The AM Forum

I'm interested in what the BC rig folks here do to turn the power down for the 1.5K limit (of course I assume that is done..).
Some transmitters have a switch, some do not.
Some have taps on the transformer primary, some use resistors..
In some cases I think I see the modulator left as is and the final reduced. I thought that would cause more distortion or overmodulation.

So, is it arranged via the simple power level switch, or another more complex method like reducing drive and modulator power?

Patrick,

Why would you want to? ? ?   ;D  ;D  ;D  QRO is always the way to go  ;D  ;D

 .....::)......

Wondering Same.

73
Jack
KA3ZLR

If you must, the B+ is dropped either via taps on the power transformer, or the inclusion of resistors in the B+ line as you suspect.

The modulator, however, is also modified in most cases by the application of an attenuator in the audio input line.  Failure to do this will result in a LOT of overmod capability.

No one I know running a BC rig uses a custom reduced power level on HF.  Most BC rigs of the 1kW or 500W class had factory cutbacks for 250W, and that is often used as all it takes is the push of a button if you find yourself in need of less that 375W carrier.

73DG

Patrick,

    You could always keep the carrier at full strap an simply reduce the audio gain to limit PEP power. For close in short skip, 30% modulation is fine when the carrier is 50 b over nine. Alternatively run NBFM!! For several years in the 1980's I ran sliver bandwidth NBFM on 7157.5  with Ralph KD60S. He did the same with a Johnson Desk on hi tap, and I was using a Globe King 500. Both had FM excitation modifications. Wedged between two big foreign broadcast stations we had near full quieting QSO's. The sideband energy from the tall ship stations on 7155, and 7160 prohibited AM, an possibly even SSB.

Jim
WD5JKO

Thanks for this insights! I just wanted to know how it was done officially and after-the-design.

The Tucker KW is not a BC unit but it has a "low power/normal" switch. That is just a couple of 750W heating elements in series with the plate transformer primaries and the regulation is terrible on low with those things in because they heat up with modulation and sag the HV down to 2KV and the efficiency is gone at that point. If there was a switch that did the right thing, it would be more or less 'calibrated' to this or that power level. That could also work with just the modulation level as suggested.

The BTA-250 is smaller but I'd like to be able to go to 100W on it and it does not have the power change option.

The NBFM is really a good idea. I like FM.

Mine has three power levels.  Vac relays on the power transformer and one with a cap.

My Friends W0VMC modified 20v2 has 4 levels and uses resistors.  HIs runs 375watts carrier on low and he keeps it legal.

C

My BC1-T originally had a big rheostat to precisely adjust +HV to the final. It could be manually adjusted at the transmitter, or remotely from the studio using a reversible motor.  I replaced it with a bank of fixed power resistors to drop the voltage down from +2600v to just over 2000. I would recommend leaving the full +HV on the modulator stage and reduce the audio input level to  maintain a maximum of 100% modulation. That will allow more audio head room and result in a cleaner signal with less likelihood of flat-topping on positive peaks. More importantly, a resistor in series with the modulator HV line will cause the class-B modulator plate voltage to sag with the variable modulator plate current, a no-no with the class B or AB modulator because it produces considerable distortion.  You want to maintain as perfect regulation of the modulator plate voltage as possible.

The low power/tune mode uses relays to switch the main plate transformer from 230 volts to 115, and automatically switches in an audio attenuation pad to maintain the same modulation percentage, and reduces the bias on the modulator tube to operate at lower plate voltage.

Patrick, the low power/normal switch may be for tuning. I have one on the HN-500 that is used in that way. It limits the DC so you aren't tuning up at full bore. I'll lay money that is what its for. I've seen many home-brew rigs with that set up.

Do not use series resistors unless you need a shack heater. Change taps or run the 240 volt tap on 120. The best way, if you must, is to put a variac on the transformer primary. Then you can dial the yield you need.

You know Frank, I never did give it a thought how the circuit is constructed in the HN-500. Usually, I'll run about 2500 VDC on the 813's. In the 'low power tap' I get about 1900 VDC and about 300 watts of carrier.

I needed to install several power levels for low power after sunset use on a Gates BC500G.  The 500 watt version of that transmitter had a stock plate transformer that could be switched to full or half voltage, which wound up at 500 or 125 watts.  We needed a 200 watt level and a 50 watt level, so I installed a variac that we switched in and out of the primary of the plate transformer.  That, of course controlled the plate voltage of the modulators as well as the PA tubes.  The rig used 833s, so no screen supplies were needed.  However, when the plate voltage on the modulators was cut in half, the grid bias was completely cutting off the modulators, and of course it sounded pretty bad.  So I used an extra set of contacts on the plate transformer contactor to insert some resistance into the grid bias supply, and voila, great sounding audio and TPO of about 50 watts.  It was kind of wasteful of energy since each 833 needed 100 watts worth of heater power, plus the 6 807's and a couple of 12BY7s heaters.  We figure that the transmitter burned at least 500 watts of AC to get the 50 watts of RF out.

