My traumatic case pre-dated the ca. 1988 heat dam issue...
During my college years, I was employed by WYBR-FM in Rockford, IL in 1979.
The station is licensed to Belvidere but serves the Rockford metro. At the
time, it was a 3KW station with a TPO of 2,250 W. Tower height was 300 ft.
and supported a 3-bay circularly-polarized ERI rototiller antenna.
Transmission line was 1-5/8 Andrew Heliax. So, taking into account antenna
gain and line loss, licensed TPO was 2,250W to make 3KW ERP from a single
Eimac 8877. The station operated 24/7 at that power level.
When I first arrived for work, the one and only 8877 was black as charcoal,
but it still produced full power although grid current was excessive owing
to the fact that it had to be driven harder to make licensed power. The
transmitter had been neglected and the air filter was extremely dirty which
resulted in low air volume and the reason for the tube turning black. I
don't recall if the PA cavity had a pressure vane switch which would have
prevented the overheating -- or if it was bypassed by a prior engineer.
Regardless, it overheated.
I drove to Richardson RF in the Chicago suburbs and purchased a new Eimac
8877 at the will-call counter. I then drove back to Rockford with the new
tube and installed it at midnight.
I could not get more than 500W from the new tube. I made a second attempt
and drove back to Richardson RF for a replacement. The replacement was
installed but had the same issue. Maybe three time's a charm? Not in my
case. Richardson was reluctant to give a third 8877 to a 19 year old kid
but after explaining my dilemma, they agreed. The third 8877 was tried but
it still could not make more than 500W TPO. I had investigated all related
components, confirmed voltages, etc. It just HAD to be the tube.
By this time, the owner of the station was concerned with my ability to
solve the problem. In a move of desperation, I contacted Eimac's Ken
Atkinson at the SLC plant. He gave me pre-paid authorization to overnight
the third tube to SLC for analysis. After cutting open the tube, they
determined that the cathode was poisoned during manufacturing. He
immediately sent me a known good 8877.
My hands must have been trembling as I inserted the tube in the transmitter.
The transmitter had the usual 5 minute filament time-delay to adequately
heat the cathode. After waiting what seemed like hours, I hit the
transmitter's "plate" switch and the transmitter easily made full power.
I was able to save my job, thanks to Ken Atkinson.
Paul, W9AC
-----Original Message-----
From: Amps <amps-bounces@contesting.com> On Behalf Of John Lyles
Sent: Wednesday, January 26, 2022 1:13 PM
To: amps@contesting.com
Subject: Re: [Amps] 8877
I learned about 8877/3CX1500A7 manufacturing woes from a friend who worked
for Eimac back then. This is a summary of what I learned:
In 1968 Bill Sain of Eimac-San Carlos developed test models of zero bias
triodes with segmented-oxide cathodes and radial bar grids as reported in
internal report TR69-2. These tubes were made from existing tube parts
initially, to prove concepts of very high gain (for a cathode driven triode)
and low grid interception current. Initially Sain used segmented cathode
tetrode parts and connected control and screen grid to ground, to make a
high mu triode. Then they developed a triode grid for several variants,
based around using 4CX350 and 4CX1500B cathode assemblies. The former tube
was useful, while the higher power tube had problems. Eventually effort was
focused on the 1-2 kW tube and the 8877 was developed in a record 10 months.
It was a very good tube from a design standpoint, originally,
There were several divisions within Eimac and the Salt Lake City operation
competed with San Carlos in some broadcast and medical tube products. Salt
Lake engineers changed the material used for the cathode heat dam, a disc at
the top of the cathode cylinder, as a result of a cost reduction program. It
saved an estimated 35 cents per tube according to former employee. It was
never tested with RF, only in DC current division tests. Kovar has iron and
is very lossy in RF, compared to the original nickel part. The result was
that the part would heat and expand, shorting the grid to cathode. ETO was
making superb rf amplifiers for MRI systems and private labeling for GE
Medical at the time. A pair of 3CPX1500A7s ran in parallel for 15kW pulsed
at ~41 MHz.
Both tubes had to be fairly close in gain, another problem so that special
tube type YU-158 was sold, a selected 8877 for low grid current.
Around 1988-89 tube failures were common in the amplifiers. After about
10 to 20 minutes of RF operation the tubes would short internally between
grid and cathode. The staff at Salt Lake City had not been able to pinpoint
a cause. 8877's returned under warranty tested good in Eimac's DC current
division tests at the factory and many bad tubes were being sent back to
customers without the defect being identified. It was a vicious circle.
Meanwhile, Erbtec, a Colorado competitor, knew about the problems from GE
Medical and developed a single tube amplifier for the same application using
a new San Carlos tube, the YC-156 or YC-179, both versions of the 3CPX5000A7
triode with much larger cathode area.
Erbtec eventually won the contract with GE Medical, and ETO took a big loss,
essentially because of a small tube change. That really hurt them, and it
was due to a small internal change at Eimac that never was tested in high
power RF situation, only in DC.
Initially the QA specification was for a maximum grid current of 50mA with 1
amp of plate current (pulsed measurement). The 8877 had to be assembled with
great care under a microscope in order to make this spec.
The alignment of the grid bars with the beams from the segmented cathode was
critical to having a low grid current zero bias triode. While visiting the
Salt Lake City plant, one of the San Carlos technicians noticed many of the
production 8877's were measuring higher than 50 mA and he asked about it. He
was then informed that the test specification for grid current had been
changed to improve manufacturing yield. It was later discovered that Salt
Lake City had a 55 gallon drum full of scrapped tubes and was not accounting
for that poor yield. It was said that this led to the Salt Lake City
division president leaving, although there could have been other reasons as
well. Tubes that shorted were probably in the 1988-89 production runs.
John Lyles
K5PRO
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