Tube Amplifiers Just Won't Die
Author:   Frank P. Morgan - MCL Inc.

For the last few years, there have been attempts to shift the focus on Microwave Power Amplifiers away from "Tube" technology to "Solid-State" technology.The thinking has centered on the perceived advantage that Solid-State appeared to offer by eliminating the need for an amplification component that contained a filament element.   The argument evolves that, without the tube and its filament to contend with, an all Solid-State amplifier can be theoretically calculated to demonstrate a "Mean-Time-Between-Failure" (MTBF) result of greater than 100,000 hours.However, this result is a theoretical value that is not, necessarily, substantiated by operational results in the real world.

Few can argue that, up to certain power levels in the various Satellite Communication bands, the advent of cost-effective lower power Solid-State amplifiers have been very useful. Consider a VSAT (Very Small Aperture Terminal) application today without a low power Solid-State amplifier for its output power stage. These amplifiers have made the VSAT technology possible and affordable for many applications around the world. And it is a fair assumption that, up to a certain power level (operating band specific), Solid-State is the more cost-effective choice to make. And, there is significant field history that substantiates that Solid-State amplifiers are the correct choice for lower power applications.   However, in any given Satellite Communication band, there is a threshold where Tube amplifiers (See Photo #1 - Typical Rackmount tube amplifier) are rightfully considered as contenders when choosing an amplifier.

For the purposes of comparison, the following thresholds are offered as the RF power levels (usable amplifier rated power output) where tube amplifiers begin to be favorably compared to Solid-State amplifiers and, therefore, can be considered as viable choices for the amplifier requirement:

•  C-band (5.725-7.105GHz): 80 Watts (Rated amplifier output power)

•  X-band (7.90-8.40GHz): 70 Watts (Rated amplifier output power)

•  Ku-band (12.75-14.50Ghz): 50 Watts (Rated amplifier output power)

•  DBS-band (17.30-18.40GHz): 40 Watts (Rated amplifier output power)

•  Ka-band (27.5-31GHz): 25 Watts (Rated amplifier output power)

The premise here is that, above these power levels, there are numerous reasons why a tube amplifier should be considered as an amplifier requirement solution.   Above the power levels indicated, the current generation of tube amplifiers offers a solution that has merit in the following categories:

•  AC Power Consumption: The current generation of tubes utilized in amplifiers are of the "Multi-Collector" type which offer significantly lower energy consumption than the older "Single-Collector" tube designs used some years ago.Therefore, the higher efficiencies of these modern tube designs will require less power per watt than before, allowing them to compete very favorably with Solid-State amplifier power consumption.

•  Thermal Management: Again, the utilization of the "Multi-Collector" tube designs also reduces the amount of thermal power generated inside the amplifier, which lowers the operating temperature of the amplifier. Under the premise that "Heat is the enemy", amplifier life is extended considerably. The drawback to a high power Solid-State amplifier is that the designer is required to remove a considerable amount of heat from a very small RF transistor case, which is not easy to do.The tube Collector surface area is considerably larger which lends itself more readily to standard heatsink solutions.

•  Rugged Design: Tube amplifiers have been utilized, and proven, in many harsh environments over the years. In both Military and Commercial applications, tube amplifiers have risen to the environmental challenges and succeeded. Consider that the typical tube amplifier manufacturers commercial product brochure will state a nominal "Mil-Spec" conformity for Shock, Vibration, and other environmental factors while most Solid-State amplifier brochures do not.

•   Proven Reliability: There is adequate evidence in the industry that justifies the statement that tube amplifiers, at the higher power levels, are very reliable for just about any application. Year after year, tube designs are optimized by the tube manufacturers and, correspondingly, tube amplifier designs also are improved on a continuous basis, as new power supply technology becomes available. With this new power supply technology, the tube amplifiers' high voltage power supplies are reliably achieving switching speeds of 100kHz and beyond with significantly improved tube DC Voltage Ripple reduction and high speed protection circuitry. The combination of the tube designers' efforts and those of the amplifier designers generates solutions that offer significant tube amplifier life benefits.

•  Packaging Flexibility: One other subtle point is that it is possible to offer more flexible custom packaging solutions with a tube amplifier since the tube is relatively compact and somewhat easier to package into unique customer volume requirements.

The range of RF output levels offered in the Tube amplifier product lines is somewhat different than Solid-State amplifiers.   Typically, except for Ka-band, Tube amplifiers do not go below approximately 100 Watts of usable RF output power but depending on the frequency band under consideration will offer products with usable RF output power levels in excess of 3,000 Watts.   In Ka-band, Tube amplifiers start as low as 40 Watts of usable RF output power.   As with Solid-State, the Tube amplifiers are offered in both indoor rack-mount and weatherproof outdoor-mount versions that allow for flexible system design solutions.  

