Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on September 05, 2015, 12:36:00 pm
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Looking for a schematic example of a Fixed bias SE amp. I was wandering the net and read an article that said something like "can be either self biased or fixed" and it was in reference to a SE amp.
thanks
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They are probably scarce. Here's one...
http://ejjamps.com/PDF/6V6SE-stereo-amplifier-project.pdf (http://ejjamps.com/PDF/6V6SE-stereo-amplifier-project.pdf)
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They are probably scarce
they are, fair amount of reading ,pros n cons, but no real schematics, thanks
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They are probably scarce
they are, fair amount of reading ,pros n cons, but no real schematics, thanks
take a cathode bias SE plan and note tbe cathode voltage. use the same. voltage for bias voltage and ground the cathodes. you may have to tweak the voltage up a couple of volts ( more negative) to keep plate dissipation around 90% of max.
--pete
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Why they are scarce:
A single-ended output stage must also be class A. Theoretically, the average current through a class A stage is equal to its idle current (distortion can cause the max signal average current to be higher than the idle current, but it won't be a large percentage higher). If average current (during driven conditions) equals idle current, a resistor can be inserted in that current path to create a constant voltage drop, which can then be used to create a steady bias voltage. The cost is 1 resistor. A fixed-bias supply could also be used to provide a steady bias voltage, but at the expense of many more parts.
And a fixed-bias supply lacks the inherent self-limiting feature of a cathode resistor: If tube current tends to runaway and become excessive, the resistor will drop more voltage, increasing the bias voltage, which reduces the tube current. So a cathode resistor will keep your tube from burning itself or a transformer up, while a fixed bias supply has no such limiting feature (without adding yet more parts), so a simple fixed-bias supply will cheerfully let you tube take out a transformer.
So the cheapest, simplest solution of a self-bias cathode resistor is also the best option unless there are additional functions the current-limiting fixed bias supply must accomplish (you might see that in some very modern circuits, but then again they tend to go for more output power and less distortion, so use something other than single-ended operation).
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> Why they are scarce: ... ... ... ... a self-bias cathode resistor is ...the best option
+1
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self-bias cathode resistor is also the best option
totally agree, honestly never thought there would be ANY, but then I read an article, which peaked my curious George side, which lead to this post, which produced a schematic!!! I like cyclic reasoning :l2:
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IRRC there's a bunch of schematics for SE amps with fixed bias over at AX84.com, although its been a few years since I had a look there.
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Thanks, I did get some pros/cons reading threads at ax84.
I parsed out the schematic and added r1 r2.
Do R1 R2 act as a split load plate R for creating the 115vdc for v1B?
Does R1 change the 6V6 operation?
Thanks Sluckey, it's a nice schematic, practical might be in the eye of the beholder.
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R1 is negative feedback. R2 is the plate load. There is a detailed circuit description in the pdf I linked.
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Thanks, reread it, found it, now I gotta go read up on DC NFB