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Adjustable cathode bias

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Posted on November 2, 2015 at 13:49:13
Frihed89
Audiophile

Posts: 15703
Location: Copenhagen
Joined: March 21, 2005
 I have an SE amp that uses a common cathode bias for the output tubes. If I can use two different output tubes in the amp, but need to change the cathode resistor to do so. Having done this, i like the sound of both tubes (1626 and 12V6). I am looking at making the bias adjustable, between 500 and 150 ohms.

What are the pitfalls in this situation, if any? Do I parallel the pot with the existing 500 cathode resistor? A setting of 250 Ohms would get me to 150 ohms. Or, do i simply replace the existing cathode resistor with a pot that i can vary between 500 and 150 ohms?

I am not a DIY-type. I learn as I go by doing small things to existing amps.

 

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RE: Adjustable cathode bias, posted on November 2, 2015 at 13:58:57
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 You might think about a 220 ohm resistor and a switch.

The switch would switch the 220 ohm resistor in/out of parallel with the 500 ohm cathode resistor.

220//500 is 152.7 ohms

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Adjustable cathode bias, posted on November 2, 2015 at 17:23:12
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 Dissipation may prevent you from using a pot successfully.

The switch is a very good suggestion.

 

RE: Adjustable cathode bias, posted on November 3, 2015 at 05:02:39
SteveSchecter
Audiophile

Posts: 52
Location: East Tennessee
Joined: November 26, 2013
 I have a 25w resistor that is adjustable. It is a wirewound ceramic tube. The windings are exposed along a narrow strip. It has a sliding contact that lets one adjust for any ohm value desired. This is great for dialing in the value you like. Ideal for your situation.

I don't know if such is still manufactured anymore but surely they can be found on eBay, etc.

 

Active Bias Regulator, posted on November 3, 2015 at 06:10:23
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 You might think about using an active cathode bias regulator for this. The LM337 can set the cathode to whatever voltage is required. You could adjust current not only for different tube types, but for an exact value within each family.

Here's the basic idea (not all parts are shown):





 

Clean, simple, and easy to use solution. Nice (nt), posted on November 3, 2015 at 07:44:21
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 nt

 

RE: A SPDT switch works perfectly, posted on November 3, 2015 at 08:48:08
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I have done this in one of my SE amps so I can use a 6v6 or 6l6 tube. I am also using a LM317t as a voltage regulator/ CCS for setting the cathode bias. I switch out the voltage setting resistor which is only 1/2 watt. I suppose you could install a pot in its location if you want to adjust the bias continuously but that means installing a test point and measuring the voltage at the cathode. cheers, Dak

 

RE: Adjustable cathode bias, posted on November 3, 2015 at 08:53:49
Frihed89
Audiophile

Posts: 15703
Location: Copenhagen
Joined: March 21, 2005
 Yes, I was worried about heattoo and thinking about that a little.

 

RE: Adjustable cathode bias, posted on November 3, 2015 at 08:58:55
BofService
Audiophile

Posts: 741
Location: Atlanta
Joined: February 28, 2003
 Those adjustable resistors are absolutely still available.

 

Thanks, posted on November 3, 2015 at 09:08:15
Frihed89
Audiophile

Posts: 15703
Location: Copenhagen
Joined: March 21, 2005
 Not knowing too much about electronics, I only know that variable resistors can get quite hot. An SPDT switch, I guess, wouldn't have that problem. I could add a 200-215 ohm, 3W resistor in parallel with the 500 ohm 3W one and use the switch to cut off the smaller resistor, right?

 

RE: Active Bias Regulator, posted on November 3, 2015 at 09:10:00
Frihed89
Audiophile

Posts: 15703
Location: Copenhagen
Joined: March 21, 2005
 Thanks, a little advanced for me; i'll have to find out what an LM337 is.

 

RE: Thanks, posted on November 3, 2015 at 10:07:42
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 If you are going to use just the large resistors then you need to wire the cathode to the center pole of the switch and run one resistor from the outside pole to ground and again the second resistor on the other outside pole to ground. And don't forget your bypass cap. cheers, Dak

 

RE: Thanks, posted on November 3, 2015 at 14:27:54
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 My suggestion was to leave the 500 ohm resistor in place and use a switch to place a 220 ohm resistor in parallel with that 500 ohm resistor when 150 ohms are wanted and out of circuit when the 500 ohms are wanted.

The 220 ohm resistor could be permanently connect to ground and the other end could be switched to connect to the cathode or disconnected from the cathode.

He uses one shared cathode resistor for both output tubes so it would only take one single pole single throw switch to get the job done for his stereo amplifier.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Yes, yours would be a simpler , easier solution, cheers. nt, posted on November 4, 2015 at 09:49:23
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 .

 

That was my plan nt, posted on November 4, 2015 at 10:15:37
Frihed89
Audiophile

Posts: 15703
Location: Copenhagen
Joined: March 21, 2005
 nt

 

RE: Adjustable cathode bias, posted on November 9, 2015 at 09:24:20
Ed Sawyer
Audiophile

Posts: 2595
Joined: November 3, 2000
 As mentioned, a switch is the best way to do this. The resistor (pot) would need to be a biggish variac-style variable resistor with heavy duty windings, and it's not going to be as quiet as a "real" switched resistor solution.

 

Dissipation may prevent you from using a pot successfully., posted on November 20, 2015 at 11:29:23
richardl
Audiophile

Posts: 3555
Joined: September 5, 2002
 this is true. I have definitely blown several trying to adjust CCS on large tubes.

 

RE: Adjustable cathode bias, posted on November 22, 2015 at 06:41:00
hifipaul
Audiophile

Posts: 735
Location: NY
Joined: December 22, 2008
I recently built a 842/10 amp that needed either 1600ohms or 3000ohms in the cathode. using a switch proved to be the best option. It also worked nicely as a tie point (solder lug) inside the amp. Using a 25W pot is good for experimenting, dialing in the best values, but not much for the final design.

 

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