power. I recall one night running so little power my wattmeter was not moving on the 100 watt scale. Yet everyone in the group could still hear me. Lots of fun.

The QRP thing was big for a while back around 96 with the PisWeaker Nets and the cake pan transmitter rage. Some attempts to restart QRP nets occurred in 2006 and 2011. Maybe it's time again

The PisWeaker Manifesto from 1996 is attached. A link to PW schematics and cake pan rigs below.

http://amwindow.org/tech/htm/pw.htm

* PisWeaker Manifesto.pdf (194.66 KB - downloaded 321 times.)
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VE3AJM
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« Reply #7 on: December 25, 2014, 02:10:26 PM »

I recall Tim WA1HLR relating a story about a 50C5 tube or transmitter, from back in his younger days. Could be a different story.

I remember Mike N3HJQ from somewhere in Philly I believe. He had a TMC GPR-90 receiver. Even with the low power, he had a very effective signal and audio with excellent articulation. No FMing or instability, and no internet CQing.

Al VE3AJM
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AJ1G
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« Reply #8 on: December 25, 2014, 04:35:45 PM »

10 mW PW Tom Vu from one of the Sunday afternoon QRP nets we had going a few years back..don't recall if I recorded this locally or someone forwarded it to me...


* K1JJ 10 mw 3880 HPSDR.mp3 (467.03 KB - downloaded 244 times.)
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Chris, AJ1G
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VE3LYX
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« Reply #9 on: December 25, 2014, 08:06:37 PM »

That is pretty good.
I heard a New Jersey station calling today obviously low power and stated he was running about 33 watts. The band was pretty noisy But I switched on the 45 tube rig tuned up and gave it shot. He said try again and he would see if he could get me in the clear. He repeated back to me some of what I said but said he had a high noise situation but we should try again when conditions were better as he was just getting snitches. I worked the BK with my CW rigs the second weekend and had decent results. Something fascinating about a glowing tube or two, a crackling set of earphones and a successful QSO. And if you built it a few days before from your junk box that is a bonus.  I still have my 50L6 rig and use it on Cw with it matching regen a bit. I built a one tube (01A)cathode modulator for the other Cw rigs and tried it a week ago on the 50L6 rig s it is also cathode keyed. It worked fine. I also have a two band 6l6 Am only rig but so far it hasn't produced.  But the right conditions should produce success. It would be exciting to get something going. Even if like the BK deal just a couple of weekends a year. I read all the cake pan and PW stuff with get interest. Of course half the battle to working low power is to hear well. Sometimes overlooked. For me here the beverage in studio B or the Loop & regen in Studio A give a big advantage.
don
This is the 50l6 tx and it regen mate along with the one tube modulator
don


* PICT0023.JPG (276.15 KB, 1280x960 - viewed 467 times.)

* PICT0024.JPG (271.92 KB, 1280x960 - viewed 473 times.)

* TNT modulator.jpg (43.22 KB, 448x336 - viewed 426 times.)
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Don VE3LYX<br />Eng, DE & petite Francais
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« Reply #10 on: December 25, 2014, 11:12:34 PM »

I had planned to build a low power rig last year, and as far as I got was trying to modulate my vfo/exciter unit with poor results as it was not built for that.

I dug out some 2E26 and 6L6 tubes and a 25 watt modulation transformer, and a nice power transformer.
Can you still get crystals?
That would make it easy, a vfo is a lot of work.
A 40 meter rig needs only 3 or 4 xtals, 7280, 7285, 7290, and 7295, or you could pull a crystal some, at least 50 Khz I think.
20 watts carrier could make some nice contacts on 40.
 
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Carl WA1KPD
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« Reply #11 on: December 26, 2014, 02:03:33 AM »

Here is my version.

One rule, had to use all existing parts from junk box.

Old BC radio chassis with PS
6AG7-6L6
Plug in coils.
BC cap for resonance.
Odd meter. Since been changed to a 30's Test Toob meter
Modulator is a 12AX7 to a 6L6

Works fine. Rcvr is a HB regen.

