FM transmitters – Page 15 – Engineering Radio

Tube transmitters vs Solid State transmitters

I thought it would be interesting to do a comparison between the two types of transmitters, both AM and FM. I have been doing this thing for 25 years and have quite a bit of experience working on all types of transmitters. Some of the broadcast transmitters I have personally worked on over the years include:

Harris: FM-20H, FM-5G, HT-35, HT-10, HT-3.5, FM-25K, FM-5K, Z5-CD, MW-50A, MW-50B, MW-1A, MW-5A, BC-5H, SX-5, SX-1A, Gates 1
Broadcast Electronics: FM-5B, FM 3.5A, FM5A, FM30A, FM35A, FM30T, FM20T, FM10S, FM5C, FM1C, AM10A, AM6A, AM5E, AM1A
Continental Electronics: 816R-2, 814R-1
Collins Rockwell: 831F-1, 838E-1
Nautel: ND-1, ND-5, XL-60, V-40, V-10, V-7.5, NV-40
Gates: BC5P, BC1T, FM5B
General Electric: BTA-25
RCA: FM20ES1, BTA5J, BTA1-AR
CSI/CCA, Visual, Energy Onix, Bauer, McMartin, QEI, some Italian something or other, etc. Various makes and models.

I think I have a fair amount of transmitter experience under my belt. What I have found is that certain brands of transmitters are better than others, regardless of whether they are tube or solid-state. There are several differences in each type, obviously. As to some blanket statement about which is better, solid state or tube, I don’t have one. My statement would be “It depends.”

Tube transmitters are more rugged and will take more abuse than a solid-state unit. Things like heat, lightning, EMP, and mismatched antenna won’t phase a well-designed, well-manufactured tube transmitter. On the other hand, they are less efficient AC to RF, have higher B+ voltages, have hard failure modes, and are more difficult to linearize, if that is required for some reason.

Solid state transmitters are more broad-banded, easier to change frequency, they have soft failure mode due to redundant amplifiers and power supplies. The voltages are lower, thus they are safer to work on.

Here is a complete list of advantages and disadvantages of each type:

Attribute	Tube	Solid State	Comment
Ruggedness	Very rugged, able to take heat, EMP, lightning, mistuned antenna, poor operating environment, etc	Not heat tolerant, lightning and EMP can damage MOSFETS, switching power supplies sensitive to AC mains issues	Advantage: Tube
Electrical Efficiency	Less efficient	More efficient	Advantage: Solid State, however efficiency gain can be wiped out due to larger air conditioning requirement
Failure mode	Hard, most often	Soft, most often	Advantage: Solid State, failure of a single module or power supply generally will not take unit off the air
Frequency agility	Difficult	Easy	Advantage: FM Solid state transmitters can easily be moved. AM transmitters still require extensive retuning.
Re-occurring cost	More	Less	Advantage: Solid State, as tube changes are required every two to three years
Maintenance	Same	Same	Advantage: neither
Servicing	Requires skilled engineers to service and trouble shoot	Modules and power supplies are often hot swappable and returned to manufacture for repair	Advantage: Solid State, however either type requires occasional measurements with specialized test equipment
Servicing safety	High voltages, contact will be fatal	Lower voltages, but can still be fatal	Advantage: Solid State
Redundancy	Low	High	Advantage: Dependant on TPO, Higher powered solid-state transmitters are much more expensive than there tube type counterparts
Cost	Less	More	Advantage: Dependant on TPO, Higher powered solid state transmitters are much more expensive than there tube type counterparts
Availability	Good used market, some new FM transmitters still being built	Good new and used	Advantage: Tube
Reliability	Dependent on brand	Dependent on brand	Advantage: neither

For some reason, the latest Broadcast Electronics tube-type transmitters seem to have very long tube life. I installed an FM20T at WYJB in Albany, New York, in early 2001 and it is still on the original tube, some ten years later. The same can be said for the 2005 FM20T and FM30T installation at WHHZ/WKZY, Gainesville, Florida. Those tubes show no sign of giving up anytime soon. I don’t know if that is an unusual trait of the transmitter or that particular tube.

The above comparison seems to heavily favor a solid state transmitter. As a general rule, brand new solid state transmitters both AM and FM have advantages in almost every category except high power FM transmitters, where tube types still make sense. From a used transmitter standpoint, there is nothing wrong with a tube type transmitter, provided it has a solid state IPA. I have noticed the 4CX250B driver tubes most often used in FM IPA stages have markedly reduced reliability of late. I would also tend away from transmitter makes and models where the manufacture is no longer in business or no longer supports the product.

New Nautel NV-40 at WVPS, Burlington, VT

Sorry for the prolonged absence. I have been, quite literally, out of reach for the last two weeks. In fact, for the entire month of July, I spent just five days at home. Some of the travel was for work and some for pleasure.

On the work side of the equation, WVPS in Burlington, Vermont has a new Nautel NV-40 transmitter. WVPS is the NPR affiliate for Vermont Public Radio and it’s transmitter site is located on top of Mt. Mansfield, in Stowe, VT. I will do a separate article about the Mt. Mansfield transmitter site because it is an interesting place. WVPS is a Class C FM on 107.9 Mhz. They have one HD subchannel for the VPR classical music format.