Several years ago we put in a BE AM1A which can be set to any power level from a couple of watts up to a KW.  It barely moves the electric meter at 50 watts.   But the only thing that glows on the new transmitter is a couple of led's.

73
Ted  W8IXY

Did the 833's ever ware out?

I have a rig that runs 833a's and I use a 240 volt variac in the plate transformer primary.  I don't go for really low power like 50 watts but it seems to work okay.  No audio distortion.

I don't remember exactly.  They used the transmitter like that for at least 10 years, and we changed the 833s and 807s about once every year or two.  They just didn't last like the American manufactured tubes from the 1960's.  Actually the 807s wore out before the 833s.  A pair of Chinese 807s would last for less than 6 months.  We foind some NOS 807s from Antique Electronic Supply, with date codes from the late 1950's, and those lasted 3 years!

73
Ted  W8IXY

I was under the impression that running a h.v. supply as a load on a variac continuously was a bad idea.  Something about the resistivity of the variac.  Okay to slowly bring up a supply with one to charge caps but then bypass it before the load comes on.  Evidently it isn't a problem?  I guess it has to do with the hugeness of the variac  but I thought it could be a problem even if it is rated for the same current that would trip the breaker.

 

Usually running the KW rig on low OR half power is close enough for government work. We won't turn you in.

Fred

Variacs in the primaries is the way to go.

On my RCA BTA1-R I removed one of the PA tubes and changed a couple of the parts as per the manual to convert the output from a 1 kW to a 500. To reduce the plate voltage I ran 120 volts to the plate transformer instead of the original 240 and that gives me a reduced plate voltage of around thirteen hundred volts and an output of about three hundred seventy five watts.
Being that the RCA uses all triodes for the modulator and PA (833) no issues with screens.
The big issue for me was trying to find a neutral return for the HV transformer, the RCA has no neutral brought out to it so in order to run the HV transformer on 120 volts I had to return one side of the transformer to ground. The transmitter is feed from a sub feed panel that I have out in the shop and being a sub feed the neutral and ground are not tied at that point. As per NEC the only tie between neutral and ground is at the main panel. I know your never supposed to run any current over the ground system but the transmitter has a #6 safety ground returned to the station ground an 3/4 copper bus bar that’s tied to the sub panel ground and the main panel ground and neutral at that point. There is a little sixteen gauge 120 volt circuit with a small cord that is used for cabinet lights in the transmitter but the neutral circuit for that appears too small for the HV transformer return.
In order to correct this I plan to install a 240 volt autotransformer (virac) in the 240 volt feed to the HV transformer and use that to control the AC input to the transformer. Looking at the RCA solutions to reducing power by inserting resistors in the output of the HV supply appear to be a huge waste of energy by developing lots of heat and basically just wasting power thru a resistive load. I am curious to see what if any issues there would be in using an autotransformer to control the AC input to the HV transformer?  Have never seen this done but common practice in broadcast transmitters to use a autotransformer to control input to the screen supply.

I've used a variac in the primary of the HV transformer on my Gates BC-1G for a couple of years now.  No problems....

I don't quite understand this? Why didn't you just hook up you 120 where the 240 used to go? Technically, one side would be at ground in this way except you would have the neutral and panel ground in parallel which is the way, I suspect that most of us are doing it with the three pin common plugs we use for 120 VAC. 

For reasons I have never completely understood NEC states that neutral and ground cannot be connected except at the main panel and all sub panels have to keep the neutral isolated from local and service ground at that panel, also think there is something about not being allowed to return any current on the ground being that’s only a safety connection. The RCA transmitter is designed for 240 volt operation being that all the transformers, the fan and all the control relays are 240 volt. So the connection to the transmitter is a three conductor cable brought out to a twist lock plug that has two wires for the 240 volt input and a safety ground. No neutral! So there is no neutral available in the transmitter to connect one side of the HV transformer to. So I connected the one side of the primary of the transformer to ground with the other side of the primary going to the 240 volt plate contactor. The other wire from the plate contactor is just tied back and taped off and that’s how I develop 120 volts on the HV transformer. The MV (600) volt transformer still receives 240 volts along with the filament and control circuits. Being that the transmitter has heavy grounding that’s attached to the radio and electrical system grounds not worried about doing this but just something about tying one side of the primary of the HV transformer to system ground is unnerving, that’s why I want to install the autotransformer and go back to not having any of the incoming AC grounded in the transmitter.

Thanks, Now I understand.