 


* homebrew2.jpg (54.04 KB, 666x292 - viewed 493 times.)
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Carl

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
VE3LYX
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« Reply #12 on: December 26, 2014, 07:36:12 AM »

I have both , rock bound and vfo. Hartley VFOs are easy to build. I just remember "grid, grid leak, top of coil, coil centre tap to cathode and bottom of the coil to ground. Don't bother with schematics anymore.  The last two were built on a Saturday each. Carl, what a cool rig!  When and where do you use it? A regen too! Way to go!
don
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Don VE3LYX<br />Eng, DE & petite Francais
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« Reply #13 on: December 28, 2014, 05:27:39 PM »

Well, this is a bit surreal, but I was discussing my high school station's transmitter (a Graymark 505) and the topic "drifted" to putting it on 40M. Even at 100mW, it might have a shot at making a contact, particularly if I can set up a sched so someone would be listening (carefully) for it.

The original circuit was transformerless with series-wired heaters and half-wave rectified AC to produce B+. The circuit is nearly identical to the Knight Broadcaster shown below. Depending on which way the non-polarized power plug was inserted, the RF common buss was either at "neutral" or 120V. Blowing a circuit breaker was a definitive test to confirm this.



A friend of mine from those days thinks I'm out of my mind to even be fussing with it today, but I was determined to make it less lethal to operate and to document its performance. The first step involved adding an isolation transformer for B+ which allowed grounding the AF and RF busses without trepidation.

The transmitter achieves a reasonably flat frequency response after the C4/R6 pair are removed. These provided a significant bass boost for turntable cartridge equalization, a nod to its use as a "wireless phonograph oscillator". This actually helped when the operator used the included crystal microphone, sounding much like a D-104 microphone without the acoustic padding!

My transmitter had a dramatic hum, and while this is usually caused by things like a hot buss or "carrier current hum" (imposing an RF signal on power lines), this time it was due to a component selection error. R13 is supposed to be two 1.8k resistors in parallel (Brn/Gry/Red) but the builder put in two 180 Ohm resistors (Brn/Gry/Brn) instead, and that defeated the isolation between the two filter caps. I wish I noticed this in the late 70s. That changed dropped B+ a bit and fixed the hum issue.

The other change is RF coupling to feed a 50Z load. The normal output is at a very high impedance point (a DC blocking cap on the oscillator plate) and that causes the frequency to wander significantly when tuning an external PI network. A much better output tap comes from the grid winding of T1. A DC blocking cap (to preserve bias) and a variable series inductor allows matching to a low impedance load much easier and with greater frequency stability. I get over 40mA of current into a 50 Ohm load!

Admittedly, this is a bit of a ridiculous project, but it's bringing back some fond memories...
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N2DTS
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« Reply #14 on: December 28, 2014, 10:04:05 PM »

What a strange design!
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KB2WIG
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« Reply #15 on: December 28, 2014, 11:41:55 PM »


     " What a strange design! "


Yeah, it lacks depth.


klc
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VE3LYX
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« Reply #16 on: December 29, 2014, 07:47:30 AM »

Ah Ha. Thanks for showing up. You certainly got my interest. Stuff like that is very interesting. Worst case you might need to build an linear. Quadfour 25l6 s? Anytime you want to test give me a heads up. I will listen "carefully" I understand completley what you mean by that. I have two 50l6 Txs hear. One is AM but is apart at the moment for mods but the other is a bare essentials 50L6 design that I have been able to cathode modulate with a 01A self designed modulator.  I run 50c6 tubes when I can. They fit but are a bit more robust. On CW with a 50l6 I have worked the east coast with good RST. Am would be a bit less but should still be doable. I have worked Maine and Virginia on 1 tube AM rigs i have with under 5 watts so should be doable on a good day. Thanks for the Schematic. As for transformerless supplies I installed Polarity plugs on all my so equipped rigs which eliminates the possiblity of backwards plug ins and a hot chassis.
don
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Don VE3LYX<br />Eng, DE & petite Francais
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