I have a design decision to make - mini guitar amps - diyAudio
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Old 10th October 2012, 03:37 AM   #1
KFW is offline KFW  United States
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Default I have a design decision to make - mini guitar amps

The situation - I found a homemade tube-based intercom at a yard sale. 6AQ5, 12AX7, 6CG7, two nice wooden boxes, a handmade sheet copper chassis, and I've got a pile of transformers beyond the ones in the box.

The decision - without buying a bunch of parts, I can build either one amp or two amps from these parts. Two amps - a "classic" 12AX7/6AQ5 3-5 watter and a mini one-tube amp from the 6CG7? Or one amp, with the 6CG7 driving a fancier tone stack?

In favor of one amp, I don't know if the 6CG7 amp will be good for anything even if it does work, and I have a really cool tonestack idea (a quasi-parametric midrange that adjusts from bandpass through flat through a notch filter), that I've used successfully in other boxes, but never in a tube version.

In favor of two amps, if the little one DOES work, the bigger one is pretty much guaranteed to be great even without a fancy tonestack, and I have a second tube amp, not to mention a one-tube amp, which is unbearably cool. And I don't know how well the midrange control works for real, as it's running the tube really lean, and spice modeling isn't particularly trustworthy under these conditions.

Schematics? Bog-standard except for the midrange control, which looks like this...
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Old 10th October 2012, 06:55 AM   #2
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I definitely would favor making one amp. Then again, I favor complexity with provisions to bypass when you please. I like two tone stacks, one before any rich distortion is generated and another tone stack after. Even if one early tone "stack" is just a bright switch across the first volume pot or even across the input resistor network. I want to control putting pre-emphasis and post-de-emphasis on the guitar, but the distortion generated between the two tone controls only goes thru the post-correction. That gives some independent tone control of the guitar and independent tone control of the distortion. Usually mfgrs boost the treble, add some grit, then cut the treble back, so the guitar comes out pretty intact (treble boosted then cut) but the distortion is all nice rich subtle lower-order stuff with harmonics beating against each other and those bass and midrange beats emphasized but less of the worst irritating treble distortion, leaving most of the treble relatively cleaner and a greater percentage of clean guitar. Maximum interesting chords and leads, minimum irritation. But if you want really bright or really deep, or deeplyu scooped, two tone stacks can be additive instead and if you keep the gain manageable you can get really shimmery highs with a brightness that emphasizes the clean-sounding highs. If in doubt whether it's getting too complicated, you can always throw in a bypass switch or two and still have the simple amp. Or bypass the stack when you want more gain. Best of the clean and best of the dirty, extremely versatile. I confess, I just like lots of knobs and switches. I wish I could hear what you're into, what you've built. My opinion matters little to your journey, and I'd like to see where you've been. I've been out of this for years now, just getting back into it, and there's whole new genres of singel-ended amps for me to discover.

Last edited by cyclecamper; 10th October 2012 at 07:02 AM.
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Old 10th October 2012, 09:52 PM   #3
KFW is offline KFW  United States
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Join Date: Mar 2008
Default I like complexity too, but two or three tubes is sort of limiting...

The thing about the two-tube amp is that it either sounds good or doesn't - there's barely enough gain for a single tone stack, and many 3-5 watters don't even bother, just a tone control. But with the right speaker, there's no reason it shouldn't sound good, even if it's just one or two sounds.

Mostly what I'm interested is how a 6CG7 works at really low currents. Usually they're higher-power tubes, which is why I find the one-tube amp intriguing. With a 12AX7 I know I could get the midrange control working, but I don't want to buy another one for this project.

The big amp design, at this point, is 12AX7 gain/midrange 6CG7/volume/6CG7 gain/james bass & treble/12AX7 gain/6AQ5 output

The simpler version is 12AX7 gain/tone & volume/12AX7 gain/6AQ7 output. Without a LPB, Tube Screamer or something similar driving the input, I just don't see a need for a master volume in this amp. And of course, if I do that one, the simpler one is a single triode driving a triode power stage, no room for tone stack there either.

If this unit has enough gain to bother, a master volume could go on the 6AQ5 input, but I expect the 6CG7 gain to just compensate for the tone control loss. In addition, I expect that 90% of the amp's use will be clean or semi-distorted. And if I want more distortion, I can always overdrive the input.

I also had this idea of running DC through an unused transformer winding so as to use generic transformers for SE amps, but that's another discussion, and only relevant here if I build two amps, because it really opens up the potential supply of transformers for tiny (<1w) SE amps. It's also got me thinking of a new kind of distortion box that uses a saturable transformer as the distortion element, but that's REALLY another subject.
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