Chinese EL34 problem

[Malcolm Irving]

I’ve just read a very interesting article in Guitarist, a UK-based magazine, March 2019, p34, which quotes Adrian Emsley – tube amp designer with Orange.

The article says that Chinese EL34s were manufactured for decades with an internal wiring error. Pin 1 (which should be the suppressor grid) went nowhere, while pin 8 (which should be the cathode) went through the suppressor grid to the cathode. Adrian, and a German retailer, pointed this out to the Chinese manufacturers who immediately corrected the problem.

The earlier tubes worked OK but suffered premature failures, especially in hot-biased amps, as all the cathode current went through the suppressor grid on its way to the cathode.

Before the real problem was realised, the Chinese factories went through a process of increasing the general robustness of the tubes: bigger plates, thicker glass, etc. to improve lifetimes. These features have been retained, meaning that the current production are now very robust.

Apparently you can tell if you have an old example simply by looking inside to see what pin 1 and pin 8 are connected to.

[Leo_Gnardo]

Chinese EL34s were manufactured for decades with an internal wiring error. Pin 1 (which should be the suppressor grid) went nowhere, while pin 8 (which should be the cathode) went through the suppressor grid to the cathode.

Thanks for passing this along. One has to wonder, what decades? Those failure prone Chinese EL34's from the 80's - early 90's, that had a splint spot welded to the plate, come to mind. I still have a couple around. Looks like what Mr. Emsley may be trying to explain is the suppressor grid was internally attached to the cathode, and brought out on pin 8. Not an awful problem except for those few amp designs that had the suppressor wired to the raw bias supply or some other circuit point.

[Malcolm Irving]

... Looks like what Mr. Emsley may be trying to explain is the suppressor grid was internally attached to the cathode, and brought out on pin 8. .

It was pretty clear in the article that he meant that pin 8 was attached to one end of the suppressor grid and the other end of the suppressor grid then went to the cathode. All the cathode current went through the suppressor grid - presumably overheating it.

Although thinking about that, the suppressor grid would normally only have one end connected. ???

I suppose it could have been that the suppressor grid was connected to pin 8 at the base of the tube and then the top end of the suppressor grid could be connected to the cathode near the top of the tube.

[Leo_Gnardo]

Although thinking about that, the suppressor grid would normally only have one end connected. ???

That's what I'm thinking, so it's not so much a big deal if it's internally connected to cathode. And nobody's much going to be using fresh 25+ year old Shuggie "splints" though I do keep a couple around just for laughs, to remind me what dreck was in circulation during the bad old days. Heck even Marshall quit installing them in their amps mid 90's, and went to Sovtek wafer base 5881 for a couple years.

[pdf64]

I guess that the current flow across g3 might set up a magnetic field there that affected electrons making their way to the plate somehow?