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Post subject: SF Vibrolux Reverb bias
Posted: Thu Dec 09, 2010 5:18 pm
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I got 78 SF Vibrolux, I would like to try some new power tubes in this.So i had some JJ's 6l6 to give it a shot.With the stock power tubes the Bias was 37.5 mA. So i turn what i hope is the bias adjustment pot counter clockwise and it goes almost to Zero so i set it back to it normal bias and turn it clockwise and notice the the bias is almost also going to zero as well.Is this a bias pot or should it be one of them balance pots i heard about


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Posted: Thu Dec 09, 2010 5:37 pm
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It's a balance pot. Most of the silverfaces converted to that from the traditional bias adjust in the late '60s.

Arjay

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Posted: Thu Dec 09, 2010 5:56 pm
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OK so if i changed power tubes to anything else i would have to change a resistor to get the bias correct then. When i threw in modern JJ's i got a bias of 25mA.I tried adjusting it and got the same result. Now how would i adjust the balance on this.


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Posted: Thu Dec 09, 2010 11:01 pm
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Joncarr, to adjust the bias the 15K resistor between the wiper and ground would need to be changed. The only schem I have is the AA270 for the Vibrolux Reverb. If you have a more accurate schem, that would be cool. Art

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Posted: Fri Dec 10, 2010 2:09 am
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This link has one method of putting a BIAS POT in an amp that has only a HUM BALANCE pot (later SF's, for example). I kinda like it because it keeps both pots, though if the power tubes are well MATCHED (and stay fairly closely matched thru use and life), the actual benefit of the HUM POT is kinda minimal.

This example is for quad-6L6GC amps. Same idea works with dual-6L6GC (and any other power tube in a FIXED BIAS setup).

http://www.diyguitarist.com/GuitarAmps/TR-BiasMod.htm


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Posted: Fri Dec 10, 2010 4:24 pm
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Yes, that would be well worth the effort. Good link, Beemer ! But I wonder why such a large(100K) pot was chosen ? Seems like the sweep would be a little touchy. And it should be a linear pot. But yeah, I like that idea. Art

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Posted: Fri Dec 10, 2010 6:50 pm
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I guess it depends on what other resistor(s) you have inline with the pot. I like pots that have the approx correct bias in about the midpoint for new tubes. Yup, should be linear.


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Posted: Fri Dec 10, 2010 6:59 pm
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I like the idea of installing a pot. I just wanna understand everything before i attempt.I get about the 15k resistor the pot witch is making up for the resistor.Now i got a question about the 3.3k or 4.7k resistor.So i would run that from the back side of the pot to a ground on the chassis.If i am understanding this correct


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Posted: Fri Dec 10, 2010 8:47 pm
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See the 15k-ohm resistor off the middle wiper of the stock Hum pot? See schematic, "1-point" on hum pot?--- (which means screwblade adjustable in schemo language)? You will replace this 15-K-ohm resistor with a bias pot (you supply). Then, solder a 4.7k to 6.6 k-ohm resistor from new bias pot (red triangle point off new bias pot, on Paul J. Marossy's diagram) to ground.

In other words just replace the stock 15-ohm resistor with a bias pot and a 4.7k to 6.8k-ohm resistor, in series. The reason to use a set resistor inline with the pot (to ground), is in case someone cranks the pot to zero ohms ... you'll still have negative voltage being applied to the grids of the power tubes.

http://www.ampwares.com/schematics/twin ... _aa270.pdf


Last edited by BMW2002Ti on Fri Dec 10, 2010 8:53 pm, edited 1 time in total.

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Posted: Fri Dec 10, 2010 8:50 pm
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Actually Jon, the resistor would be taken off the leg of the pot that is going to ground(in-line). If the pot was mounted through a hole in the chassis, then it could go from same leg(tap) to the body. It's just a matter of how you choose to ground it. If it were me, I'd go with a 50K-L/ 10K through the chassis. But like Beemer said, you'll want to check the available adjustment before hard wiring. Oh yeah, be sure the balance pot is centered before testing. JMO Art......... OOOPS ! Beemer took care of it, whilst I typed !

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Posted: Fri Dec 10, 2010 8:56 pm
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Art, You're absoultely correct. Me make wording boo-boo (hum pot for bias pot). I EDITTED it. Hum pot in midway position, as stated--- provided power tubes are somewhat matched (15-20%).

Thanks!


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Posted: Sat Dec 11, 2010 4:17 am
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Thanks guy i get it now.remove resistor,put in pot where resistor use to be and run a new resistor to ground from leg of pot. then i have adjustable bias.


NOw say i just wanna change resistors.instead of doing the pot what kinda values do i need to go up say 10mA


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Posted: Sat Dec 11, 2010 5:41 am
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Jon,

It's not that straight forward. The amount of current your power tubes pass at idle is dependent on the tube as well as the negative voltage applied by the BIAS POWER supply. Not all tubes will pass the same amount of current, even with a set plate voltage & negative grid voltage.

If you can find specific current draw for your tubes and know the specific negative voltage applied to the grid... you can GUESSTIMATE the corresponding new negative voltage needed to change the idle bias current using proportions, BUT tubes are not linear beasts. So, proportions may get you into a ballpark, but you may need to solder resistors in parallel and series to get the exact idle bias current (or watts of dissipation). That's is, fine tune the resistance.

Better to use the pot... then substitute a fixed resistor, once you've adjusted the bias point to what you want. You'll need to do this each time to rebias the beast.


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