Question about biasing heater above cathode in a direct coupled circuit.

G · 26th May 2007, 08:27 AM

Hi all. I have a direct coupled version of a 12B4A amplifier that I would like to build. The cathode of the output tube(12B4A) is sitting at 174v above ground. The input tube has the cathode sitting at 6v above ground. I would like to bias the heater of the input tube above the cathode to cut down on hum. Since the cathode of the ouput tube is so far above the heater relative to ground do I need to worry about it producing hum? I suppose I could just use DC on the heaters but I would prefer not to. I think biasing the heaters up to around 50v will be ok but I would like some opinions. Thanks.

EC8010 · 26th May 2007, 12:42 PM

The ideal is to make the heater 40V positive with respect to the cathode. You need two heater supplies; AC will be fine so long as you do it properly. You definitely need to bias the 12B4A heater to > +174V, otherwise you will get noise (even on an insensitive stage) and possibly eventual heater/cathode insulation breakdown.

26th May 2007, 08:27 AM	#1
G diyAudio Member Join Date: Apr 2002 Location: Belleville, IL.	Question about biasing heater above cathode in a direct coupled circuit. Hi all. I have a direct coupled version of a 12B4A amplifier that I would like to build. The cathode of the output tube(12B4A) is sitting at 174v above ground. The input tube has the cathode sitting at 6v above ground. I would like to bias the heater of the input tube above the cathode to cut down on hum. Since the cathode of the ouput tube is so far above the heater relative to ground do I need to worry about it producing hum? I suppose I could just use DC on the heaters but I would prefer not to. I think biasing the heaters up to around 50v will be ok but I would like some opinions. Thanks. __________________ Gavin

26th May 2007, 12:42 PM	#2
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	The ideal is to make the heater 40V positive with respect to the cathode. You need two heater supplies; AC will be fine so long as you do it properly. You definitely need to bias the 12B4A heater to > +174V, otherwise you will get noise (even on an insensitive stage) and possibly eventual heater/cathode insulation breakdown. __________________ 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...

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
cathode biasing question	puginfo	Tubes / Valves	7	3rd June 2009 04:24 PM
Direct-coupled cathode/source follower driver in PP?	ray_moth	Tubes / Valves	83	17th July 2008 03:53 AM
newbie asks question :cathode biasing	garbage	Tubes / Valves	9	17th April 2007 10:20 AM
6DJ8 cathode heater voltage question.	Morse	Tubes / Valves	13	30th January 2003 08:59 PM
Can someone explain parafeed & direct coupled & a.c. coupled?	mwmitchell	Tubes / Valves	2	11th September 2002 02:10 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)