digital pre-amps with vacuum tubes 1

[jemmme]

a relatively new trend, voice and guitar pre-amps are digitally manufactured, but with the use of a vacuum tube in the circuit. i asked why to some musicians, they said it´s because tubes add a warmer sound. is there some foundation of truth to it and if yes how does it work? in other words, how can a mere switch alter sound?

 

[zeitghost]

Fashion?

You can fool some of the people all of the time?

For the same reason as people pay £1000 for an IEC mains lead (oxygen free copper, gold plated etc. etc.)?

Most likely it's because the valve generates nice 2nd harmonic distortion...

 

[BrianG]

Are you sure the pre-amps are actually digital? There is some truth in the preference for conventional analog tube amplifiers amongst some guitarists I know. This is because of the characteristic "soft" distortion tubes exhibit at high drive levels, whereas transistor amplifier stages clip the waveform when they run out of available voltage swing. This sounds much harsher, as the signal is bordering on sqaure waves, and is thus to be avoided - except when done intentionally as in the "fuzz box" special effect.

 

[MikeHalloran]

Could "digitally manufactured" mean "by fingers", i.e. not automated?



Mike Halloran
Pembroke Pines, FL, USA

 

[bklauba]

It all has to do with clipping, and the nature of the distortion that is produced when you clip. And BrianG is right to ask if the preamp is digital. The tube portion, which is the 1st and likely 2nd stage is analog; thereafter the processor is digital.

Tubes (and you gotta remember the crowd that really believes in these are a tad, should I say, "culty", whether the hi-fi crowd, or the guitar amp crowd) supposedly produce dominantly even-order harmonics, as opposed to odd-order harmonics, when you overdrive (clip) them. This makes for a smoother, controlled, more mellow (not "grainy" or harsh) sound quality. (I must confess, I have a McIntosh MA-230 at home, that I recently refurbish, now that the Russians are making 7591A's again, and it sounds SUPERB! Rated 30 watts a channel, it tests at 50+, and if I put out my old pair of EV 12TRX's during one of my cookouts, neighbors become pretty well acquainted with my musical tastes, which sorta runs towards non-commercial rock, blues, and whack (Zappa).

Typical tubes used for preamp, whether guitar or hi-fi, are the famous 12AX7, although you do see some 12AT7's around. Normally these are used in the classic "push-pull" arrangement, where the positive and negative portions of the waveform amplified by different stages of the tube.

After the preamp, where this overdrive can take place (or not depending on your settings, and the preamp's design: Blues guitar amps have less overdrive available), the digital processing takes place, which is where the "whah-whah" and 57,000 other settings are available.

You can get guitar preamp processors, for not a lot of dough (maybe 300-400 USD), with tube preamps in them, and then send this signal to a nice power amp (Crown, QSC, or whatever is available locally), which is the way it is often done, again, for less $$$. The all in one units are convenient, packaging-wise, but you gotta pay.

BK

 

[ScottyUK]

In my younger days I've repaired quite a few Marshall guitar amps for various bands I was acquainted with. The even order harmonics during overdrive are a major contributor to the audibly more pleasing characteristic of the valve amps at higher outputs. From an engineering perspective they were inferior to solid state in almost every technical specification, but really only 'came alive' once they were running into their overload region which is the last place you want a solid state amplifier to operate. The Marshall amps were THE rock band guitar amp of choice over here: ugly to look at but fantastic sound. Happy memories of a career path I didn't take (to the benefit of both my finances and my health).


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[OperaHouse]

I had to work on a tube/fet amp for one of my musician friends. Todays joke.....

What do you call a bass player without a girlfriend?

HOMELESS

Anyway, tube circuits have almost no feedback so when they do distort there aren't all those nasty odd harmonics. This 300W bass amp drove the fets through a capacitor from the tube circuit. The only feedback was a little op amp circuit that corrected the DC offset voltage at the speaker terminals. My repair was finding a set of tubes matched enough to clip evenly. These circuits are very sensitive to tube condition.

 

[bklauba]

OK, OperaHouse:

How do you get a guitar player off your front porch?

[Pay him for the pizza!]

I used this on one my sons, somewhat mercilessly, until he (finally) got a real job. He still plays very well, but I don't have to house or feed him anymore!

BK