Davy's KEL84 Grid Voltages

[Richard]

Hi Davy, All,

This amp is built fine, works to spec and sounds great. Before switch-on I checked round and re-did chassis earth which was stood off 2 ohms by paint and tidied a few signal wires; in partic the unscreened end of the R input which was lying alongside the fb wire at the pcb end. Davy's not sure if it's always been like that but the amp hasn't misbehaved at all in many hours testing and that could certainly account for positive feedback. So, nothing exciting but it is often something simple isn't it.

He reminded me about the EL84 grid voltages and I said I'd check them out. I'd checked all voltages at switch on, they were all close to spec, and tested the valves which were all fine and paired up.

Sure enough after 1 min all grids are 0V but,

after 5 mins they varied .01V to .48V
after 20mins they were .01V to .83V

Thereafter they stabilised and even dropped back a little when the mains dropped. Cathode voltages stayed the same at 11V even on the 2 worse valves showing .47V and .83V on their grids.

So, removed the EL84s and checked gridstopper and gridleaks were correct values and they were. Switched on with the valves out and checked for DC on the grids but there was none, just flickering mV which would account for nothing so the coupling caps (plastic) are fine.

Replaced the valves back in the the same pairings but opposite channels of the amp. Lo, and those exact same grid voltages followed the valves.

Researched gridleak values and found KEL84 uses same 470K as Leak used. Mullard used 820K in their 10W amp. EL84 data sheet says max 1M with cathode bias so gridleak value doesn't seem to be the issue.

Google search found one ref to a tired EL84 showing 1.5V on it's grid. Conclusion then is that this might be an ageing thing with these valves.

It does not affect the bias voltage at the cathode but I'm not sure about the actual bias. The voltage showing on the grid must be created across the gridleak and so will be a minute current through the grid of .83/470K = .0017mA. If it is the ageing routine for these valves and that positive grid voltage continues to rise then the valve will gradually pass more current till it self-destructs.

I don't know how many hours are on these valves (Davy?) but I'd check grid voltages in a few weeks time. (Valve marked 4 shows .83V and 6 shows .47V but interestingly these 2 don't draw the most current on test.)

Does anyone have exp or further info?

Rich