ruby amp problem, no distortion

[Ghost Planet]

I built a ruby amp following the per board layout on ROG. It works, but it only gives a clean signal. The gain pot only adds more volume. While its a nice clean sound, I would like some distortion. Any ideas?

[GibsonGM]

Hi....first, you should always post a link to where the schematic is so people can jump over to see what you're working on  ;o)

The Ruby is a pretty clean 'micro-amp'...you might want to build something else that is dirtier if you're looking for distortion...maybe the Little Gem?     Or you could use a pedal in front of it, or BUILD a clipping "preamp" before the 386 amp....You're not likely to get great distortion from the 386, there are too many limits involved.  But you CAN make it sound better.

From the ROG Ruby description: "Ruby has many shades of sparkly clean all the way up to a great, natural overdrive. The input buffer helps to retain treble detail going into the 386 chip. As you increase the Volume, you will start getting nice breakup. Where the Little Gem is similar to a "master volume" amplifier, Ruby is more like the "non-master" volume amps. If the Little Gem can be considered Marshall-ish, Ruby could be called Fender-ish or like a JTM 45."

[Thecomedian]

it could be an issue of current gain with the fet. afaik, BJT and FET and even tube/valve are similar in that taking signal from the side near to the voltage, whether it's positive and negative is ground/reference, or the voltage is negative and positive is ground/reference, is for moderate voltage/current gain, while buffering via emitter/cathode/source follower which is the BJT/FET/Tube terminal near ground does not provide voltage gain but still provides current gain, and at a transformed impedance level from high to low.

If the mpf102 has a low value of this gain (is it it's transconductance value, gm?), then it would have low voltage gain on the drain side or current gain on the source side, I believe.

It's a bit of a quandary, fets and tubes are similar devices in terms of voltage controlling current, which leads to amplification, yet tubes are always rated for their gains in hFE but FETs never are, even though they cause gain. I don't understand that reasoning.

[psychedelicfish]

You're not likely to get great distortion from the 386, there are too many limits involved. 

There are loads of great sounding distortion circuits out there using a 386 for the distortion, so I disagree.

It's a bit of a quandary, fets and tubes are similar devices in terms of voltage controlling current, which leads to amplification, yet tubes are always rated for their gains in hFE but FETs never are, even though they cause gain. I don't understand that reasoning.

My guess would be that it's something to do with the wild variability of FETs. Try getting the max and min IDSS and VGS_off values for any common JFET and stick them in to the Fetzer Valve calculator, you'll find that you'll come out with a huge range of gains.

[GibsonGM]

"There are loads of great sounding distortion circuits out there using a 386 for the distortion, so I disagree."

Not the one he built, Psyche...ROG even tells you it'll do a little breakup/OD if pushed, but you're going to be on the Fender side of things, if we're using amps as a 'dirt reference'.   IMO, when you crank a 386 a little too much, it just goes right to a strange fuzz/phase sort of nastiness.  Yes, it can be done - if you WANT to.

You might improve things by looking at the FET, sure.  If you're not expecting to get a JCM800 out of the thing ;o)