Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JCSIFU on March 17, 2012, 04:24:28 pm
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Hi all,
I am trying to add a to a jcm600 with an already existing bias level pot. I have been searching the forum for info as well as the web and can't find a reason that this idea would not work. Can anyone see anything problematic that might occur? I was going to use something like a 100k trim pot for this, what do you guys think?
thanks,
jc
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(http://s2.imagestime.com/out.php/t671353_Williamsonwith6550.jpg) (http://www.imagestime.com/show.php/671353_Williamsonwith6550.jpg.html)
That was used in Williamson amps and other
however I prefer a double separated control
Kagliostro
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(http://image72.webshots.com/172/5/4/96/2951504960102108523XlUgSO_fs.jpg)
That was used in Williamson amps and other
however I prefer a double separated control
Kagliostro
Ok, so it would work fine then. I'll give it a go. Thanks.
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Because the bias supply has about -0-current, series resistors have near -0- effect on dropping voltage, prr ohm's Law. Your varying pot values may have little effect. You will probably need a voltage divider, with shunt resistors to ground off the outer pot lugs, to get a voltsge drop. Taht in turn may require boosting bias voltage to the balance pot.
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Because the bias supply has about -0-current, series resistors have near -0- effect on dropping voltage, prr ohm's Law. Your varying pot values may have little effect. You will probably need a voltage divider, with shunt resistors to ground off the outer pot lugs, to get a voltsge drop. Taht in turn may require boosting bias voltage to the balance pot.
Ok, I think I see your point. I'll look into finding a dual bias layout with the resistor to ground as a voltage divider. I've used those before with pedal constructions, not with an amp so I'll have top look up some values. Thanks for taking time.
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Your varying pot values may have little effect. You will probably need a voltage divider, with shunt resistors to ground off the outer pot lugs, to get a voltage drop.
jjasilli knows a lot more than me about amps, but FWIW, I think he's right. Maybe to put it another way, the bias balance pot needs something to work against to make a change in the bias voltage, because of no current draw or very, very, little current draw at best.
Brad :icon_biggrin:
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Your varying pot values may have little effect. You will probably need a voltage divider, with shunt resistors to ground off the outer pot lugs, to get a voltage drop.
jjasilli knows a lot more than me about amps, but FWIW, I think he's right. Maybe to put it another way, the bias balance pot needs something to work against to make a change in the bias voltage, because of no current draw or very, very, little current draw at best.
Brad :icon_biggrin:
I think I am going to do this:
http://ampgarage.com/forum/download.php?id=11224 (http://ampgarage.com/forum/download.php?id=11224)
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Regarding your original diagram:
Fender did it differently. Where you have the raw (or adjusted) bias entering the wiper, Fender used a pot with a mid-tap, and brought the bias in there. The wiper of the Fender pot was connected to ground. Tweaking the pot move the ground point closer to one side of the pot or the other.
The genius of the Fender plan is the ground connection from the wiper causes some current flow through the resistance of the pot. As a result, when you move the wiper, one end of the pot winds up with more negative voltage than the other end.
I don't know where you'd find a pot with a mid-tap; maybe some high-volume electronics distributor.
Or you can make life easy. Add a second bias adjustment pot and associated resistor(s). Normally, a single adjustment pot would feed the joined ends of a pair of 100-330k grid reference resistors. Separate the joined ends. One adjustment pot feeds one resistor, the other adjustment pot feeds the other resistor. You now have independant bias adjustment for each side of the push-pull stage.
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I don't know where you'd find a pot with a mid-tap
Not strictly necessary - this is a possible solution - the same used by Williamson
(http://s3.imagestime.com/out.php/i671349_BiasBalancepot.png) (http://www.imagestime.com/show.php/671349_BiasBalancepot.png.html)
Kagliostro
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(http://s2.imagestime.com/out.php/t671353_Williamsonwith6550.jpg) (http://www.imagestime.com/show.php/671353_Williamsonwith6550.jpg.html)
That was used in Williamson amps and other
however I prefer a double separated control
Kagliostro
Ok, that looks like a good idea. I'll map this one out and try it. Thanks! :occasion14:
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I don't know where you'd find a pot with a mid-tap
Not strictly necessary - this is a possible solution - the same used by Williamson
(http://s3.imagestime.com/out.php/i671349_BiasBalancepot.png) (http://www.imagestime.com/show.php/671349_BiasBalancepot.png.html)
Kagliostro
I think this is the way I will go. Again, thanks!
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I would pull the output tubes and measure the negative bias voltage on pin 5 of each tube before doing the mod. This will be your pre-mod reference level. Then install the balance pot. Set it to center of rotation. Then adjust the bias voltage on pin 5 of each tube to be the same as your pre-mod reference level. Test to be sure the balance pot can adjust the bias voltage above and below your reference level for each tube. Finally, reinstall the tubes and test live.
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I have a thread on this somewhere. I'm away for the weekend, but trying to keep up. Realy the simple solution, and the one I finally succumbed to, is to add a second bias adjust pot, instead of a bias balance pot. This was in my el84 Traynor Bassmate thread. Still, to my surprise, I found that the 2 adj pots were interactive.
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I have a thread on this somewhere. I'm away for the weekend, but trying to keep up. Realy the simple solution, and the one I finally succumbed to, is to add a second bias adjust pot, instead of a bias balance pot. This was in my el84 Traynor Bassmate thread. Still, to my surprise, I found that the 2 adj pots were interactive.
I tried it with 2 independant bias pots and I noticed the interactivness as well. I think I like the idea of using the level pot as was already on the amp as a main level control, then ballancing the 2 sides. These JCM600 circuit boards are VERY fragile and I am trying to do the minimum amount of desoldering and resoldering that I can. The common bias feed (from level trim pot) has already lifted on one eyelet from 1 quick desolder braid time and I am now using only one of the two on the board. (same trace but 2 eyelets.) I could always run a wire from the previous component, but I prefer to not have wire routed all over the place. These amps also have 3 boards and are horible to work on. I am leaning towards the ground referenced regular pot for a bias ballance kagliostro posted, less desoldering needed.
sluckey:
I will be sure to follow your check list, sounds like a solid plan of safe attack.
The reason for this mod is that I have a pair of NOS Mullard XF1 EL34's that don't quite match up and I would really like to hear them in this amp. I'll post the results after I do this mod.
thanks for everyone taking time to write.