Ah. I missed that. Don't have much familiarity with European valve designations.
Only one way to find out if you're happy with it, particularly when it comes to "tone", aka, 2nd harmonic content. 🙂
But the very low rp (6k - 7k) of the 12AU7 might be a slight advantage here - it can very easily drive two paralleled pots (for your two channels) without any significant loading, because of the very low output resistance (compared to a 12AX7 triode, with 60k rp).
I have a similar low rp triode as V1 in my little DIY amp (it's half of a 6JW8 valve). It works okay, but the tone is very, very clean. Not a problem, I made sure that subsequent stages in the clean channel generated the tone I was looking for.
I was after the same thing, and I found that it took a lot more "uncleanness" to achieve this fattened-up tone at low volume.
I think it has to do with the way the human ear responds to quiet sounds (Fletcher-Munson curves). We're less sensitive to treble at low volume, so we don't hear a tiny sprinkling of harmonic distortion in the mix. So the amp sounds sterile-clean unless there is a significant amount of harmonic distortion in the mix.
Because gain is cumulative as you go through the amp, normally the last stage is the first to distort, and the preceding stages are quite clean when this happens.
This doesn't seem to produce very rich-sounding distortion, though, more of a Jimi Hendrix fuzz-box nastiness.
The way around this dilemma is to insert voltage dividers between stages. These will throw away some gain, but also allow preceding stages to distort to some degree without overdriving the heck out of the subsequent stage(s).
Leonidas Fender, the penny-pinching former accountant, would never have done this (gain costs money, so you can't throw it away, right?). But if you look at some newer and higher-gain valve guitar amp schematics, these interstage voltage dividers are very common.
The thing is, clean amps don't need FX loops! Because the (clean) preamp doesn't distort, the sound of FX pedals at the input will come through unscathed. So I kinda agree with AquaTarkus, the simplest place for FX pedals in a clean amp is at the input, and there is no real downside to having them there.
Things are different with a "dirty" preamp. If you put FX pedals at the input, the sound of the pedals is distorted and modified (and maybe wiped out) by the preamp. So it's better to have an FX loop, to get the distorted signal from a "dirty" preamp to go through your FX pedals, rather than the other way around
I agree with AquaTarkus that it's turning into a more complex design, but I also agree with the reasons you want to do it this way, and I like your plan to build it in a rather modular way, one piece at a time.
I did that with my one and only entirely original guitar amp build. I started by building just the power amp. I used a Danelectro Fish-n-Chips graphic EQ/clean boost pedal as the preamp at first, and a $5 thrift-store loudspeaker as the "cab". So I could already play music through my newborn amp, even at that very early stage of construction.
Later, as time permitted, I added one clean preamp input stage, to replace the Fish-n-Chips. Then I added tone controls and a gain make-up stage. And so on.
At every stage, I had an amp I could actually make music with. That kept me motivated and moving forward.
-Gnobuddy
I agree here, and I also don't find 12AU7s very useful in guitar preamp stages. They do make great power tube drivers.
In theory that's true. In practice I have found that it's perfectly fine to place delay and modulation effects at the front of a distorting guitar amp (assuming its reasonably pedal friendly in the first place). The Marshall 18W is a really good example, as almost all its distortion takes place in the power tubes, and I've never encountered any issues running delay or chorus pedals straight into the input jack.
+1
Sent from my phone with Tapatalk. Please excuse any typpos.
Personally, I hugely prefer delay / chorus / modulation after distortion, rather than before. I guess it's just a matter of personal preference, then.
-Gnobuddy
For a "clean amp", 12AU7 stages are pretty awesome. To make them clip or break up substantially running off a guitar using only 2 stages of gain, the second stage has to be biased cold. Either that or reduced power supply voltage. At the typical (250 - 300 VDC) supply voltage, they have too much headroom to distort much. They also make good tone stack, output tube, and even output transformer drivers. For such a little package, one bottle wired in push-pull can make an efficient cabinet growl and scream above "comfortable" bedroom listening levels.
With "high-gain" amp channels, I prefer to use a 12AX7 (2 stages) or some sort of pentode (single stage) at the front end. For eq drivers, fx loop buffers and phase inverters further downstream, the AU bottles work just fine. Be warned, however, that the AU tubes are all over the place as far as amp factor and tonal character compared to the AX models. Rolling a few different tubes through a certain socket is very productive.
