Armstrong FM tube transmitter efficiency

[bossbill]

Anyone else have a 10kw one-tube Armstrong (from Italy)? Last time I checked with Armstrong, they had no clue as to why the transmitter is not nearly as efficient as their brochures suggest. It works great, but puts a real heavy load on the 125 amp breaker on the wall. I'm thinking it runs about 10 amps over the published number. It's been like that from the beginning about two years ago, we're getting ready to move it, and sure would like to find out if anyone actually knows anything I should look for. Yes, it's tuned for efficiency.

 

[frankberry]

Is the line voltage within specs? If the voltage is low, the current will go up.
It's possible that their specs are "the best" that the transmitter can possibly do. Not necessarily "real world" numbers.

 

[bossbill]

Line voltage is about 230 volts. Armstrong claims power consumption 17 kw for 10k out. It's running closer to 21 kw. Armstrong basically said "it is what it is". I should have pressed the issue two years ago. Now that we're getting ready to move into a new building with new service, I thought it would be a good time to re-address the issue, since I have a couple of options. I'm hoping someone out there may have had some experience with this transmitter. The final tube is a 3CX15,000A7. Maybe this points out a problem with buying from an importer. At least with a domestic manufacturer, there might be some kind of experience base to draw from.

 

[frankberry]

I once had an issue with a Harris AM transmitter. The PA efficiency wasn't nearly what it should have been. I got pretty much the same answer from them.
Could be a bad LC combination which is giving low efficiency. Grounded grid? The RF choke on the Cathode might need a bit of tweaking.
Just some random thoughts.

 

[rfry]

I once had an issue with a Harris AM transmitter.... Grounded grid?

Just to note that Harris did NOT supply grounded-grid r-f amplifiers in the final stage(s) of any of their AM or FM broadcast transmitters for at least the last 30 years.

RF (Harris Broadcast Division FM Product Line Applications Engineer, retired)

 

[rfburns]

Is this a single phase or 3 phase unit?

RFB

 

[Tom Wells]

Is this a single phase or 3 phase unit?

RFB

Thank you rf.
I would sorta expect 3 ph at 10 kw.
If 3 ph, amp clamp all 3 and see if they agree.


Next is how far away is the dist xfrmr? Sometimes all three phases don't balance because of what
another customer is pulling from the line. Or you've got one of those funny hi-leg situations.

Next if three phase, is there a delta to wye xformr there and/or do you know if your feed is wye or delta?

You're moving, so it really may draw less power under a better supply.
Make a slew of readings now, so you can compare operation and voltage/current in the new location.

 

[littlejohn]

Do you have the initial test data for the box? How do your readings compare to it? Is it running drive starved? i.e. cathode to ground RF impedance too low, or drive amp output too low or improperly coupled to the cathode? Armstrong's numbers - a bit under 60% efficiency - seem about right for a grounded grid at VHF using a 3CX series tube. Bit lower at the top of the band, bit higher at the bottom. You're doing something under 50%, which seems low to me. Something is mis tweaked in there, seems like.

 

[bossbill]

Single phase, not drive starved. Solid-state exciter, driver, I need to go back and check the "factory" numbers.

 

[frankberry]

I once had an issue with a Harris AM transmitter.... Grounded grid?

Just to note that Harris did NOT supply grounded-grid r-f amplifiers in the final stage(s) of any of their AM or FM broadcast transmitters for at least the last 30 years.

RF (Harris Broadcast Division FM Product Line Applications Engineer, retired)

R. Fry.

I didn't say that the AM transmitter was grounded grid. I was inquiring if the Armstrong FM transmitter was grounded grid.
The AM transmitter in question was an MW1. It operated at 540kHz. The low frequency may have been a problem for the transmitter as it never met the Harris efficiency specs. It did sound good though.

 

[rfburns]

Single phase, not drive starved. Solid-state exciter, driver, I need to go back and check the "factory" numbers.

My experience has been that the single phase units at that power level are somewhat less efficient. I had a 25kw CCA single phase transmitter that was terribly inefficient compared to the 25kw CCA 3 phase unit they replaced it with.

I'd see what the factory numbers are - you might also check with Armstrong and see if they can put you in touch with some other users of that transmitter and see if they are getting similar results using it in a single phase configuration.

RFB

 

[BobOnTheJob]

The thing that sticks out to me is why they chose to use a 3CX15000A7 in a 10KW transmitter? Talk about overkill. The extra filament juice alone reduces the AC/RF efficiency. The CCA 20KW rigs used a 3CX10000A7 whereas the 25KW version uses the 3CX15000A7. I'm not sure if the 3CX10000A7 and 3CX15000A7 will fit in each other's socket/blocker, but if they will, any chance that someone before you plugged the wrong tube in?

 

[bossbill]

I agree that it seems to be overkill, but that's the tube they're using in the 10k and 15k model. The only good news is the tube should last a while...

Good news is that I'm on a flat rate electrical. Bad news is with the move I'll be metered separately...

 

[K4WRF]

3CX15000 operating at 10kw is the big suspect item for me. Tube must be operating at the bottom of the load line. Second, single-phase power supplies are power hogs by their very nature. Without mentioning names, some of these “box builders” have been known to publish three-phase AC to RF numbers on the single-phase boxes. Make sure power company does not under rate the service drop. If you don’t already, use a meat thermometer to find sweet spot in the PA tuning. I have seen this make quite a difference in the stack temperature thus eff.
Best,
w/

 

[bossbill]

Thank you for the reminder about stack temperature. When we move the transmitter, I'll have some time to tweak the PA stage. It is interesting that we have two QEI transmitters at our other sites, and QEI performs as advertised with 17 kw in for 10 kw out. The "Armstrong" spec sheet states 17 kw in for 10 kw out, but that's not what's happening. The QEI uses a smaller final tube.

 

[rfry]

Just to note that the tube/cavity FM transmitters manufactured by Harris (and probably some others) were rated for the same AC input to r-f output efficiency no matter if their a-c input was supplied for single phase, or 3-phase (as ordered).

I can personally attest to this, as I was part of the team responsible for measuring and publishing those specs in support of the FCC type notification, and equipment brochures.

//

 

[bossbill]

I didn't recall there being a significant (if any) difference between single-phase and 3-phase. I thought a larger issue had to do with the reliability of the plate transformer. Seems like some like QEI were making 20 kw in single phase and having transformer issues.

 

[bossbill]

So the transmitter has a 125 amp breaker. It is fed from a subpanel with a 125 amp breaker which feeds from a 125 amp breaker on the subpanel. The subpanel is fed from another building, from a... you guessed it... another 125 amp breaker.

The transmitter is configured to turn back on following a power failure.

So about twice a year, the transmitter trips all three 125 amp breakers, but not the one on the transmitter, and no faults.

Last week, there were several power bumps with in two hours, and each time the transmitter came back on.

Then last night, the transmitter dumped and all three breakers were tripped (not the transmitter breaker however). This time, the breaker that feeds the transmitter would not reset.

Aside from the fact that it doesn't sound reasonable to have three identical breakers in a row, I'm thinking that the transmitter is running too close to the range of those breakers. The breaker on the transmitter is maybe a bit slower, and the inrush current makes the breakers all trip, especially after having a few recycles.

Does any of this make sense?

 

[rfburns]

You might want to check the breaker temps with an infrared thermometer. I had a similar breaker trip issue - turned out to be a burned pole on one of the breakers. The hot spot only showed up under load.

RFB

 

[necrat123]

What happened to my whole reply??

Radio-info, your board software screwed up.

I had typed a good reply to this thread and it got completley wiped out.

I will retype it later.