12sn7 heater-cathode voltage - 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 5th May 2008, 05:50 PM   #1
alexg is offline alexg  Philippines
diyAudio Member
 
alexg's Avatar
 
Join Date: Jan 2005
Location: Philippines
Default 12sn7 heater-cathode voltage

I built a 12sn7 linestage, first triode is a grounded cathode DC coupled to the second triode, configured as cathode follower, measured voltage as follows:

Triode 1
* Plate voltage = 121.4
* Cathode voltage = 4.16
Triode 2
* Plate voltage = 254.2
* Cathode voltage = 126.7

Heater Voltage 12.2VAC

Am using a toroidal transformer with 0-12V secondary, both legs of the secondary have 100R resistors to ground.

Sylvania tube manual on 6sn7GT and 12sn7GT regarding Heater-Cathode voltage:

Heater Negative with respect to Cathode - Total DC and Peak - 200V Max
Heater Positive with respect to Cathode - DC- 100V Max; Total DC and Peak -200V Max.

With a cathode voltage of 126VDC on my second triode and my heaters at 12.2VAC, do I need to use a voltage divider to meet the heater-cathode voltage difference?

I tried using a 100K-33K voltage divider and I am getting serious humming problem.

Thanks.
  Reply With Quote
Old 5th May 2008, 06:07 PM   #2
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
It's never a good idea to have a significant voltage between the cathode and heater (especially when there's signal on the cathode). Assuming you used one dual triode as the (stereo) input stage and another as the (stereo) output stage, elevate the heater of the second triode to 160V and bypass it to ground at AC with a large capacitor.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 5th May 2008, 07:27 PM   #3
diyAudio Member
 
kstagger's Avatar
 
Join Date: Dec 2003
Location: Grand Rapids
as EC8010 noted, try a bypass capacitor on your voltage divider. Been there, done that!
  Reply With Quote
Old 5th May 2008, 09:30 PM   #4
Nordic is offline Nordic  South Africa
diyAudio Member
 
Nordic's Avatar
 
Join Date: Sep 2005
What do you do where the two triodes in the same tube run at significant difirent cathode voltages...?

My current project has one such a tube, 1.6V on one cathode 145V on the other... wich still falls in the 200V cathode to grid limit specified....
  Reply With Quote
Old 5th May 2008, 09:48 PM   #5
GordonW is offline GordonW  United States
diyAudio Member
 
GordonW's Avatar
 
Join Date: Mar 2005
Location: Marietta/Moultrie GA
Quote:
Originally posted by Nordic
What do you do where the two triodes in the same tube run at significant difirent cathode voltages...?

My current project has one such a tube, 1.6V on one cathode 145V on the other... wich still falls in the 200V cathode to grid limit specified....

Split the difference. In this case, you would like to consider the swing of the voltages too. You need to make sure that the highest swing of the bigger voltage, does NOT go over the peak AC difference for the H-K voltage too, on the 12SN7... which, IIRC, is 200v.

So, the way to do that... consider the max voltage swing of the second tube, and add that to the DC voltage. Say, it's a 150v p-p swing... or +- 75V centered at 145v. That would give 220v for the max voltage on the cathode. In this case, with a split difference of 145v and 1.6v (73.3v) would still work... the max AC H-K voltage would be 220 - 73.3, or about 146.7 volts. That's safely within the 200V max swing...

Sometimes, if you're pushing the limit, you have to fudge the voltage to where it meets in the middle somewhere... you may have to push the DC heater offset higher, in some situations.

I've had cases where I was right on top of a max negative DC difference in one direction, and positive max AC difference in the other direction, even after putting the DC heater offset as expeditiously as possible. In a perfect world, that's when you start looking at possibly separating the functions into two bottles...

Regards,
Gordon.
__________________
Speaker Design, Restoration and Repair- since 1985.
  Reply With Quote
Old 6th May 2008, 01:18 AM   #6
alexg is offline alexg  Philippines
diyAudio Member
 
alexg's Avatar
 
Join Date: Jan 2005
Location: Philippines
Thanks for the capacitor bypass tip, I will try that tonight.

  Reply With Quote
Old 7th May 2008, 11:40 AM   #7
alexg is offline alexg  Philippines
diyAudio Member
 
alexg's Avatar
 
Join Date: Jan 2005
Location: Philippines
Thanks for all the tips.

Got it working last night. I will rewire it so that the first stage for both channels is on one tube and the CF on the second tube, easier to lift the ground and also better for tube rolling.
  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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Heater-Cathode Voltage bobbyq Tubes / Valves 11 11th August 2009 06:45 PM
Heater to Cathode max voltage specs, including signal swings? flysig Tubes / Valves 2 28th June 2009 09:22 PM
Variable 6922 heater to cathode voltage limit...? ashok Tubes / Valves 2 13th April 2007 08:16 AM
Heater to cathode voltage limits in cascodes??? grhughes Tubes / Valves 1 10th October 2006 03:48 PM
6DJ8 cathode heater voltage question. Morse Tubes / Valves 13 30th January 2003 07:59 PM


New To Site? Need Help?

All times are GMT. The time now is 04:19 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