The Nautel NV-40 transmitter is greatly updated from the V-40, which was installed at WHUD. Basically, the V-40 is four ten-kilowatt transmitters combined. It is a novel approach and offers quite a bit of redundancy as entire transmitters can be switched off and worked on with the other three remaining on the air at full power.

The NV-40 is a single large chassis with internal combining networks. It uses different RF modules but the same power supplies. The entire thing is controlled by a fancy GUI on the front of the transmitter but also has the ability for manual control if the GUI fails. That is a key feature not seen in other transmitters which simply won’t work without the fancy computer. Other things that I like, are the ability to control all of the biasing and other options via the GUI and things like a spectrum analyzer and Lissajous display. The ability to look at several graphic displays at once makes it easy to configure and monitor.

The transmitter arrived at the top of the mountain via a local moving company. After unloading it on the loading dock, it took some amount of doing to get it down the hall into the transmitter room. The thing weighs in at 1,600 pounds after being uncrated.

Nautel NV-40, Mt. Manchester transmitter site loading dock — Nautel NV-40, Mt. Mansfield transmitter site loading dock

Unpacked:

Nautel NV-40 uncrated and read to move down the hallway

Moving into the final position in the WVPS transmitter room.

Movers putting transmitter into final location and removing pallet jack

The connections were made to the transmitter, including connecting grounding strap to the back, 200 amp electrical service and the RF output connection via 3 inch rigid coax.

The remote control consists of basic transmitter functions going to a dial up Gentner remote control and a Network connection going to the GUI. The network connection allows persons on the network to use a web browser to look at the GUI. The HD radio connections are made via a HD radio importer and exporter, located at the studio, which also uses the network, via a connection on the exciters, to send the HD subchannel. The analog main channel is via an AES/EBU connection from the STL.

All connections go through large toroids to help isolate the transmitter from any lightning-related surges.

Before I left, we tested it at full TPO into the dummy load. All worked well, the only outstanding issue was getting the HD radio importer/exporters to work over the network, which was out of my jurisdiction.

Here is a rather blurry picture of your author standing next to the NV-40 with the exciter and GUI turned on. There are to IEC power connectors at the top of the transmitter that go to the GUI and exciters. This allows those part of the transmitter to run on UPS’s, which is nice, being that the GUI takes about a minute to boot up after power failure.

All is not well in Paradise

If one considers paradise an FM35A. Going through another iteration of blown transmitter fuses for WEBE, Bridgeport, CT. Yesterday, I spent the afternoon examining the transmitter and found several interesting things:

Fresh arc tracks on the PA cavity and PA loading capacitor
The shoes and bars in the high-voltage contactor were severely pitted
One of the mains phases (middle) in the high voltage supply appears to be heating up, likely due to a loose connection.

I checked and re-tightened all of the mains connections. Apparently, this is an old problem, as the Allen screw was tight. Interestingly, the fuse that was blown was on the red phase, which is different from what it was last time.

I spent the afternoon filing and sanding off the arc track marks in the PA cavity. It is very important to file flat all sharp points that were the result of arcing. Any sharp points will induce corona. I also filed down all of the contacts in a high voltage contactor, which took a fair amount of time. These are soft copper shoes and bars that had so much pitting and carbon I wonder how they didn’t catch on fire. I filed them flat. We were back on the 35A transmitter at full power by 4:30 pm.

If this happens again, I will bring my megger out and check the insulation on the wire between the disconnect switch and the HV power supply.

When I left the site at 5:30, I felt like we did some good work.

What happens next

It’s the middle of the night and the phone is ringing. That is never good. The transmitter is off the air. You call the remote control and try to put the main transmitter back on the air. No good. The backup comes up, no problem. Shaking off the sluggishness, you get dressed and head out the door. The transmitter is about 30 miles away, but it’s in the middle of the night, so there is no traffic. While driving, you are thinking of all the things that could be wrong. The blower motor was sounding a little loud last trip. The exciter has some reflected power. The PA tube is two and a half years old.

Upon arrival, there are several overload lights lit, including the driver’s plate. An investigation is in order. You turn everything off and open the doors. The trouble seems to be a bad IPA power supply. There are spares on the parts shelf, so you put one in. Put the transmitter into the dummy load. You turn on the filament and the transmitter comes to life again. Reset the overloads.

Broadcast Electronics FM35A transmitter ready to be turned on

Now you are standing there looking at the plate-on button. Was it really only the IPA or was that just a symptom? Was there something else that took out the IPA power supply? What will happen when I press the plate-on button? Will it come on normally or go BANG! I hate BANG! By the way, my tradition in a situation like this, if on a mountain top somewhere, I go outside and pee. I give the situation one more run through the mental checklist, then come back inside and press the button.

Broadcast Electronics FM35A transmitter high voltage on button

Please excuse the blurry picture, it is hard to take a picture of yourself turning on a transmitter…