Need help for a bias circuit

jonas86 · 20th October 2010, 07:08 PM

Hello!

I'm trying to build a bassman100 amp but i can't find the 10k pot with 4 terminals to set the bias.
I was wondering if i can do it in an other way with 2 normal 10k pots (Attched file),
could somebody tell me if it is wrong and maybe how it could be done?

I will appreciate any help,

Jonas

DF96 · 20th October 2010, 07:56 PM

The original circuit is strange. It adjusts the AC balance a little as well as the DC balance. I don't know how critical this is.

Doz · 20th October 2010, 09:55 PM

How many knobs on this pot ... ? 2 wipers would tend to infer 2 knobs... some Alps pots have 4 terminals on them, but only one knob. Chinese copies are available, and probably better quality than the original.

DF96 · 20th October 2010, 10:01 PM

One wiper, plus a fixed centre tap?

Palustris · 20th October 2010, 10:39 PM

That is not the way I would have done it, but that's not the first time I've seen that bias setup. Its real advantage is that you have the current, in this case about 3.5mA, only going through a small portion of the pot, therefor it drops little current and can be low wattage and cheap. The double wiper pot really has only one function: to set the current through each tube to the correct value.

You could do a slight re-engineer to provide the same functions using two separate pots. I would try two 20k pots in parallel; one end to B- (omit the 3.3k resistor but not the cap) the other to the 15k resistor. The wiper goes to the 68k resistor. The pot will have to dissipate 2.7mA * 35V = .1W, so any well constructed 1W or 5W, 10k linear pot should do. I can't guarantee that the voltages will work out exactly as I described, but by juggling the 15k resistor, maybe 7.5k on either side of the pot, you should be able to get the pot in the center of its range when the amp is biased properly.

Enzo · 21st October 2010, 02:02 AM

Leo Fender never paid extra for matched power tubes when he built amps. Random tubes were installed. The balance style bias control was there for hum null. It let you balance the amp with the random tubes, at least improve the balance. Set the control for least hum.

Many many guys have rewired the thing to a straight bias level control for all the tubes, and they just install matched sets.

You can find that odd pot and duplicate this circuit, or build something else for bias. You can make something that adjust each tube separately, or something that has an adjustment for each side, or just one overall control. Only thing you MUST do is wind up with proper bias voltage on pin 5 of each power tube.

jonas86 · 21st October 2010, 07:15 AM

Yes the original is a pot ( so 1 knob) with 4 terminal, 1 wiper and a center tap.

Ok i will try to build a bias circuit with 2 pots with the values that Palustris has proposed.

Thank you all for yuor help!!!

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
Bias Circuit Calculation	blue andry	Tubes / Valves	26	6th July 2010 12:55 AM
Need confirmation on LED bias for circuit	phrarod	Tubes / Valves	56	10th May 2010 02:03 PM
Help needed with bias circuit	Rob11966	Tubes / Valves	18	1st March 2009 03:07 PM
cathode bias circuit question	gary h	Tubes / Valves	14	26th August 2008 12:36 AM
bias circuit for mosfet help	AudioGeek	Solid State	2	29th April 2006 11:22 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


	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

20th October 2010, 07:56 PM	#2
DF96 diyAudio Member Join Date: May 2007	The original circuit is strange. It adjusts the AC balance a little as well as the DC balance. I don't know how critical this is.

20th October 2010, 09:55 PM	#3
Doz diyAudio Member Join Date: Jul 2010 Location: Sat Down	How many knobs on this pot ... ? 2 wipers would tend to infer 2 knobs... some Alps pots have 4 terminals on them, but only one knob. Chinese copies are available, and probably better quality than the original.

20th October 2010, 10:01 PM	#4
DF96 diyAudio Member Join Date: May 2007	One wiper, plus a fixed centre tap?

20th October 2010, 10:39 PM	#5
Palustris diyAudio Member Join Date: Aug 2006 Location: Cape Cod	That is not the way I would have done it, but that's not the first time I've seen that bias setup. Its real advantage is that you have the current, in this case about 3.5mA, only going through a small portion of the pot, therefor it drops little current and can be low wattage and cheap. The double wiper pot really has only one function: to set the current through each tube to the correct value. You could do a slight re-engineer to provide the same functions using two separate pots. I would try two 20k pots in parallel; one end to B- (omit the 3.3k resistor but not the cap) the other to the 15k resistor. The wiper goes to the 68k resistor. The pot will have to dissipate 2.7mA * 35V = .1W, so any well constructed 1W or 5W, 10k linear pot should do. I can't guarantee that the voltages will work out exactly as I described, but by juggling the 15k resistor, maybe 7.5k on either side of the pot, you should be able to get the pot in the center of its range when the amp is biased properly.

21st October 2010, 02:02 AM	#6
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	Leo Fender never paid extra for matched power tubes when he built amps. Random tubes were installed. The balance style bias control was there for hum null. It let you balance the amp with the random tubes, at least improve the balance. Set the control for least hum. Many many guys have rewired the thing to a straight bias level control for all the tubes, and they just install matched sets. You can find that odd pot and duplicate this circuit, or build something else for bias. You can make something that adjust each tube separately, or something that has an adjustment for each side, or just one overall control. Only thing you MUST do is wind up with proper bias voltage on pin 5 of each power tube.

21st October 2010, 07:15 AM	#7
jonas86 diyAudio Member Join Date: Oct 2010	Yes the original is a pot ( so 1 knob) with 4 terminal, 1 wiper and a center tap. Ok i will try to build a bias circuit with 2 pots with the values that Palustris has proposed. Thank you all for yuor help!!!