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Old 6th January 2004, 08:26 PM   #1
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Question push pull bias servo

i've heard its possible to use a transistor or op amp to create an automatic biasing servo circuit for a push pull tube amp. ive searched around and i can't really find anyone who knows or is willing to show the world how to do it because its a special exclusive feature of whatever amp they're trying to sell me. i dont really see how its possible though. i mean, the only thing i can come up with is a current regulator and that doesnt really provide much benefit (... and requires a bias pot or resistors to set the bias.) does such a magical thing exist really? any one? oh and i definitely realize the whimsical nature of such a disgustingly unneccessary circuit, but now curiosity has gotten the best of me and i wont rest until i build one into one of my guitar amp output stages. so just humor me =)

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Old 6th January 2004, 08:43 PM   #2
agent.5 is offline agent.5  United States
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servo circuit is at the bottom of the page.
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Old 6th January 2004, 08:59 PM   #3
SY is offline SY  United States
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That circuit will not work on a guitar amp unless it's a class A amp. I don't think that's likely.

It's possible to design a servo for an AB1 amp, but it's a lot more tricky. You either have to do a "inhibit and hold in the presence of signal" (like in the old Audionics BA150) or adapt a fancy rectifier circuit like in Chater's 40W solid state amp from Audio Amateur.
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Old 10th January 2004, 07:40 PM   #4
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Okay ... our forefathers already worked on the description you give.....an article appeared in (UK) Wireless World..... April 1976 pages 36-40, by S. Berglund; < transistor driver for valve amplifiers >. This was a servo for a triode config Wiliamson. It was a compound design using Siliconix fets and discretes.
A fragmented circuit dia is the only hardware bit I have. However, I believe UK wireless world ceased in 1983 or so...info could be hard...perhaps others know better.
I have no proof if this gismo ever worked. The awkward part is going to find someone or in website with info. Sorry I couldn't be more assist.

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Old 10th January 2004, 09:42 PM   #5
EC8010 is offline EC8010  United Kingdom
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Wireless World now calls itself Electronics World and is a pale shadow of its former self.

The design of a bias servo suitable for Class AB is discussed on pages 418 - 420 of "Valve Amplifiers" 3rd edition, but it's quite complex. Menno van der Veen also mentions a proprietary bias servo in "Transformers and Tubes".
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Old 2nd May 2013, 04:23 PM   #6
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Howdy all: don't know if anyone is still viewing this thread given that it's 10 years old but, I am working on a full OP Amp circuit that will handle Class AB operation as both a bias closed loop servo and a bias balance servo. My goal is that it set and maintain the idle bias and enforce that even while under dynamic conditions of making power.
On paper only for now, not ready for publication.

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Old 2nd May 2013, 04:36 PM   #7
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Hi Rene

great you are working on it! John Broskie and Morgan Jones published some schematics of fixed bias circuits for AB circuits but both are quite complex and therefore complex to put together on proto board (I tried it). Menno vd Veen and Guido Tent have a ready to go module, but quite expensive. What the world needs - that is a big expression coming from me - is someone offering a PCB for these servo circuits, so we can put them together ourselves using whatever material we have available (which is usually a lot!)

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Old 2nd May 2013, 04:43 PM   #8
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would a handful of commonly available opamps, a small bipolar (+/- 15) supply and a few small signal BJT, together with a few resistors and caps be considered "quite complex"? I've just described the extent of the circuit. On the other hand, I don't foresee any complex or critical adjustments.
Thanks f or the encouragement.
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Old 2nd May 2013, 05:19 PM   #9
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Hi Rene,
No, the 'complexity' lies more in taking care that all these components don't get mixed up when laying them down on a protoboard. It is doable to assemble MJ's or Broskie's fixed bias circuits on a protoboard, but nowadays I see PCB's for 'simple' circuits as power supplies (4 diodes, big caps, some bleeding resistors) so I imagine a PCB for a fixed bias would be quite doable and payable, and with these nice drawings (silkscreens ?) showing exactly where and how to assemble each component, failure ratings would drop considerably compared to protoboard.

I think that as tube/valve amplifiers fans we get quite spoiled by the relatively low number of (passive) components that go in the circuits and the resulting possibility of P2P wire everything!
my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
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Old 2nd May 2013, 06:38 PM   #10
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I had like 50 or 100 PCBs made of Broskie's circuit. I have not finished testing it yet, but I would be willing to send some out if someone is willing to give me feedback after building it.

It is mostly surface mount, though. It made the board way smaller and way easier to route. I could post a schematic when I get home.
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