Some year ego I've build SE EL84 guitar amp I never use. I had an idea of putting 8ohm resistor on output trafo and just direct line out to use it as mic preamp. But I dont' see it done so before I start messing about I thought I'll ask opinion on that. What cons or even pros it might have? I thought it might give me nice saturation without too much distortion which is what I'm after and not getting enough of that from just regular triode preamps (apart from the distortion when driven hard).
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Do you want a mic preamp for real, or do you want a harp amp? Usually when we discuss mic pres, it is in the context of good clean signal transfer. Stepping up the relatively weak mic signal to a stronger level, but without coloration.
But now you speak of overdriving it and distortion, which leads me to think you might be chewing on a harmonica with a bullet mic cupped in your hands. Guitar amps are not designed to be flat or hifi, so if that is your goal, don;t use a guitar amp. But if your goal is to go for an overdriven distorted tone like most harp players use, then why not try it. Although part of the sound of a good harp amp is the speaker's very important contribution.
An 8 ohm resistor is not expensive, and a couple resistors to knock the signal level from that down to something a mixer can use, also cheap.
But now you speak of overdriving it and distortion, which leads me to think you might be chewing on a harmonica with a bullet mic cupped in your hands. Guitar amps are not designed to be flat or hifi, so if that is your goal, don;t use a guitar amp. But if your goal is to go for an overdriven distorted tone like most harp players use, then why not try it. Although part of the sound of a good harp amp is the speaker's very important contribution.
An 8 ohm resistor is not expensive, and a couple resistors to knock the signal level from that down to something a mixer can use, also cheap.
I couldn't leave it alone so have tried it. It's not for harmonica, I just wanted to see how EL84 and it's output trafo will sound as a mic preamp. It actually works quiet well, in fact I'm getting about 5x the gain which is great but also 5x the noise which is not, mostly hiss tho.
I do get a nice saturated tone, not audibly distorted (there is a difference between those two). But the overall sound quality is not as good, could be the output trafo or the tone of the EL84 I don't know. I think a mic preamp using pentode and quality output trafo would be very handy in a studio but just a basic guitar amp is not sonically good enough. So in case you wonder what's it like, I tried it.
I do get a nice saturated tone, not audibly distorted (there is a difference between those two). But the overall sound quality is not as good, could be the output trafo or the tone of the EL84 I don't know. I think a mic preamp using pentode and quality output trafo would be very handy in a studio but just a basic guitar amp is not sonically good enough. So in case you wonder what's it like, I tried it.
I remember reading an old WW2 era Argentine electronics book, whern NO new parts were available, because half the World (Europe/Japan) was "the enemy", and Allied nations used 95% or their tube production for Military needs, and barely suppying their own civilian market, so full recycling was the way to go.
So this book clearly stated that *any* tube available "except a diode/rectifier" could be used as an audio gain stage in a pinch.
FWIW even in modern days a few designers use "TV tubes" to build Guitar amplifiers.
Here´s a friend from Rio de Janeiro who makes 6EM5 and 6BQ7 based amps:
https://www.youtube.com/watch?v=5suXvD7zFoM
As smooth and tasty sounding as amps based on "guitar tubes" , go figure.
So this book clearly stated that *any* tube available "except a diode/rectifier" could be used as an audio gain stage in a pinch.
FWIW even in modern days a few designers use "TV tubes" to build Guitar amplifiers.
Here´s a friend from Rio de Janeiro who makes 6EM5 and 6BQ7 based amps:
https://www.youtube.com/watch?v=5suXvD7zFoM
As smooth and tasty sounding as amps based on "guitar tubes" , go figure.
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