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Series vs Parallel tubes...

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Looking over more "Adding a 12AX7 tube" stuff and finally starting to get it. I had some interesting ideas; see my blog around paragraph 5-7 on this post:




To summarize, I've noticed that adding a second 12AX7 involves dropping the new tube circuit in in series with the old 12AX7 to EL84 path. Effectively, you cut one wire (trace) and plug one end into the Input and the other end into the Output and then jack Power into the tube and you have a fresh 12AX7. You can totally bypass this by breaking Input and Output and shorting the original trace back together, so you can switch-control this new tube.


In this design, the first 12AX7 (Tube 0) acts like "gain" on modeling amps (which just adds an overdrive effect). Overdrive it, pass it to the Tube 1 circuit, and have that immediately attenuate the signal. Now you have blues or overdrive going into Tube 1 to get amplified (as I understand, the EL84 doesn't overdrive without... effort...).


In theory (this is a really bad hypothesis based on no EE background), if you split the signal across two tubes, amplified both to half volume, and ran the signal together, you'd get the same thing as when you do this with two pickups (or three, or five); the sound is the same and the same volume as original. If you split the signal across two tubes, amp it to full clean volume, and run it out, you get double clean volume. This is interesting, of course: more volume, same headroom. Further, if you break their input and output of either circuit, your signal goes to only one, and at full strength.


Now the nasty parts. First, the 12AX7 has a volume control pot (should be trapezoidal/logarithmic?) after its first amplification stage (look at the diagram, signal goes to tube first); each tube thus gets its own volume control. This means, yes, you can overdrive Tube 0 and play blues or clean on Tube 1, or vice verse. This of course means some very interesting sounds could come out. It also means our bypass mechanism for either tube is the push-pull on the volume pot (all pots have push-pull DPDTs in theoretical discussions).


Next, let's add a voltage divider between the tubes. Input -> Pot.Pin2, Pot.Pin1 -> Tube0, Pot.Pin3 -> Tube1. Now you can not only control individual volume, but also input signal level distribution. This means if half the signal won't overdrive Tube 0, you can divert 75% signal to Tube 0 and then max out Tube 1 to get that sweet blues tone. This is good because you might not actually be able to get past a sweet bluesy overdrive on 50% input signal (my amp overdrives at less than that though).


Good idea, bad idea, any thoughts at all? Am I better off building my own amp from scratch for this one?

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Sounds like you need to root around in the HO and SEL projects at AX84. They've got some very good, tried and true preamp and power amp ideas over there. http://www.ax84.com/


Check out the October, too. It's got a neat variation on the parallel input stage of the crunch preamp. There's also a cascade mod for that by Zaphod_Phil over at 18watt. Parallel or cascaded stages by the flip of a switch. Very cool.



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