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fragman56 20th December 2003 10:02 PM

Push pull class A bias question
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My first push pull attempt; will this work? I am most concerned about the driver tube biasing. Not looking for the highest quality amp--perhaps for a guitar.


fragman56 20th December 2003 10:19 PM

2 Attachment(s)
trying again to attach

Sch3mat1c 20th December 2003 10:31 PM

First of all: driver will *NOT* bias. Grid has to be allowed to float, so put a cap between VR and V1 grid.

Two. Grid needs to be able to move for amplification to occur. Put a resistor between the grid and the R1-R2 node, rather than a jumper.

Three. You'll need at least 8 volts drive for full signal. More if VR isn't at max. You'll need at least one pentode amplification stage, or two cascaded stages to run from an unamplified guitar.

Four. If you run in pentode mode, you'll get more than twice as much power, which corresponds to as much more volume. Unless you want a practice amp, this will surely be important for guitar.


SY 21st December 2003 12:40 AM

Why Class A for an electric guitar amp in the first place?

Also, the grid of the input tube has a problem- it's going to want to sit at roughly 130VDC. That means you'll have to isolate it from the pot by a capacitor. I'd junk this design and start again. Use a simple diff amp in the first stage.

fdegrove 21st December 2003 12:43 AM



Not looking for the highest quality amp--perhaps for a guitar.
Either way that circuit won't fly as Tim pointed out.


fragman56 21st December 2003 04:49 AM

Thanks for the comments!

Let's forget the guitar amp idea for a moment. Let's say the input is from CDP which I guess has a cap output? Still need a cap between VR and V1?

Also, if the input is floated with this cap, won't the grid be slightly negative with respect to the cathode? Say, 130v compared with 132v on the cathode? Then, with an AC signal of 2 vrms...still won't work?

Won't this driver have plenty of gain for 8 volt drive to the output? Perhaps too much; like 20v or more to each output grid given a 5v pk-pk?

I would like to get a push pull, class A, 12ax7 with 6bq5 triode to work even if it won't sound very good. I would like to get the split load inverter design to work, too. I thought the 12ax7 would have plenty of gain even using the concertina, class A is simple, triode has less distortion and CDP has plenty of voltage.

I know this is not a good driver tube but would like to hear it...if possible.


fdegrove 21st December 2003 05:09 AM



Let's forget the guitar amp idea for a moment. Let's say the input is from CDP which I guess has a cap output? Still need a cap between VR and V1?
Two caps in series do what? C1+C2=?

No, you don't need an extra cap...If the source is cap coupled, you don't want a second cap.

The rest of the question isn't too clear to me, so I'd leave it to someone else to answer....

Cheers, ;)

Sch3mat1c 21st December 2003 05:21 AM

Indeed the cathode will be floating at a fraction of B+. Ideally, for maximum output and minimum distortion using this stage, it should be biased about 1/2B+ across the tube, which leaves 1/4B+ and 3/4B+ (measured vs. ground) for the cathode and plate resepectively.
132 is... uhm what's B+, 250V? Then unless there's a current somewhere else, there is -14V across the tube... wtf! :cannotbe: Clearly this voltage is impossible with this type of stage.

BTW, R1 will have to be larger, more like 1k. R2 should be around 100k for a 12AX7.

If the AC signal were applied between grid and cathode, yes, it would have enough gain. But it is very difficult to apply it this way, since the signal is ground referenced. So the signal as the tube sees it is stacked on top of the voltage which appears at the tube's cathode... think about it a while and you'll see.

Fortunately, a 12AX7 (actually, a 12AU7 would have more suitable gain levels for this project) has two triodes so you can tack on an amplification stage.


SHiFTY 21st December 2003 08:24 AM

How about this one? A nice simple design, should sound pretty decent

Jax 21st December 2003 10:28 AM

One 12AX7 and a pair of 6BQ5? Made me think for a while and came up with an idea using the old paraphase splitter.


It's neither tested nor simulated, just designed it a few minutes ago. It may need tweaking. The R2-R3 junction should have around 200V. Cathode bias is individual to lessen the matching requirement. Adjust R18 for AC balance between the halves.

No feedback. Some of the distortion from V1A will propagate through V1B. Gain is highish.

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