[Amps] MKS brand 8877 test

[Jim Barber]

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