The 120V 20A variacs in the Tucker KW were good to less than 5% regulation in any position, comparing input sag to output sag under a 13-18A load. This was measured as part of some very detailed troubleshooting to find the voltage drop of the B+ supply under load. It was not the variacs.

I am an idiot, or maybe senility is starting early. My transmitter is a RCA BTA1MX and not an R, should have remembered this because the MX with its 833 tubes is way better looking than the R with the 4-400 tubes. Pictures of the transmitter:
 http://staff.salisbury.edu/~rafantini/RCABTA.htm
Look and judge for yourself, sorry don’t have a picture of an R series.

Ahhhh, don't beat yourself up too badly.  As they say, "Beauty is in the eyes of the beholder."  I think the 1R1 is quite nice looking, with its symmetry of appearance with the nice window in the center and the large cast aluminum "RCA broadcast transmitter" just beneath.  "6 of one and a half dozen of the other, I suppose.    ;)

The "safety" ground is not supposed to carry any of the load current. The neutral is supposed to be treated the same as a hot, everywhere past the entrance where neutral and ground are bonded together. One of the reasons is that without the separate safety ground, if the neutral connection were lost, the entire unit, case and all could become hot with 115v. The same could occur if one of the hots shorted to a case left floating with no safety ground. Keeping ground and neutral separate beyond that one bonding point also reduces the tendency for ground loops to occur.

Some older 230-volt appliances like electric kitchen ranges make no distinction between neutral and ground. The same goes for broadcast transmitters.  I separated the two in my BC1-T by lifting  the original ground/neutral from the cabinet, and running a separate ground wire from the transmitter cabinet to the shack ground. The shack ground is connected to a 8' ground rod, some ground radials at the point where the rod is driven in, plus a #6 wire, buried a couple of inches in the soil in the same manner as a radial wire, bonds the shack ground to the main service entrance ground at the house.

I have noticed that when I temporarily short the neutral wire in the shack to ground, there is enough current sometimes to cause a visible spark.  There may be a volt or so measured difference between ground and neutral.  With the short in place, the hum level in my audio equipment increases by several dB.

If you pull a heavy 115v load off one leg of the line, a voltage drop will occur in both the hot and neutral. That will cause the voltage to sag.  But the voltage drop in the neutral is additive to the other side of the line.  Assuming the same size wire is used for the hot(s) and neutral, the voltage at a 115v outlet on the opposite leg will increase by approximately one-half the voltage sag on the side under the load.  I take advantage of that phenomenon in my HF-300 rig, in which everything runs off 115 volts (the transformers in the rig have 115v-only primaries). The filament line is connected to the opposite leg of the power mains as the plate transformers. So when I transmit, and draw plate current, that causes the filament voltage to increase slightly, rather than to sag.  That helps maintain emission from the tube filaments under heavy plate current load.  OTOH, the modulator bias supply connects to the same side as the plate transformer right at the transformer terminals, so that the grid bias voltage sags in step with the plate voltage, reducing distortion and maintaining more constant static plate current under variations in line voltage.

Because of the slight variations in potential of the neutral wire as the circuit is placed under load, if the ground and neutral were bonded together at the appliance in use, the safety ground would share some of the load current with the neutral wire, which it is not supposed to do.

http://en.allexperts.com/q/Electrical-Wiring-Home-1734/Neutral-vs-ground.htm

I thought I read somewhere that hams who were already running AM at the old 1 KW DC input power rules were "grandfathered" when the new 1500 PEP rule went into effect, and they didn't have to reduce their power to comply with the new regulations. Is that true?

Not so. We all had to cut the power back. I just bought a Gates BC1-T and I am trying to find the ER article on it by AA8A. Seems like there are two 710 ohm,high power resistors in the primary of the power transformer that are added to drop the power bak to 375 watts carrier output.

Much ado about nothing.

Indeed. 

The 'law' is one thing, but just who is going to accurately measure your output? ???

Why would they be there in the first place? ???

Who would rat you out anyway? >:(

The Commission has a lot more to do with few staff to delve into the arcane world of HF ham ops. :-\

73DG

And to boot, they deleted the old rule that said amateurs had to have the instruments on hand to "accurately measure" their power.

The article was actually by W4MEW in ER. They were 710 ohm @190 watts. But much ado about nothing.The fee-cee's can't find time to get rid of 11m scofflaws running >kW's. I don't think they would worry about a few more watts on HF AM on the amateur radio bands.

Fee Sees busts people based upon the amount of complaints, and the 'pull' of those making the complaints.

I've seen 11 meter operators busted for talking crap about another 11 meter ops wife the NEXT day.  Of course, said wife worked in the same building as the LA field office, 1 floor below.

Other idiots have been on the air for years, and never had any issues.