Amp voicing

[Mr.Curwen]

I've been looking at a tube bass amp, the Selmer Bassmaster 50. Everything else is pretty much straightforward about it, but I'm a bit puzzled as to the voicing of the preamp.

The (half passive) tonestack has a significant signal drop on the bass frequencies, a mid notch and relatively very high treble content. This to me doesn't make any sense in a bass amp.

After all the preamp stages, there is a treble pot and a low pass filter to tame the highs again, and sure enough the amp sounds like a normal bass amp (I've played a vintage 60's Selmer Bassmaster 50 at a local shop, truly a sweet thump).

I understand that for example in the Express, you do this kind of thing (except the other way around, tonestack emphasises bass, later it's filtered down) to avoid blocking distortion by getting away with a small coupling cap between the second and third stage, but the Selmer way you don't get a smaller cap, and you don't have any distortion to begin with.

So why would anyone design a bass amp this way? I'm puzzled.

[Structo]

I'll take a stab at it.

I think even though the audio spectrum of a bass guitar is an octave below a six string guitar, there are still overtones that contribute to the tone of it.

If all you heard was the primary frequency of the bass notes, I think it would sound sterile and lifeless.

It appears to be a cathode follower tone stack, which I think is much less lossy as a plate loaded one.

The controls that appear later in the circuit are probably there for exactly the reason you thought of.

[jaysg]

Adam Grimm (SatelliteAmps at TGP) has number of Selmers. You might ask him about it. It's possible that an odd tonestack is used to compenstate for a cheap output transformer. dunno

[Ears]

Without getting into the particular cct psycho-acoustics also have a bearing on the issue.

The human ear and brain can "fill in" a lot of low end information, based on the acompanying overtones/spectrum, without much of the fundimental frequency being actually present. This allows for acceptable bass definition from a filter that one would intuitively think lacked sufficient information. Composers, arrangers and instrument builders have used this effect throughout the centuries whether knowingly or otherwise. Amp builders too.

I'm sure there is plenty of information on the web about this but haven't any links at hand.

[Mr.Curwen]

Structo, I absolutely agree that harmonics are very important for a bass guitar amp, but what's the point of 'boosting' the treble (or rather attenuating less but anyway) for those two stages after it, and then filtering it down again? The harmonic content that is emphasised with the tonestack is being attenuated back to normal anyway, never reaching the power section (even with that treble control at full, there's that 10nF to ground taming the highs pretty much away), so I'm guessing it's got something to do with what those two stages do, but I don't see anything special about those.

jaysg, thanks for the tip. As for the OT, the 'boosted' highs never reach it, because of the treble control and the 10nF to ground after it, so the tonestack couldn't help out the OT in that department.

Ears, totally agree on the psychoacoustics. However, the amp actually does sound like a balanced, 'normal' bass amp, because the hi-lo balance is restored after the last two preamp stages. My question is why is it first whacked out of balance (emphasis on highs in the tonestack), then after two more stages returned to balanced with a lo pass filter.

There must be something to this, hopefully somebody can figure this thing out!

[jjman]

Maybe it has Dolby Noise Reduction.

[John C]

I'll try to throw out some more comments, based only on the schematic posted here.

The intent of the attenuation of the bass at the very front, I believe, would be to lessen the build of harmonics in the bass frequencies. Rather than sounding "lifeless", the bass could then sound defined and clear. Allowing the upper mids and trebles to pass through that first stage would give them a chance to build up harmonics - harmonics that would still be present, in part, after attenuating some of the top-end in the final stage or pot. So the idea may be to have clean bass coupled with some harmonic build up of mids & trebs for a better sound than an "over-warm" or muddy tone - the result of too many harmonics in the bass.

Maybe someone who owns one of these amps would have something very different to say; my comments are only my immediate reaction to the info posted here.

[Mr.Curwen]

jjman, forgot all about Dolby! =D

John C, makes sense.


Apparently the Fender Bassman 6G6's have an almost identical solution, so I'm guessing Selmer borrowed that topology.

[VacuumVoodoo]

Try 250k Bass pot. I thinks 25k as in original schematic is a mistake.