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Problems with E88CC / 6922 drawing grid current on power-up

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I'm using E88CCs in a cascoded differential pair as the input stage of a fully-balanced power amplifier. Twin muting relays pull the signal ends of the grid stopper resistors to 0V until the valves have warmed up, and there is an initial time delay to allow the heaters to get up to temperature before the HT is turned on. During this time, the grid acts as the anode of a diode, and current flows from 0V through the muting relays, into the grids and down the tail resistor to the negative bias rail. This is not totally unexpected and shouldn't cause a problem because the grid current flow should cease as soon as the HT rail comes up. At least, that's the theory - and in most cases this is exactly what happens. However, I've found that with certain individual valves, grid current continues to flow for a while even after current starts flowing from the HT rail, and this means that there is an audible "thump" when the muting relays open.

I need to find a solution to this that doesn't involve testing and selecting valves that don't exhibit the problem. Has anyone else come across this phenomenon or got any ideas about how I could get around it?
You may be heating the grid so it then emits for a while under 'normal' operation. Try adding small silicon diodes between grid and cathode, so they are forward biased while warming up but reverse biased during normal operation.

The ECC88 and its cousins has a very fine grid, not designed to cope with any grid current at all.

Alternatively, it could simply be that some of your valves are a bit gassy. They may settle down with use, or they may not.
Yes, I was thinking along the lines of reverse-biased diodes between grid and cathode - I will give this a go. The grid-stoppers are 4K7, so the grid current is hopefully limited to a below-damaging level. It's interesting that most of the valves I've tried are completely okay. I've been using brand new JJ E88CCs, so maybe it's a gassing issue as you suggest. Hopefully the getter will carry on doing its job. Many thanks.
At normal conditions the -Vg is there thanks to the fact that the cathode is at +1...3V above ground,the grid is at 0V.During warm-up and/or without Va the cathode is pulled below ground,causing gridcurrent.Simply put a diode (1N4148)between cathode and ground (anode diode on ground).Now Vgk may go negative but only 0.6V positive.
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No mistake :mad:
Do it like this :D


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A diode from the cathodes to ground won't work because the voltage on the cathodes sits at around 1.6V when the HT comes up. (The cathodes are returned to a -15V rail via a BJT current sink that sets the tail current at 12mA). The cathodes sit at around -12V when the grid current is flowing with no HT present.

I've been trying to recreate the fault condition this morning but the grids keep returning to 0V whenever the HT comes on. Typical - I hate intermittent faults!
I think I'm finally starting to get to the bottom of this as I have a valve on the bench that is repeatably demonstrating the problem. I fitted the diode between the cathodes and 0V, and with scope probes on the cathodes and one of the grids I see that (as expected) the cathodes are initially clamped by the diode to about -0.7V. As the heater warms up, the grids start to draw current and their voltage slowly drops to the same potential as the cathodes. On a good valve, when the HT comes up, the cathode voltage immediately jumps to about 1.3V to set the bias current at the required 12mA. However, on a faulty valve, the cathodes - and hence the grids - only rise to about -0.5V. Over the course of a further minute or so, the cathode voltage slowly increases, eventually reaching 1.3V. The muting relays open 55s after the heaters came on (and 25s after the HT came up), so with the faulty valve, there is a loud "pop" from the speaker. With a good valve, the grids are sitting at 0V, so the relays open silently.

It seems odd to me that the faulty valve is taking so much longer to warm up (or at least to start passing anode current). Does anyone have any experience of this?
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