Red plating NOS Tesla hexplates

[ajkimba]

I have a pair of EL 84 NOS Tesla hexplates that are red plating in my slll. I raised my biasing resistor to 150 still red plates, guess I could higher.

But if I lower the screen grid voltage by changing the 100 ohm to 1K would this help with the red plating?

The reason I don't want to go too high with the biasing R is because all my other NOS tubes run fine with 150 ohm.

Just wondering if lowering the screen grids would help with red plating?

[18lee]

I have seen this a few times with Stereo Repairs that I have done. Several of the manufacturers are requiring different bias voltages than specifications. I would test the tube off-line with a control grid voltage low enough to guarantee no conduction. Then I would raise if until the current begins to flow.

I don't like to diss tube manufacturers so I don't mention names but the available 7189's from some manufacturers are simply 5881 (6L6) tubes with a different print-out. With a fixed bias amplifier the crossover distortion is obvious audibly and on the scope. Good luck.

[markh]

The EL84s will certainly be happier with less voltage on the screens, but that won't make a big difference in the steady-state bias conditions. Try 180.

Remember, 125 or 150 isn't some magic number (unless you're using Mullards made in the late 60s ), the resistor just sets the DC bias, and different manufacturers tubes sometimes need different bias. All the audio signal will be going through the bypass cap.

--mark h

[zaphod_phil]

I don't like to diss tube manufacturers so I don't mention names but the available 7189's from some manufacturers are simply 5881 (6L6) tubes with a different print-out.

IMO if they're misleadimg people, then you really *should* name and shame them. I've also heard of KT66s which were just 6L6 guts place inside a fat bottle. OK, maybe given a softer vacuum, they might sound a bit more like real KT66s...