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Lifetime Analysis of the 7835 Power Amplifier: Insights and Challenges

This document provides a comprehensive analysis of the 7835 power amplifier, focusing on its operational lifetime, shortcomings, and historical data. Originally developed by RCA and later by Burle Industries, it operates at 201 MHz and supports bursts of up to 2 MW. The average lifetime of the amplifier is approximately 11,000 hours, with recent trends indicating a decline in longevity. It highlights the maintenance and rebuilding challenges faced by users like Fermilab, Brookhaven, Argonne, and Los Alamos, emphasizing the need for a reliable supply of spare tubes to ensure stable operation.

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Lifetime Analysis of the 7835 Power Amplifier: Insights and Challenges

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  1. 7835 Lifetime Analysis Elliott McCrory December 15, 2004

  2. 7835 Basic Information • 5 MW, 201.25 MHz Power Amplifier • Original model 4617 • Difference: Thoriated filament • Matched driver, model 4616 • We use at 175 kW, 201 MHz • Capability: • Up to 2 MW for very short pulses • 195 to 600 MHz • Burle Industries, Lancaster, PA • Spun off from RCA in 1987 • Remember: An average year is 8766 hours

  3. The 7835: Sliced! Ion Pump would go here Anode Getter Cathode/Filament Pinch-off

  4. Current users of 7835 • Fermilab • 5 sockets; 8000 hours/year • Brookhaven Lab • 9 sockets; ~4000 hours/year • Argonne • 1 socket; <1000 hours/year • Los Alamos • 5 sockets; ?? • Quadulan Islands (DOD) • 4 sockets; 8000 hours/year?? • 4617 model tube

  5. Costs/Failures • Costs • New: $150,000 • Rebuild: $50k to $120k • Num rebuilds: Have seen R9 • Rebuild as good as new • ~1/5 attempts to rebuild fail • More or less independent of rebuild number • Failures • Half fail due to emission limit • Half fail for other reasons • Water leaks • Irreparable sparks • We can repair some sparks locally • Vacuum failure

  6. Date Received vs. Hours to EOL Reached End Of Life In Service Burle RCA 11/1968 11/1972 11/2000 11/2004 Date Tube Received by Fermilab

  7. Tube Lifetime Histogram

  8. Number Tubes Received Per year Average: ~6/year

  9. Date Received vs. Hours to EOL Median life: 11791 hrs; RMS = 8000 hrs, Standard Error = 8000/sqrt(n) = 1153 hrs

  10. Brookhaven’s Data (Thru 4/03) • Average Tube Lifetime at Brookhaven • 12203 hours

  11. What Has Happened?? • RCA became Burle in 1987 • Some expertise retired • This tube is half engineering and half artwork (it seems) • Many (most?) of the original experts/artisans have now retired • Some (most?) apprentices were properly trained, but some not! • Example: Anode temperature • Example: Getter decision

  12. How Many Burned Up Per Year? • Assuming • Median life of 11800 hours (~25000 T1) •  1.48 (3.0 @T1) years of tube life • 1 year = 8000 hours • Or ~4.5 weeks off per year • Then we need • N = ΣS [ 1 / LSi ] • S is the number of sockets • L is the median lifetime of the tube in a socket • (4 tubes/1.48 yrs) + (1 tube/3 yrs, T1) • 3.04 tubes/year • Assuming EOL at 5500 hours (0.69 years): • 6.13 tubes/year

  13. Number Tubes Needed vs. Lifetime But Might Be Here We Are Here Burle Average Flux

  14. Stable Operation Today? • 2001: • March • 5 spares and 2 deliveries imminent—Wow! • Division Management pleads to reduce expenses • Decision by Webber and me (~march 01) not to order rebuilds • Oops! • June 22 through October 1, 2001 • Replaced 6 tubes, leaving 1 spare • Were forced to borrow tubes from Argonne and from Brookhaven • Moral: We want ~6 good spares at all times • Required flux: At least 6/year.

  15. Conclusions • Historical median lifetime • About 11000 hours (1.4 years) • Need at least 6 good spares for stable operation • Longevity of 7835 power tubes has decreased since 1990 • Some indication (hope?) this has turned around • Must get at least ~6 tubes/yr from Burle • If lifetime is under 8000 hours? • Then Burle needs to improve flux

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