In the previous issue of SatcomPost, we looked at the financial savings and performance benefits and applications for employing pre-owned TWTAs. In this Part 2, let’s look at the management of risk.
How do you assess the risks in buying a used TWTA? According to manufacturers, TWTAs can operate for 100,000 hours (more than 11 years) of continuous service at their rated RF output power 
As engineers know, the heart of a TWTA is the tube. When the tube reaches life end, the cost of a replacement tube can range from $17,000 to $27,000. As a buyer, it is vital to know the condition of the tube in the amplifier that you are considering for purchase.
Fortunately there are some very good parameters that give clear intelligence about the condition of the tube. Clients are typically interested in three items:
- The total hours of operation;
- The “beam on” hours; and
- The Helix current.
The hours of total operation and the beam on hours are of lesser significance but certainly they do give an indication of the age of the amplifier. The most important parameter is the Helix current. When a tube is in good condition, irrespective of its hours of operation, if the Helix current at designed output is less than 5 mA (milliamps) then the tube is in good shape. It can be predicted to offer several more years of service. Conversely if the Helix current is in the 9 mA range, for instance, then the future of the particular tube is very cloudy.
Expiration Date vs. Life
It has been said that the manufacturers of amplifiers are similar to canned food manufacturers in one respect: they love to put a “best used by” expiration date on the box, as a prompt for us to buy more product.
According to manufacturer statements, the vast majority of TWTAs live up to and past the planned lifetimes of the systems they inhabit. Cooling fans and power supplies of both systems are likely to fail first, before the tubes . There are electronic control circuits in TWTAs that age, and eventually deteriorate. The overall reliability of a TWTA is affected by the failure rates of both the tube and the power supply , so an estimate of reliability must include the MTBF of the high voltage power supply. But advances in power supply reliability have in part been the result of a large market for high voltage power supply circuit components, and high production and improved quality control .
Each manufacturer has its own expiration guidelines, but most manufacturers recommend about 45,000 hours of use on a TWTA. Although a manufacturer may specify a set number of hours of operation as the “expiration date” for the unit, that date not may reflect the capabilities of the HPA to continue to provide reliable service. Well-designed TWTs have been known to supply a beam of electrons in excess of 100,000 hours of continuous operation . At the other end of the scale we have seen amplifiers with a few thousand hours on the clock with tubes that are clearly destined for imminent failure.
We believe that manufacturers are much too cautious. This is based on our field experience with customers who buy TWTAs with 45,000 hours or more, which run successfully for years to come.
To be fair to TWTA manufacturers, the hours of operation are obviously important because a TWTA does not run by tube alone. However, failure of the control circuits is normally not the catastrophe that would be caused by a tube failure. We have purchased and resold amplifiers with as much as 90,000 hours on the meter. And yet with good tube characteristics they can be expected to last for many more years.
Original vs. Replacement Tubes
Even if the heater hours (total time operated) indicator reports 80,000-90,000 hours, the actual Beam On hours for the specific tube could be much lower. It may have been a replacement part installed later than the first tube that was initially shipped from the factory with the unit.
It is a good practice to ask your used equipment suppliers what testing they perform, and what guarantees they offer on the items they ship. We perform diligent testing to ensure the unit is in good condition and can be predicted to deliver the expected performance for the planned term of use. Many of our customers are overseas, and because we guarantee our items’ performance, if an amplifier were to fail during warranty, the cost for us to return, repair or replace is very high. Therefore, we make every effort to ensure perfect condition prior to shipping. Careful packing is also essential to avoid damage in shipment.
Delivery and Proper installation
When you purchase a used TWTA, it’s essential that the equipment be properly commissioned into service. Manufacturer’s safety precautions exist for storage, set-up, testing, and operation of TWTAs and their components, all of which should be observed.
When you stand to save over 50%, taking a quick look at your options with pre-owned TWTAs can make a lot of sense. Even in today’s world of eBay deals on used equipment, when it comes to something as specialized as earth station HPAs, your best bet for acquiring used TWTAs is to work with an experienced, reputable supplier, and one that follows test procedures, and provides delivery and performance assurances you can rely upon.
- TWTAs are typically the most cost-effective and reliable solution compared to other earth station HPA (high power amplifier) technologies for applications requiring medium to high power level requirements across a wide bandwidth over commercial satcoms bands and less commonly used frequency bands.
- Used earth station TWTAs (travelling wave tube amplifiers) can offer very large capital savings (e.g. 50%-plus discount) compared to new units.
- Used TWTAs can be delivered within days of placing an order, compared with months from the manufacturer.
- Pre-owned HPAs can work for a variety of business cases such as permanent services, low-budget services, test and trial services, Occasional use or ad-hoc services, backup systems, short-term contracts (e.g. 1-3 years), Emergency services, urgent interim services needing fast turn-up.
- This article explains what to look for in a used TWTA.
- Helix current is a key health indicator of travelling wave tubes, while total hours of operation, and beam on hours are other indicators.
- Typical manufacturer’s guidelines 45,000 hours (over 5 years), but with good bill of health it can last much longer, over 11 years.
- Follow manufacturers guidelines for turn-one, warm-up, storage, and temperature.
- Ask your pre-owned satellite equipment supplier about testing, packaging and delivery guarantees.
 Source: http://www.cpii.com/rffactcheck/,
 Via Satellite, October, 1998, “Earth Station High Power Amplifiers KPA, TWTA, or SSPA?”by Robert A. Nelson
 Source: Howard Hausman, VP Engineering, MITEQ, Inc. ,”A Comparison of High Power Amplifier Technologies: TWTAs vs SSPAs”, MPDigest.com, January 2008, Page 1. Article states that tubes have a typical life limited to 70,000-100,000 hours of continuous operation.
Also Read: Smart Savings with Used Uplink TWTAs - part 1
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