Vg-k max - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th August 2006, 06:00 AM   #1
lndm is offline lndm  Australia
diyAudio Member
 
Join Date: Mar 2006
Location: nsw
Default 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?
  Reply With Quote
Old 28th August 2006, 06:17 AM   #2
diyAudio Member
 
Tubes4e4's Avatar
 
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.
  Reply With Quote
Old 28th August 2006, 06:22 AM   #3
lndm is offline lndm  Australia
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.
  Reply With Quote
Old 29th August 2006, 05:00 PM   #4
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
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..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 29th August 2006, 11:51 PM   #5
lndm is offline lndm  Australia
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.
  Reply With Quote
Old 30th August 2006, 12:00 AM   #6
Tweeker is offline Tweeker  United States
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.
  Reply With Quote
Old 30th August 2006, 12:26 AM   #7
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
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.
  Reply With Quote
Old 30th August 2006, 11:07 AM   #8
lndm is offline lndm  Australia
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.
  Reply With Quote
Old 1st September 2006, 12:33 PM   #9
lndm is offline lndm  Australia
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 12:11 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2