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Re: Parallel triodes

3/11/1999 8:34 AM
eth Re: Parallel triodes
One extra comment about the parallel tube/starved plate thing. Parallel tubes also have more current through them. Stages with more current are often described as "smoother, rounder..etc"(Kevin O'conner in TUT#1 for ex.) I would verify this to be my liking. I have also found that running one stage with low current(non-parallel/higher Rk) early in the chain to give a nice defining edge to the sound. In Kevin's book there is a High voltage-voltage regulator (mosfet) that takes very few parts. The one I built uses a 1M pot as a VD and varies the voltage between 90-330v. Clean is fixed at 320v. Yeha!-Eth
3/11/1999 9:57 AM

A differential or balanced configuration can cancel hum if everything is really balanced and matched. But it would also cancel even order harmonic generation (distortion), and I don't know if this would be a nice thing in a guitar amp...  
Perhaps you would have to introduce an "unbalanced" stage somewhere in the signal chain just to be able to generate some assymetrical distortion...  
3/11/1999 4:24 PM
ken gilbert

GFR said:  
Perhaps you would have to introduce an "unbalanced" stage somewhere in the signal chain just to be able to generate some assymetrical distortion...  
You hit it right on the head... If you go fully balanced you're going to cancel all even harmonic distortion. What's left will be all odd, and that may not be the most euphonic guitar sound!  
Perhaps throw a couple of 12DW7's in there to skew things deliberately. This is assuming you're using the same twin triode tube for both paraphase lines. If you're not, then you could use different tubes entirely...  
Interesting concept though. It COULD give you great control over the distortion.  
3/12/1999 1:58 AM
Good points ! Killing 2nd order harmonics would be the last thing I'd want to do. Interesting thought k.g about the "balanced-but-unmatched" topology. Guess it's that kind of maverick attitude that can bring about breakthroughs in sound.
3/12/1999 4:36 AM

You can put a pot at the cathodes of the triodes in a diff pair, and have "ajustable" matching. Ends of the pot to the cathodes, wiper to the shared cathode resistor (or current source).  
3/15/1999 7:44 PM
Steve A.

... running two 12AX7 triode stages in parallel ?  
    Bruce Collins of Mission Amps has suggested tuning each of the parallel stages differently for a fuller sound. Like using an Rk/Ck of 1k5/22uF for one stage and 2k7/0.68uF for the other (to use the classic choices in vintage Fender and Marshall amps). A less extreme example like 1k5/4.7uF and 2k2/1.0uF might produce more musical results. Throw in a 3 position switch (one, the other or both) and you'd get a nice selection of tonalities from a single preamp channel. (BTW for the smaller Ck caps, a poly cap sounds a lot better than the cheaper electrolytics.)  
Steve Ahola
3/15/1999 8:26 PM
Great idea Steve!  
I definitely will have a crack at that. Then the amp would kind of be like a guitar with a 3-pole selector switch.

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