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Dummy Load Transmitting Legality

A

AMRadioGuy

I'm curious... Say I were to buy an old 250 watt tube transmitter as a project and get it running... Could I legally run it on a low setting (the Gates unit I'm thinking about would do 50w at low power I believe) into a dummy load?

I'd really love to run my Audimax/Volumax pair through a tube rig and be able to hear what it would've sounded like on the air back in the 60s, and thought this might be a potential way to do that (yes, I do realize it would be a pretty complex way).

The answer is likely no, but I thought I'd ask anyway...
 
Oh... to clarify... the idea here would be to have a very minimal coverage area, literally enough to pick up the signal from a radio right beside the unit and monitor/record the result.

The idea is not to try to "broadcast" with this setup at all, or do pirate radio.
 
Could it be possible to analyze the sound coloring that a vintage tube transmitter adds and recreate it in a legal, 100mW circuit?

I note that tube-based Part 15 transmitters already exist, with a wide variety of different designs, so in principle it shouldn't be too difficult to compare it against a genuine vintage commercial unit and modify the modern tube unit so it sounds similar?

On another track, for a station with a proper AM license, are these transmitters are still technically legal for them to use (if not particularly efficient) provided their RF outputs can be filtered down or adjusted such that they comply with modern standards as closely as possible?

I don't think anyone in their right mind would want to run a vintage 50kW unit, given how inefficient they are compared even to their contemporary solid state versions, but I don't see how it would be meaningfully hard for a Class C or D station to manage.

c
 
@cc333: Old AM transmitters remain legal to operate on the AM broadcast band. The rules have not meaningfully changed in decades.

The main reason the really old transmitters aren't used much anymore is because of parts availability, not so much efficiency. Essentially all tube manufacturing in the US was gone by the mid 80s, and most overseas is now defunct as well. Tube transmitters require new tubes once a year (rough generalization), so if those aren't readily available, replacing the transmitter is the only choice.
 
Some hams have also modified old AM broadcast transmitters for use on the 160M band. Of course they can't play music, but a few have even implemented AM Stereo on their transmissions, and used it for stereo reverb and sound effects.
 
This (absolutely amazing) Canadian electronics engineer has one such beast in his home:


I can't imagine that any engineering types who might be reading this would be unfamiliar with Paul Carlson, but if not, check out his channel post-haste.
 
kevtronics said:
Yes, as long as it does not exceed FCC Part 15 limits:

Federal Register :: Request Access

www.ecfr.gov www.ecfr.gov
Click to expand...
IIRC, you can't just modify any old transmitter to be less than or equal to 100 mWs and call it Part 15 compliant. The device itself (or a representative sample) has to be submitted for evaluation and then type-approved. If my memory's accurate, the OP's idea of running a 250W device into a dummy load wouldn't be strictly legal unless he put himself and his xmtr through such a fire drill.
 
There are still lots of those tube rigs in service, mostly as backups. WXXI 1370 in Rochester still has its 1955-vintage RCA BTA-5G in great working order, though it only gets very occasional use as relief for the Nautel XR6 solid state main.

The power bills are much lower, especially with MDCL on the Nautel, and if the AM still has any listeners I doubt they can tell the difference.
 
Lots of interesting conversation here...

Yes, I realize that those old rigs are power-hungry beasts and that tubes can be difficult to get.
cc333 said:
Could it be possible to analyze the sound coloring that a vintage tube transmitter adds and recreate it in a legal, 100mW circuit?

I note that tube-based Part 15 transmitters already exist, with a wide variety of different designs, so in principle it shouldn't be too difficult to compare it against a genuine vintage commercial unit and modify the modern tube unit so it sounds similar?

On another track, for a station with a proper AM license, are these transmitters are still technically legal for them to use (if not particularly efficient) provided their RF outputs can be filtered down or adjusted such that they comply with modern standards as closely as possible?

I don't think anyone in their right mind would want to run a vintage 50kW unit, given how inefficient they are compared even to their contemporary solid state versions, but I don't see how it would be meaningfully hard for a Class C or D station to manage.

c
Click to expand...
I'm sure that would be possible... but I'd need to be able to run a tube rig for an extended time to analyze the sound, which would either require a willing station owner let me play stuff over the air with their old tube rig, or would require something like what I was proposing.
fybush said:
There are still lots of those tube rigs in service, mostly as backups. WXXI 1370 in Rochester still has its 1955-vintage RCA BTA-5G in great working order, though it only gets very occasional use as relief for the Nautel XR6 solid state main.

The power bills are much lower, especially with MDCL on the Nautel, and if the AM still has any listeners I doubt they can tell the difference.
Click to expand...
I was curious about that. The stations I work for have several tube rigs at various sites, but only one (supposedly) is still in operating condition and was last on the air within the last decade as a backup.
 
