reducing output on a KW BC rig.

[Opcom]

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?

[The Slab Bacon]

Patrick,

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

[KA3ZLR]

 ...........

Wondering Same.

73
Jack
KA3ZLR

[W7TFO]

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

[WD5JKO]

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

[Opcom]

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.

[ke7trp]

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

[k4kyv]

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.

[W3SLK]

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.

[WA1GFZ]

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.

[W3SLK]

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.

[W8IXY]

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

[W2PFY]

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.

Did the 833's ever ware out?

[KB5MD]

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.

[W8IXY]

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.

Did the 833's ever ware out?

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

[K5UJ]

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.

 

[flintstone mop]

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

Fred

[WD8BIL]

Variacs in the primaries is the way to go.

[KA3EKH]

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.

[Jeff W9GY]

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....

[W2PFY]

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.

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. 

[KA3EKH]

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.

[W2PFY]

Thanks, Now I understand.

[KB5MD]

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.

 That's odd, I have a BTA 1R1 and it uses (4-400a)'s rather than (833)'s,  Maybe there were two different models of the BTA 1R1?

[Opcom]

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.

 

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.