Should 2 bias controls be independent of each other?

[pbman1953]

Years ago I was given this diagram, here,  for 2 bias controls. One control per tube pair. I thought they were working fine but if I adjust one the non-connected tube pair change.


Is that normal or should each pair not feel the other control's change?


Sunn 2000

[triode]

I have repaired a few Sun 2000s amps, and have done bias mods which are like that and also one for each tube.

I would note that the other tubes will move "a little" in nearly all amps when you first start biasing for example if you have two new tubes both sitting at say 12mA and you jack one up to 32mA,then put the second one at 32mA, the first may now sit at 29mA and you have to turn that one up, then recheck the second one again. Are you seeing something like that?

I find that big changes will in fact move the bias on other tubes, I have several home stereos with independent tube bias and if one new tube is off a lot (like 10mA) I will have to goback and "tweak" the others.

OR are you saying something like "if I increase the bias on this pair 2mA the others take a nose dive of 10mA"?

[sluckey]

If there is only a slight interaction don't worry about it. It's normal to have to go back and forth with both pots a few times to get everything exactly matched.

But if you have a big interaction, then the mod is not done in accordance with your schematic. It's easy to mess this simple mod up due to the way Sunn wired up those two long terminal strips. Post a couple hi-rez pics that show the two pots and the two long terminal strips.

Also, look at my Scepter bias mod. It's only two output tubes but may help.

     https://sluckeyamps.com/sunn/sunn.htm

[pbman1953]

First off a very cool , mod!




I took a closer look to be sure the the bias points were isolated and they were. I gave each pit a spritz of Deoxit and I'm back in biz.


Thanks Guys!

[sluckey]

I took a closer look to be sure the the bias points were isolated and they were. I gave each pit a spritz of Deoxit and I'm back in biz.

I don't see how that could have any effect on the issue you stated. I'd still like to see some hi-rez pics of this mod.

[pbman1953]

I took a closer look to be sure the the bias points were isolated and they were. I gave each pit a spritz of Deoxit and I'm back in biz.

I don't see how that could have any effect on the issue you stated. I'd still like to see some hi-rez pics of this mod.

The white bottom pot is the stock one


Let  me know if you a different angle

[sluckey]

I need one hi rez pic that shows all four output tube sockets, both bias pots, all bias caps, and the top five lugs of the two long terminal strips that straddle the PI tube.

[pbman1953]

I need one hi rez pic that shows all four output tube sockets, both bias pots, all bias caps, and the top five lugs of the two long terminal strips that straddle the PI tube.

I sent pictures of-


4 Power tube sockets
2 bias pots
1st bias cap
Center section

[sluckey]

Looks like the mod is wired correctly. Did you say that the bias pots act normally after the deoxit?

[pbman1953]

Looks like the mod is wired correctly. Did you say that the bias pots act normally after the deoxit?

Thanks , but I think the Deoxit wasn't a huge help. After the spray and  moving the the pots may have helped them track better. I got the sides pretty close