The KT-66 has a Vg-k max (design centre and absolute) of 200V. Can I push this in practice? Will it survive a temporary journey to -600V at turn on or must it be clamped, and if so where to?
Hi,
it just is the same with other ratings, they should not be pushed. If you do, it is your own risk.
Once the heater-cathode isolation is pierced, you can forget about that tube.
And usually noise rises with rising Vh-k, too.
Tom
it just is the same with other ratings, they should not be pushed. If you do, it is your own risk.
Once the heater-cathode isolation is pierced, you can forget about that tube.
And usually noise rises with rising Vh-k, too.
Tom
Probably a good idea to clamp vg-k to something less than -200V using a combination of diodes and neon lamps. I would bet that -600V could cause some pretty spectacular arcing between the cathode and grid..
Diodes and neons? May I ask how?
This has been driving me batty for a few days, I'll explain.
I have a mu stage directly coupled to the grids in question. The driver has its own B- which is silicon rectified, and the B+ is vacuum rectified. The B- supply is CRCRC. At no load I have -600V but loaded, it comes up to -250V, all in the time it takes for my 5V4G to come up.
I'm at least glad that the driver DC starts low and comes up rather than the other way round
I have considered back to back zeners grid to ground, but when simming with a pair of 180V'ers they seem to change the bias point.
I have considered shunt regulation, but I'd rather not double the supplies load, especially as I am dropping 350V on the supply.
I have considered a switchable dummy load, and switching the B+ late.
I am really looking for the simplest solution.
BTW, I do want the grids to be allowed to be driven positive.
This has been driving me batty for a few days, I'll explain.
I have a mu stage directly coupled to the grids in question. The driver has its own B- which is silicon rectified, and the B+ is vacuum rectified. The B- supply is CRCRC. At no load I have -600V but loaded, it comes up to -250V, all in the time it takes for my 5V4G to come up.
I'm at least glad that the driver DC starts low and comes up rather than the other way round
I have considered back to back zeners grid to ground, but when simming with a pair of 180V'ers they seem to change the bias point.
I have considered shunt regulation, but I'd rather not double the supplies load, especially as I am dropping 350V on the supply.
I have considered a switchable dummy load, and switching the B+ late.
I am really looking for the simplest solution.
BTW, I do want the grids to be allowed to be driven positive.
A bleeder on the B- supply to keep it from being fully unloaded? It may drag it down some more though, retune the dropping resistors could fix. And it increases load. Maybe a PTC thermistor as the bleeder?
In similar situations I use the old standby, neon bulbs. In this case, I would hang two in series, in the grid circuit. If your driver can handle the added capacitance (a few pF) put them from the grid of the KT66 to ground. These things are a virtual open circuit (plus the few pF) until they conduct, then they become like mini 0A2's. The breakdown voltage varies quite a bit, so test them with a power supply and a series resistor before using. I got a bag of them surplus several years ago, they run from 75 to 90 volts each.
I have three 50V neons and can get 90V neons.
I have a concern with over-current on the neons.
The driver stage is not grounded, but sits between B+ and B-. The current path for the neon at turn on, would be through the lower triode (DHT so already emitting), then the neons and ground. If the neons strike, I figure the lower triode's Vgk will kick in and the current will settle at maybe 20mA, but there could be a rush at first.
Even if the B- comes up to just a couple of hundred -ve volts, I imagine the current path would still be through the neons, pumping 20mA for more than 10 seconds.
I feel that a series resistance could be a complication/problem.
I have a concern with over-current on the neons.
The driver stage is not grounded, but sits between B+ and B-. The current path for the neon at turn on, would be through the lower triode (DHT so already emitting), then the neons and ground. If the neons strike, I figure the lower triode's Vgk will kick in and the current will settle at maybe 20mA, but there could be a rush at first.
Even if the B- comes up to just a couple of hundred -ve volts, I imagine the current path would still be through the neons, pumping 20mA for more than 10 seconds.
I feel that a series resistance could be a complication/problem.
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