The indoor rack-mount versions (excluding Ka-band) will typically start at power levels of 200-400 Watts and, depending on the operating band, go as high as 3,000 Watts of output power (see Photo #2 - 400W Indoor amplifier example). The applications for this type of amplifier are usually in fixed or truck-based systems. More often than not, the transmitted signal composition is 'multi-carrier' so Spectral Noise and Intermodulation are the primary concerns for signal transmission purity. One of the important options available to optimize performance in these concern categories is a sub-assembly called a "Linearizer" which actively pre-distorts the signal content to inject out of phase cancellation content which acts to significantly suppress the Spectral Noise and Intermodulation distortion problems. The use of this device will typically allow the user to operate the amplifier assembly to within less than 3dB of its rated power output and still achieve acceptable signal parameters that will successfully interface with the intended satellite. So, under the assumption that a two carrier signal would normally have to be transmitted from the amplifier at a output power backoff of -7dB from rated power, the "Linearizer" option would allow the same signal and amplifier to operate at an output power backoff of -3dB from rated power. This provides a significant increase in usable amplifier power for these applications. Coincidentally, the use of this option produces operational results very similar to the equivalent Solid-State amplifier for 'multi-carrier' operation.

The outdoor weatherproof versions (excluding Ka-band) will typically start at power levels of 200-400 Watts and, depending on the operating band, go as high as 750 Watts of output power (see Photo #3 - Outdoor amplifier example). The applications for this type of amplifier are usually in fixed, mobile, or flyaway systems.   For these amplifier types, the transmitted signal composition will be either 'multi-carrier' or single carrier.   In addition to the "Linearizer" benefits as described in the previous paragraph, this amplifier family also can benefit from an optional "BUC (Block UpConverter)" option. This option allows the system designer to apply a lower frequency L-band input signal which the "BUC" converts to the proper operating band frequency applied to the input circuitry of the amplifier. This is a very useful feature when there is concern about input signal degradation in fixed applications with long interconnects between the amplifiers and their RF drive source.

For Ka-band frequency applications, most of the requirements mandate outdoor weatherproof amplifiers that are mounted very close to the antenna interconnection point due to the high losses of interconnecting waveguide. The tube amplifiers available for this band vary from 40 Watts up to 250 Watts output. As with other band amplifiers, there is also a "Linearizer" option available for these amplifier types which significantly boost the usable output power for "multi-carrier" applications. Also note that, at the present time, there are no "BUC" options available for this frequency band as an option for these Ka-band amplifiers. Since the frequency coverage of a Ka-band application is much broader than at lower bands, tubes are an especially good choice as they are naturally broadband by design. There are actually two sub-bands associated with Ka-band applications that include 27.50-30.00GHz for commercial and 30.00-31.00GHz for military applications although there are tubes that cover the entire Ka-band (27.5-31.0GHz).

Over the years, sensitivity to the customer operating environment and availability needs have driven all tube amplifier designs towards higher reliability and easier service accessibility.   Customer feedback forced tube amplifier manufacturers to realize that the customer parameter that carried the most weight and concern was the 'availability' of the amplifier in the customer's operating environment. If a problem developed, the overriding concern was the return of the amplifier to an operational state in the minimum amount of time and, preferably, without the necessity of returning the amplifier to the manufacturer for a time-consuming analysis and repair. The recognition of this customer need led the tube amplifier manufacturers in two directions. First, a design philosophy criteria for 'conservative' design parameters was reinforced. In today's tube amplifier designs, virtually all circuit components are de-rated to establish higher reliability results. Few, if any, components are operated at their maximum capacity.   By relieving this component stress, the overall amplifier failure rate is decreased leading to fewer failures after delivery to the customer. Second, a re-structuring of the thought process in the packaging of the tube amplifiers has led to a much more 'modular' packaging concept which allows much quicker fault analysis and isolation of failed sub-assemblies. The other advantage of this interchangeable 'modular' thinking (see Photo #4 - modular amplifier construction example) is that there is a much higher probability that a module substitution at the customer's site will allow the tube amplifier to return to service very quickly. The benefits of this thinking, in these two design categories, also accrues to the tube amplifier manufacturers by reducing warranty costs thus lowering the 'risk' factor in the calculation of selling price. The benefits are spread to both sides of the table - customer and manufacturer.

Although the intent of this article is particularly focussed on the standard frequency bands utilized for Satellite Communication purposes, it is worthwhile to note that there are literally hundreds of other specialized tubes that have been developed over the years to cover from DC to above 90GHz. These tubes apply to specialized communications, medical research, and industrial applications. These other applications are worth mentioning if only to demonstrate the versatility of tube amplifiers in a wide spectrum of applications. From 'ground-based' to 'space-borne', tubes have been there and will continue to be the best solution for these specialized requirements.

The purpose of this article is not to persuade, or dissuade, a potential customer requiring a final high power amplification stage from choosing either a Tube or Solid-State solution.   Rather, it is hoped that the information contained in this presentation will allow for a more educated selection process to occur.   The goal of any system design is to achieve the best solution for the application under consideration.   With both Tube and Solid-State amplifier deliveries at, or above, record levels for the past year, the argument is rather compelling that "Tubes Just Won't Die" as they do offer advantages in many applications.   The elimination of either technology limits the customer to a choice that only covers 50% of the possibilities.   The recommendation is to look at all amplifier possibilities and rate them for their strengths and weaknesses for the particular application.   Only then will the 'right' choice be made.

Photo 1: MT4000	Photo 2: MT3200A	Photo 3: MT3400	Photo 4: Modular Amplifier Construction

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