With "high-gain" amp channels, I prefer to use a 12AX7 (2 stages) or some sort of pentode (single stage) at the front end. For eq drivers, fx loop buffers and phase inverters further downstream, the AU bottles work just fine. Be warned, however, that the AU tubes are all over the place as far as amp factor and tonal character compared to the AX models. Rolling a few different tubes through a certain socket is very productive.
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As far as that goes - I was just thinking that I might want to run an fx through one amp but not through the other. Impossible to do that with just one input jack. But it's not a vital question so I'm gonna forget about it for now.
@jeff5may - sadly most of what I found was Sylvania 12au7s - I think there was something like 15 of them. I only found 2 old 12ax7s and one of them was clearly well used. I did come up with a pair of 12bh7s and (I think) maybe a 5751? No 12ay7 or 12at7s.
Still - I didn't do too bad. And the gem of the find (from my perspective) was the pair of GE 6L6s, so I guess I'm complaining more than I have a right to.
@jeff5may - sadly most of what I found was Sylvania 12au7s - I think there was something like 15 of them. I only found 2 old 12ax7s and one of them was clearly well used. I did come up with a pair of 12bh7s and (I think) maybe a 5751? No 12ay7 or 12at7s.
Still - I didn't do too bad. And the gem of the find (from my perspective) was the pair of GE 6L6s, so I guess I'm complaining more than I have a right to.
You are in the rarest of rare categories: electric guitar players who understand the awesome tone-shaping prowess you get from having an EQ pedal or two in the chain!
It still amazes me that people will go out and spend another $150 or more on yet another minor variation on a Big Muff - when the only difference is usually a minor change in the built-in EQ in the pedal.
The pedal I use most consistently is a Danelectro Fish-n-Chips graphic EQ / clean booster. I bought a second one to use exactly the way you do, but the day after the second Fish-n-Chips arrived in the mail, someone knocked my first one off a shelf, onto a granite floor, where it sustained enough damage to kill it. 😡
I use that Fish-n-Chips with my electro-acoustic guitar, too, to dial out that bright piezo sound, and replace it with a much warmer timbre.
-Gnobuddy
Don't get me wrong. I'm not against this approach. All I was saying is that it's not absolutely essential to use an FX loop, and in the case of many vintage amps, not very practical. IMO both approaches have their relative benefits.
Yes, the 12AU7 is a really versatile tube. One thing about bias points, since you mentioned it, many people don't understand that 12AU7s bias very differently to 12AX7s. As many people like to experiment with placing a 12AU7 in a 12AX7 socket, they will often not realise is that their 12AU7 will be biased very warm with a 12AX7's typical cathode resistor value.
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The PTs arrived sooner than expected so I started assembling the cabinet. As you can see - there's a left and right set of controls. You can also see my little full-range speakers mounted. It's not lightweight at all - Im thinking around 20 lbs-ish? But it's not obnoxiously heavy either - considering how much stuff I'm going to be putting in it.
Did you build that cabinet? Nice work, if you did! It reminds me of some jewelry cabinets I've seen. A very unique look for a guitar amp!
Yeah, unfortunately, "light weight" and "valve guitar amp" are rarely heard together in the same sentence. 🙂
I amplified a signal today but it sounds terrible.
I think maybe I'm having some fairly significant lead dress problems. I can hear it well enough to say that all parts of the circuit are probably working, but I'm afraid that I've got way too much gain and way too much noise. It only sounds like a guitar amp at extremely attenuated signal levels.
I know you guys would probably be happy to offer some advice but I think I need to draw up my actual schematic first, and measure some voltages. I'm just happy to be this far along.
I think maybe I'm having some fairly significant lead dress problems. I can hear it well enough to say that all parts of the circuit are probably working, but I'm afraid that I've got way too much gain and way too much noise. It only sounds like a guitar amp at extremely attenuated signal levels.
I know you guys would probably be happy to offer some advice but I think I need to draw up my actual schematic first, and measure some voltages. I'm just happy to be this far along.
Agree, and congratulations on making the beast live!
Don't forget, you can lift one end of a coupling cap and remove preceding gain stages from the chain - then tweak what's left to sound good, before re-adding the preceding stage by putting the capacitor back.
Rinse and repeat, working from the output back towards the input. By the time you get back to the input, the whole amp should sound good, if everything went as planned.
Also, don't forget, you may need to attenuate the signal between stages to produce the tone you want, without having too much unwanted gain. You can temporarily insert pots between gain stages during testing. Once you find the right amount of attenuation, replace the pot with two fixed resistors.
In my limited experience, you are now at the most time-consuming part of the DIY valve guitar amp process: starting to tweak a design that makes noises, but doesn't sound good (yet)!
-Gnobuddy
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