A number of these older transmitters have an output that was for the mod monitor and frequency deviation monitor and has a widely adjustable output power. You could run the transmitter into a dummy load and take your antenna feed from that output. You must remember the antenna bandwidth can play a significant role in the overall sound of the station. I have a 6 tower array that is extremely narrow banded. There is a noticeable difference in the sound of the station when I run it in the non-DA mode using the same transmitter and power. Yes, the antenna system could be re-designed to have a broader bandwidth. That would be a considerable expense and with the prospects of AM in the future would not be worth the trouble or expense.
 
fybush said:
The power bills are much lower, especially with MDCL on the Nautel, and if the AM still has any listeners I doubt they can tell the difference.
Click to expand...
Are there any airchecks around that demonstrate the differences in sound between modern DX50s and Nautels and these great old RCA tube beasts like the 5G? I'm imagining airchecks made specifically by repeating the same audio through one transmitter at a time, perhaps during a late night test.
 
AM receivers back then had much wider receive bandwidth. AM transmitter bandwidth wasn't limited to 9.5kHz either. A solid state AM transmitter would sound as good or better than an old tube rig if it was fed with non NRSC limited audio into a well matched antenna system.
 
yeoldeschool said:
Are there any airchecks around that demonstrate the differences in sound between modern DX50s and Nautels and these great old RCA tube beasts like the 5G? I'm imagining airchecks made specifically by repeating the same audio through one transmitter at a time, perhaps during a late night test.
Click to expand...
Usually you only work on one transmitter at a time. A back up tube transmitter might be tested once a quarter just make sure it still works but depending on your power company there can be surcharges if you "surge". If you are a customer of a TVA electric co-op, cold winter nights can be peak power hours thanks to heat pumps having to switch to the emergency electric coils if it is too cold for the heat pump to work. The new heat pumps do a much better job but 20 or more years ago that wasn't the case.
 

Dummy Load Transmitting Legality?​

Absolutely Legal provided you do not radiate. But then you're not Transmitting.

Do you want to play part 15? Just follow the rules and produce a quality AM signal. better than most Broadcasters, but don't expect it to go far. Bet you can make AM sound really good though. They do it across the pond but not so much here. The normal here is about 3 to 4 kHz audio response and it does sound like crap.

In fact, most AM Broadcasters don't seem to care about the service. Just look at their Transmitters and Antennas. Here we have an AM that has a night time power of 28 watts and a compromised antenna system. We lost 2 50,000 watt AM Stations after they were purchased and sold off their optimum antenna sites and were operated at reduced power and at compromised antenna sites before going dark for good. They cashed them out to prop up their financials for the Corporations. Great for local public service.

Real story here. I was driving down the road during the daytime and a Public Transportation Bus drove next to me and totally wiped out the Local AM Station that I was listening to. Plasma TVs were another fine product as are PWM Solar Charge Controllers, and the list goes on and on and on for noise affecting AM Broadcast. AM radio on a Hotel room clock radio way back in the 1980s was always wiped out.

Just who was supposed to protect the AM Radio Service? Guess they failed all of us.

That's ok, FM is next to go. The band is over crowded, over compressed and lacks quality content, and that's the Digital FM. The Analog FM sounds just as bad.

Think streaming will save us? Not if it's hacked and goes down, but at least they will get rich off all the subscription fees.

Just remember that cellphones and the internet always get overloaded and fail during a disaster.

73
 
fybush said:
There are still lots of those tube rigs in service, mostly as backups. WXXI 1370 in Rochester still has its 1955-vintage RCA BTA-5G in great working order, though it only gets very occasional use as relief for the Nautel XR6 solid state main.

The power bills are much lower, especially with MDCL on the Nautel, and if the AM still has any listeners I doubt they can tell the difference.
Click to expand...

Given the number of FM translators with an AM station as primary station, having a back up "STA ready" AM transmitter is wise.
 
Greg Strickland said:
Given the number of FM translators with an AM station as primary station, having a back up "STA ready" AM transmitter is wise.
Click to expand...

Good point, Greg. As long as the backup is licensed as an aux, it counts as being on the air for purposes of the translator operating.
 
PTBoardOp93 said:
Tube transmitters require new tubes once a year (rough generalization), so if those aren't readily available, replacing the transmitter is the only choice.
Click to expand...
That is too much of a generalization. Glass envelope tubes like the 4-400A (QB4-1100) can average around 3,000 to 3,600 hours. Old triodes like the 833 can give several years of service run 24/7. Ceramic and metal tubes can see 10,000+ hours in some cases. The tuning, amount they are pushed, cooling and other factors can help lengthen the life of tubes. And some, just to be ornery, can die young.

Good luck finding any of those tubes today. I so loved my 4-400-A tubes that I had one enveloped in clear plastic for my desk... I built 6 1kw transmitters with 4 of those tubes, and a whole bunch of FMs with just one 4-400-A.
 


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