[Amps] MKS brand 8877 test

[John Lyles]

There was a production issue related to Eimac and the two MRI equipment 
makers who used the 8877. I attached some information here that I got 
from another friend who worked in the tube biz then:

ETO was at the forefront of the new MRI diagnostic equipment development 
and ETO chose to use a pair of 8877 tubes in parallel in their rf 
amplifier. Their customer was GE Medical. GE Med was having difficulties 
with the 8877 in their MRI amplifiers: after about 10 to 20 minutes of 
operation 8877 tubes would short internally between grid and cathode. 
The staff at Eimac in Salt Lake City had not been able to pinpoint a 
cause and tubes were being returned as meeting all electrical tests but 
the would again fail when in MRI systems. It turned out that a small 
part had been changed in the 8877, a thin cathode heat dam, it was made 
of nickel in the original design but had been changed to Kovar, a less 
costly alloy with a low coefficient of expansion but containing iron as 
one of its constituent metals so rf energy would cause heating and it 
would expand and touch the grid cap, shorting the grid to the cathode. 
8877's returned for warranty tested good at the factory where they 
passed current division tests (not RF power). As a result many bad tubes 
were being sent back to customers without the defect being identified. 
Economic losses at ETO were considerable and ultimately a competitor 
Erbtec was awarded the rf amplifier contract by GE Medical using a new 
tube that Eimac developed, a bigger triode. That really send ETO a bad 
message.


MKS bought one of those companies, sorry I don't remember more. I assume 
they never used Eimac 8877s after that event. I have a friend who 
retired from MKS, Colorado Springs. They bought one of the 
abovementioned MRI machine makers. Eventually had equipment made in 
China so it was likely a Chinese source triode too. The MRI companies 
had stringent specs on the tubes when they ran them in parallel. This 
was something that Eimac in Salt Lake was having trouble making yield 
with a lot of rejects. Who knows where those tubes ended up too.

73

John Lyles

K5PRO




> Message: 1
> Date: Sun, 29 Jan 2023 09:32:01 -0800
> From: Jim Barber <barberaudio at gmail.com>
> To: Amps group <amps at contesting.com>
> Subject: [Amps] MKS brand 8877 test
> Message-ID:
> 	<CAO3SKEFP=uYxW+zutjq7t1=7GPQuU8Cy7uJAz9vKL=rzVPFQog at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> I recently bought a ?mks? branded medical pull. This is my first non-Eimac
> 8877, so I?m interested in how it performs.
>
> For the sake of the curious, here?s what I?ve found so far:
>
> I gettered it overnight, then put it on my homebrew hipot tester. No
> leakage at 8KV, which is unfortunately as high as the box will go.
>
> I installed it in my restoration-project 8877 deck, said a fervent prayer,
> then turned it on. No bang, which is always good. The second thing I
> noticed is that the heater draws a little more current than my reference
> Eimac - It drew ny filament supply (at the tube) down to ~4.8V. After a few
> minutes I adjusted the onboard Variac and brought it up to 5V. I checked a
> few times after drawing anode current and I haven?t had to change it.
>
> I happened to hit the grid at a low-load time, and the unloaded B+ read
> 4060V with a Fluke HV probe.
>
> Here?s where it starts to get interesting, for me at least. I keyed the
> deck, which applied 12V operating bias to the tube. The ZSAC (idling
> current) jumped up to just about exactly 200mA. My reference Eimac draws
> 150mA under the same conditions.
>
> I went on to do some relatively low power tests, then had to switch tasks.
> I didn?t have time to do any structured testing, but I will say that so far
> the tube shows noticeably higher gain than the Eimac. How much higher will
> be a question for a future session.
>
> The Eimac is, BTW a true NOS specimen and follows the published curves
> pretty well. I don?t remember the date code offhand, but can provide it if
> anyone is interested.
>
> I found this intriguing enough to order a second tube from the same
> supplier. Is the first one a fluke? We shall see.
>
> Jim N7CXI