More tube amps!

Earlier I got a Valve Junior Combo tube amp, which I modded with a few quick changes and new tubes.  Somehow I got stuck with V1 of the amp and needed to add a DC rectifier, extra power filtering, and a reworked ground circuit; plus I got the transformer from V3 and that made a huge difference.  I also dropped in a fresh gold-pin matched/balanced JJ 12AX7 from Eurotubes, which gave me great sound and even further reduced hiss.  All in all, a great project.

I have my eye on the Epiphone Valve Junior Half Stack now.  This came out with V3 of the Valve Junior, so the head uses the proper OT and a DC rectifier on the filaments, as well as a more stable board layout.  The amp comes with a 1×12 speaker cabiner, giving better low frequency response (we call this a “fatter low end” because that totally makes sense right?).  I may need to get a THC Hot Plate to run it with good overdrive; the Valve Junior Combo I have uses an 8 inch speaker and I need ear plugs (Hearos Hi-Fi, worth the $15).

Of course I’ve got the soldering iron all heated up and ready for this too.  This time around I’ll give it a BitMo TRIO Mod to make it more similar to a modeling amp (like the Roland Cube).   This mod gives you tone and gain controls and a voicing switch to produce a huge range of sounds, from clean to blues to dirty overdrive.  I might add a bypass for the 3-way voicing switch, possibly by using a 4-way or 5-way switch, to have an Original setting.

I still want to do a couple mods of my own to the poor thing.  I may wire the output from the BitMo 3-way voice switch (post-bypass) to jumper to itself with a spade connector, or just twisted wires.  Later I can build a 3-band Fender/Vox/Marshall (FVM) tone stack for bass/mid/treble control, with its own bypass.  I could wire this so that yanking Bass bypasses the 3-way voice switch, and yanking Treble bypasses the tone stack itself.

I also want to see about adding a second 12AX7 tube to the circuit, with a bypass on Mid (in this case, pushing in the Mid dial would turn the second 12AX7 off).  This would act as a gain control, overdriving the pre-amp 12AX7 and then attenuating the now overdriven output for re-amplification.  The tube circuit drops between C1 and R19, but has a lot of grounds; a full double-pole double-throw switch (push-pull pot) would break both the input and output of the circuit and short them together, since a direct bypass will leave too many ground paths (like, more than zero).

I question if wiring the tubes to run in parallel with a divided signal would produce better results; that is, input signal to the center pin, and tube input from pins 1 and 3.  Turning to 0 would put 100% signal on Tube 0, while turning to 10 puts 100% signal on Tube 1.  This plus separate volume controls for each tube–each with a full bypass amounting to opening the input and output of each tube circuit–would allow me to drive signal more towards one tube or another and overdrive them separately.

It may seem silly to split the signal in such a way; each set of volume and bypass settings with the signal split below five has a set of settings above five that produces the same output.  This doesn’t, however, account for mis-matching tubes (a JJ and a Tung-Sol) or for sliding the overall tone from a blues-with-some-clean setting at 3 to a clean-with-overdrive setting at 8 by interesting volume settings.  These are things that make sense in parallel but not in series.

I will probably hold back on the tube mod and save it for yet another amp in the future.  The 3-band tone stack I want to go ahead with on this amp in the future though.  Definitely hitting up the BitMo TRIO mod on this one to start.


~ by John Moser on March 4, 2008.

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