Sidebar Sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Getting a new TX for 80m AM

Captain Kilowatt

Captain Kilowatt

Professional Amateur
Staff member
Apr 6, 2005
17,260
12,139
823
60
Nova Scotia,Canada
For some time now I have been dabbling a bit with AM mode especially on 80m.We have a net here every Sunday morning on 3735 and I do try a bit with the crowd on 3885 but I have a QRPppp station compared to those guys. I just use a DX-60B and all of it's 50 watts. That is about to change. Some of you may recall that I was a commercial broadcast engineer for 22 years, well a former colleague has offered me a working RCA BTA-1S commercial AM broadcast transmitter for FREE! All I have to do is make the 600 mile round trip to Prince Edward Island and pick it up. I hope to do so the last week of the month. It will need extensive work to convert it to 80m but nothing beyond my abilities .It will run 1 KW carrier power at 100% modulation in continuous service which is what it was meant too. It runs a pair of 4-400's in the finals which are plate modulated by another pair of 4-400's. Walter says he has lots of spare tubes as well. I will of course have to throttle it back to 750 watts carrier power to meet the max power regulations here for AM service. ;) Still, being able to say "The rig here is an RCA BTA-1S with 3000 watts pep" sounds all too appealing. :D I have lots of other projects in the works so this may take a while but it will be worth it I think. I have a pro Sennheisser MD-421 microphone and a Gates limiter/amp as well as an Altec- Lansing microphone preamp so I should sound as good as the stations I used to keep on the air. This is going to be the longest three weeks coming, I know it will. Now if it just didn't weigh in at 900 lbs. :?

RCA BTA-1S info

Can you hear me now? :LOL:
 

Beetle said:
Probably wouldn't do too badly down at the OTHER end of the band either :D . What does that take as far as mains power?
Click to expand...

Yeah but keying that beast on CW would beat the relays out of it. :shock: For mains it requires 220 volts ac single phase at 20 amps,exactly what I had wired into my shack when I built the house. ;) Full load draws 16 amps with audio peaks.It consumes 3900 peak watts running full bore.
 
Umm, 'scuse me, but a 750 Watt carrier will exceed the 1500 Watt PEP limit fer sure.

In the old days, the "1 kW input" limit from the 1934 regs would allow an approximate 700 Watt carrier power. With textbook 100% modulation, PEP was approx 3 kW.

When they revised the regs to specify 1.5 kW PEP OUTput in the 80's, (errr, late 70's?) this sliced 3db off the top for the "full-gallon" AM operators. Seems to me I can still hear their screams echoing faintly on winter nights when the noise level is low on 75m. Talk about Long Delayed Echoes.......

Working backwards from the 1500 W. PEP limit, your carrier output would be limited to roughly 375 Watts, more or less, with modulation limited to 100%.

But if you're in California, they say that "Kilowatts" are measured differently there.

Not sure it would be worth moving there just for a few measly decibels.

Just be thankful the power supply is single-phase. Just about every broadcast xmtr I've ever seen larger than that requires 3-phase AC. When I was young and impressionable, I was told that the threshold from single-phase to 3-phase was around 1 kW. You could specify a 3-phase supply for a 1 kW, but the default design would be single. Anything larger was 3-phase. If you complained, the salesman would recommend a rotary converter.

But regulations aside, what you need now is a limiter/processor so you can run 125% postitive asymmetrical. Should sound genuinely sweet. Just how long would it be before some holier-than-thou comes to look at your wattmeter, anyway?

Umm, whatcha gonna use for a mike preamp? Input "line" level is way above what any low-Z mike delivers on its own.

An amplified D-104 would probably drive it directly.

73
 
Nomadradio,where am I? Check my avatar.The FCC means NOTHING to me. :p I can run any carrier mode at 750 watts be it CW,FM,or AM.In the case of AM that means 3000 watts pep.For SSB MY regulations allow 2250 watts pep.As for driving the line input of the TX,it will require approx. a +8dB level at 600 ohms impedance.I mentioned that I had an Altec-Lansing microphone preamp as well as a Gates audio limiter/amp as well.It allows for assymetrical modulation.I did this sort of work for a living for 22 years and installed a few AM and FM tx during that time. ;) Actually CCA made a 2.5 Kw AM TX that was single phase as standard configuration as well as a FM-25SP 25Kw FM transmitter which was also single phase. Also Continental made a 316F TX which was a 10 Kw AM tx and it was single phase 208/240 vac.It did have a three phase option. That beasts plate transformer delivered 10,000 Vac at 3 amps continuous commercial service. :shock:
 
Please let us know when you are ready to smoke test it as I want to be listening, and who knows, maybe Mr. Walsh will be out there to sing a few bars of 'Life's been good to me so far'... ;)

"Can you hear me now?" ...well, if you're all about tongue-in-cheek humor, you could name it 'The Big Verizon' :p
 
Oh this will be a winter project for sure.I have some time off work the end of the month and all is set for me to go and pick it up on the 28'th of this month.I will have to completely change the output tuning and build one from the parts already in it.Since a broadcast TX is single frequency there is no variable output tuning like on a normal HF amplifier.There is a tune and load control but the tune control has very little range.It uses fixed capacitors in the output tuning section.I hoope to be able to drive it with an external frequency source like my TS-820S possibly using the transverter out port and feeding the RF into the grid of the 6146B driver tube.All the HF rig would do is provide the carrier frequency,all the modulation and amplification will be done by the RCA transmitter.I will try and include some pix of the beast after I get it home.It weighs in at 900 lbs. BTW. :shock:
 

dxChat
Help Users
  • No one is chatting at the moment.


      Top Bottom
      Back