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, 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 broadbanded, 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:
|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|
|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: Solid State, multiple power amps and power supplies give solid state units more redundancy|
|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. Same can be said for the 2005 FM20T and FM30T installation at WHHZ/WKZY, Gainesville, Florida. Those tubes shows 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.