Do I need two bias taps to run two output transformer in a stereo amp

[Mike_J]

I am planning an amp that will have one 100 watt PT and two 50 watt OTs.  The PT has a dedicated 0 - 100 volt bias winding.  Can this one winding feed the bias circuits for both of the OTs or do I need separate bias windings for each OT?

Thanks
Mike

[shooter]

are your referring to fixed bias on the power tubes?  If so I believe there is very low current, but you want a stable voltage, so I would think as long as you can achieve the correct voltage for the tubes, 1 bias supply would work.

[Mike_J]

are your referring to fixed bias on the power tubes?  If so I believe there is very low current, but you want a stable voltage, so I would think as long as you can achieve the correct voltage for the tubes, 1 bias supply would work.

Shooter

Thank you for the reply.  Yes I am referring to fixed bias on the power tubes.  The 0 - 100 bias tap is for EL34 tubes.  My amp will run two EL34s and two 5881s or KT66s.  Do you think I could run the EL34 bias at 100 volts and use a dropping resistor to 60 volts and use a blackfaced Fender type bias circuit for the 5881s or KT66s?

Thanks
Mike

[eleventeen]

Current requirements for bias supplies are generally tiny, 1 ma or so.  I don't see why a single bias supply couldn't handle the task. I would use fairly large (maybe 220K) resistors to separate the different sections being biased, though. 

[Mike_J]

Current requirements for bias supplies are generally tiny, 1 ma or so.  I don't see why a single bias supply couldn't handle the task. I would use fairly large (maybe 220K) resistors to separate the different sections being biased, though.

Eleventeen

Thank you for the reply.  Do you mean placing a 220K resistor on the wire for the 100 volt tap running between the two bias circuits?

Thanks
Mike

[eleventeen]

Imagine a pitchfork. The common point, on the left, fresh out of the rectifier, feeds all ckts. Along the tines, my suspicion is that you'll want to place resistors feeding the separate circuits. You do not want adjustments made to one circuit to "talk to" the other circuits. That implies "separation" or "isolation", a function perfect for resistors.


I have never built such a thing, but what is different about one rectifier feeding three bias circuits is the notion of "adjustment independence". if you only had a single conventional voltage divider, you wouldn't much care about signal from one output-tube section coming back and influencing another, because you would not have another. There would be only one output section.
 
Perhaps the smart thing would be to try to find some hi-fi stereo amplifier schematics where dual fixed bias circuits were used. As I look at upper-end McIntosh and Harmon-Kardon stereo tube amps, they do not appear overly concerned about this, so maybe I am being alarmist.

[sluckey]

Using separate diodes will give you very good isolation between the bias circuits.

[Mike_J]

Using separate diodes will give you very good isolation between the bias circuits.

Sluckey

Thank you for the reply.  I have prepared a layout showing two bias circuits.  Could you tell me where to place the diodes in the layout and how to orient them (which side is the cathode supposed to be on).  I knew you would have the solution to this dilemma.

Thanks
Mike

[sluckey]

The rectifier diodes provide all the isolation you need. Your drawing is correct except you need a jumper between the left end of the 47K and the left end of the top 10µF on both bias boards.

[Mike_J]

Sluckey

Thanks for the catch.  My Sluckey inspired Plexi/800 ... also was missing the jumper so I am glad we did this.  I found a few small changes in the layout for the Plexi/800 and will post a new layout when the amp is completed and all tweaks have been made.

Thanks
Mike

[Mike_J]

The rectifier diodes provide all the isolation you need. Your drawing is correct except you need a jumper between the left end of the 47K and the left end of the top 10µF on both bias boards.

Sluckey

I was looking over my Sluckey inspired Plexi/800 build and compared the bias supply to the 2203 amp on the Ceriatone site.  They place the jumper on the entrance of the 50K pot.  I put mine on the ground or other side.  Do you think it makes a difference?  I Xed out the way I had it and wrote the jumper in red on the attached layout to correspond with Ceriatone's method.

Thanks
Mike