Old newbie looking for help on 12ay7/EL84

[ValveMan]

This forum is awesome. Such a wealth of info & great advice. My engineering days are ~20 years in the past, and although I studied valves at school, (I'm originally from Scotland, where the word 'tube' = 'jerk' when you want to insult someone ;-) I'm very rusty, and therefore a little dangerous.

One of my friends plays mostly acoustic guitar (I'm mostly electric) and he badly wants a low power tube-amp (~5W) that'll stay clean even at max volume. He wants the warmth without the breakup/distortion. I know, some of you might think it's pretty dumb, but he knows what he wants.

My initial thinking is to keep it very simple (kinda like the GA-5 or Champ), but with tubes that give more headroom. For that reason, I'm thinking 12AY7 for the preamp then EL84 for the power, biased to keep them both in the linear range. (also using a 5Y3GT for power)

Trouble is, with all the info, schematics etc. I've found so far, I can't find anything with that particular combination, and even looking at circuits that use one of them, the gain's naturally set up to give a nice level of distortion at the right point (for electric guitar).

Can anyone point me in the right direction?
Thanks in advance!

[txstrat]

Welcome to the forum.
If I had to design a clean amp I would go for a standard blackface channel (due to the scooped mids). As you might know the mids are for the "dirty" sound (I love it, though).
It's no big deal to build an amp with this circuit. The power amp could be one or two EL84 (or 6V6 which I prefer). SE design might be to close to power amp saturation. If you choose the push pull design, I would go for fixed bias, since it keeps the amp in a cleaner range than cathode bias, although cathode bias sounds "warmer" IMHO.
Have a look at the princeton AA964 schematic. If you want it cleaner still, you could add a tube (or leave the vibrato out) and use a long tail PI instead of the split load.

Hope this helps

[Enzo]

Be aware that there is a fundamental difference between a guitar amp and a PA amp, which is pretty much what an acoustic guitar needs. And there is also a difference between frequency response and distortion. And amp can be clean, clear of distortion, yet not be flat in response at all.

A guitar amp is not made to REproduce cound, it is a part of the instrument and is intended to contribute to the sound. it is an original PROducer of sound. That is why someone "plays a MArshall," or "plays a Fender." They don;t sound alike. COmpare that to a PA system. PA systems are supposed to sound alike, they are supposed to faithfully reproduce the sound signal that is fed to them. generally, amplifying an acoustic guitar is an exercise in making it sound the same but louder.

"Acoustic amps" are basically little self contained PA systems with a few extra features useful to the guitarist - reverb or whatever.


That is not to say an acoustic player won;t be happy with a guitar amp, it is up to the individual what sounds best. But it might be a good idea to audition some existing amps to be sure.

[ValveMan]

Thanks Txstrat - good feedback. I'd an inkling a single power tube could be close to saturation, but in wanting to keep it as simple as possible I'd hoped to get away with single SE approach. I'll take a better look at the AA694 - could be with the vibrato out (& maybe a tone mod) . . .

Thanks to Enzo too. No argument from me. I've not performed serious surgery on an amp for a long time, but I am a long-time player, know what you're talking about and still want to take a crack at finding a sweetspot in between. My buddy's way more knowledgeable than myself on guitars & amps but has no engineering background.
It'll be a fun project, even if it doesn't work out perfectly.
Cheers

[tubeswell]

What about a 5F2A for 6L6 (with a 3k2 - 4k load OT) which could be pretty clean sounding, and also use a GZ34 with 60uF reservoir to get higher B+ (and more headroom in the pre-amp), and a 12Ay7 pre-amp tube?

[jimboyogi]

What about a 5F2A for 6L6 (with a 3k2 - 4k load OT) which could be pretty clean sounding, and also use a GZ34 with 60uF reservoir to get higher B+ (and more headroom in the pre-amp), and a 12Ay7 pre-amp tube?

Sounds like great advice.

For extra clean headroom, how about a solid state rectifier?

[ValveMan]

Thanks everyone - sounds like there's some good advice coming in. I'll not be able to weigh it all up until the weekend, but keep it coming

One more Q . . . . I'd like to have 2 inputs to allow for using either piezo pickups or mag. Assuming I use an arrangement where plugging into 1 cuts of the other, would using a load somewhere in the 2M - 3M range for the piezo be ok, or is it more complicated than that? I've read somewhere that piezo pick-ups need to see a larger input impedance unless you want cut off all the low-end.

[tubeswell]

Experiment with different input impedances and see. Assuming that its just a guitar pickup level signal, there is virtually no grid current to speak of in the input stage so I guess you could try anything between 1M and 3M3 and get away with it (on a 12A_7 input stage).