Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Underwood on November 30, 2011, 09:09:12 am

Title: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 09:09:12 am

  I am checking my bias now on a 62 blonde bandmaster  and when I set the bias to 35 ma. on the right  tube the left tube is 17 ma.  It stays about 18-20 ma. off whether I go up or down. The coupling caps are new. The 220 resistors are up to spec. The blue and brown wires going to pin 3 of each tube, is that from the OT?  Could the OT be going out? The voltage on both sides is even at about 448V. The negative on pin five I think was also the same.

Title: Re: One tube has way lower bias current than the other
Post by: rzenc on November 30, 2011, 09:22:14 am

Change tube position, measure again. If measurements stays with tube, try another.

Title: Re: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 09:56:21 am

I've tried switching them around and also another set of tubes.

Title: Re: One tube has way lower bias current than the other
Post by: sluckey on November 30, 2011, 09:59:34 am

How are you checking the current?

Title: Re: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 10:14:10 am

  I have a 1 ohm resistor on pin 8 to ground.

Title: Re: One tube has way lower bias current than the other
Post by: jim on November 30, 2011, 10:24:22 am

Make sure the screen voltage is the same between the tubes.  Remove the tubes and check for identical negative voltage swing on both pin 5's when you twist the bias pot.  If these are OK it has to work.   Jim

Title: Re: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 11:19:40 am

The voltage at the point where the 2 220K resistors meet on the board is -64.2. The right tube pin 5 is -62.8. left tube pin 5 is -60.4.  They are slightly different. This is with the pot all the way up. They seem to move up and down identically when I turn the pot. The reading at pin 3 is identical at 450V

Title: Re: One tube has way lower bias current than the other
Post by: Danskman on November 30, 2011, 11:31:51 am

I would replace the two power tubes by a burned-in and paired set...
Best regards and take care,
Danskman

Title: Re: One tube has way lower bias current than the other
Post by: sluckey on November 30, 2011, 11:35:51 am

Screen is pin 4. What is the voltage on each pin 4? Are the 470Ω/1W screen resistors OK?

If still no joy, try this... Pull the PI tube to kill the NFB loop. Now swap the OT plate leads (blue and brown leads on pin 3 of each output tube). Does the low current reading move to the other tube?

Title: Re: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 11:42:38 am

  I tested it with matched SED winged C and with perfect pair of NOS GE 6LCgc and got the same results. The tube on the left was getting a lower bias than the right tube. Also, the 470 resistors are good. I will check pin 4. Then I will try to switch the OT wires and pull the PI tube and let you know what happens.Thanks!!!

Title: Re: One tube has way lower bias current than the other
Post by: tubenit on November 30, 2011, 11:49:21 am

Can you create two bias pots. One for each tube and then a "matched" pair is less of a concern?

With respect, Tubenit

Title: Re: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 11:53:37 am

  I checked pin 4 and it is 450 at both of them.  It doesn't seem to be the tubes being matched, I have tried different tubes and still getting way different bias current 27 in left and 43 in the right.  I have also swapped these tubes in another amp and they are able to be biased correctly in that one.

Title: Re: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 12:35:29 pm

 Now I am stumped.  Just put the same tubes in the same sockets and the bias is higher in the left at 44 and the right 35. One thing I did find is that one of the two green wires that attaches to pin 2 and pin 7 that connects all the tubes is not even soldered on, it is just flopping around in the pin 2 hole. Maybe that is something. Also, I just plugged my tele in for the first time since doing all this and it sounds great!! Now what? Is it safe to be running at such a different bias?  Maybe I am getting wrong readings because of that loose wire? Also, with the bias around 30 it seemed to breakup quicker, and at 44 it seemed to stay clean with a little more power but with nice breakup when turned up a little more. I always thought it was the opposite. THANKS for everyone's help!!   I will do a few more tube swaps and bias measurements before I button this one back up.

Title: Re: One tube has way lower bias current than the other
Post by: rzenc on November 30, 2011, 02:11:36 pm

Try what sluckey told you. Also check for bad solder joint/bias resistor (220K pair).

Glad to know it sounds great!! But tubeheads are never satisfried until purfection!! :l2:

Best regards
Rzenc

Title: Re: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 03:23:03 pm

  I now have it to 38ma and 42ma on both tubes. Will check the solder joints on the 220K's. If I pull out the PI and switch the OT wires should I have the standby on or should I power it up? THANKS

Title: Re: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 03:26:37 pm

 I also have some NOS 7581's coming in the mail today. Might give them a try also. Any suggestion on biasing those 7581's.

Title: Re: One tube has way lower bias current than the other
Post by: sluckey on November 30, 2011, 03:27:55 pm

Quote

I now have it to 38ma and 42ma on both tubes.

I consider that balanced. Button it up.

Title: Re: One tube has way lower bias current than the other
Post by: Underwood on November 30, 2011, 07:35:20 pm

  Thanks for everybody's help! I redid the solder joints on the 220K resistors and the caps that they are coupled with and now I am getting 40ma for both tubes.  Put her all back together and just got done playing through it for about an hour.  What a sweet amp! Now onto another project!