Vg-k max

lndm · 28th August 2006, 06:00 AM

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?

Tubes4e4 · 28th August 2006, 06:17 AM

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

lndm · 28th August 2006, 06:22 AM

Hi Tubes4e4, I should have mentioned that the cathode will be grounded and so will the heater.

kevinkr · 29th August 2006, 05:00 PM

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..

lndm · 29th August 2006, 11:51 PM

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.

Tweeker · 30th August 2006, 12:00 AM

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?

tubelab.com · 30th August 2006, 12:26 AM

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.

lndm · 30th August 2006, 11:07 AM

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.

lndm · 1st September 2006, 12:33 PM

The problem is solved...and well.

My concerns were unnecessary, I was in good hands. Thanks guys

28th August 2006, 06:00 AM	#1
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	Vg-k max 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?

28th August 2006, 06:17 AM	#2
Tubes4e4 diyAudio Member Join Date: Jul 2005 Location: Leverkusen	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 __________________ If in doubt, just measure.

29th August 2006, 05:00 PM	#4
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	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.. __________________ www.kta-hifi.net

30th August 2006, 12:00 AM	#6
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	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? __________________ Be sure your foil hat has a good low impedance ground.

30th August 2006, 12:26 AM	#7
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	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. __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

28th August 2006, 06:22 AM	#3
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	Hi Tubes4e4, I should have mentioned that the cathode will be grounded and so will the heater.

29th August 2006, 11:51 PM	#5
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	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.

30th August 2006, 11:07 AM	#8
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	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.

1st September 2006, 12:33 PM	#9
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	The problem is solved...and well. My concerns were unnecessary, I was in good hands. Thanks guys

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