how do i add adjustable bias on amp?

AudioGeek · 25th September 2005, 06:09 PM

i have a PP 6B4G amp that i just got working.

Problem is all the tubes are running at different points i.e. 61v 55v 45v and 62v across individual 1K cathode resistors.

is the idea to get the same CURRENT through the tubes or the same VOLTAGE across the cathode resistors?

anyways should I just start paralleling resistors across the 1K's till i get same current through all tubes?

or add an adj pot? if so how should i configure it around the 1K cathode resistor?

Thankyou!

Tom Bavis · 25th September 2005, 08:59 PM

You want equal currents, which is equal voltages if the resistors are equal - I asume they're 5%.

If your 6.3V supply is grounded, you can get +/-9V from a doubler connected to it - if it has a grounded center tap, you can get only "+/-4V or so - but that's probably enough. I'd filter it with 100 uF/220 Ohms/470 uF. Connect 4 20K pots pots across +/-voltage, and return the four grid resistors to the wipers. You now have combination bias, adjustable for each tube.

skyraider · 26th September 2005, 02:27 AM

If you dont mind SS in the amp, try using constant current source to 'force' the tubes to behave.

Use a balance trimpot to divide the current between the tubes.

25th September 2005, 06:09 PM	#1
AudioGeek diyAudio Member Join Date: Jan 2004 Location: Minnesota	how do i add adjustable bias on amp? i have a PP 6B4G amp that i just got working. Problem is all the tubes are running at different points i.e. 61v 55v 45v and 62v across individual 1K cathode resistors. is the idea to get the same CURRENT through the tubes or the same VOLTAGE across the cathode resistors? anyways should I just start paralleling resistors across the 1K's till i get same current through all tubes? or add an adj pot? if so how should i configure it around the 1K cathode resistor? Thankyou! __________________ It is not you... you are it.

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
Rheostat for adjustable "fixed" bias?	porkchop61	Tubes / Valves	11	25th April 2011 08:57 PM
bias supply question / sanity check - adjustable resistor?	ilted	Planars & Exotics	0	5th May 2009 02:12 PM
Stray hum pickup preamp cathode R-bias vs. battery bias & 12AU7vs.5687	awedio	Tubes / Valves	2	19th April 2008 01:30 AM
Adjustable Fixed Grid Bias	dsavitsk	Tubes / Valves	5	4th December 2007 03:58 PM
LM385 Adjustable voltage ref. diode as LED bias?	mus	Tubes / Valves	2	30th March 2007 04:27 AM

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


	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

25th September 2005, 08:59 PM	#2
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	You want equal currents, which is equal voltages if the resistors are equal - I asume they're 5%. If your 6.3V supply is grounded, you can get +/-9V from a doubler connected to it - if it has a grounded center tap, you can get only "+/-4V or so - but that's probably enough. I'd filter it with 100 uF/220 Ohms/470 uF. Connect 4 20K pots pots across +/-voltage, and return the four grid resistors to the wipers. You now have combination bias, adjustable for each tube.

26th September 2005, 02:27 AM	#3
skyraider diyAudio Member Join Date: Jan 2004 Location: malaysia	If you dont mind SS in the amp, try using constant current source to 'force' the tubes to behave. Use a balance trimpot to divide the current between the tubes.