Mesa/Boogie Question

Jon Light (deceased) · Post by **Jon Light (deceased)** » 16 Apr 2003 11:05 am

Looking at the specs and descriptions of 50/50 and 2:90 power amps, I'm seeing that besides the 6L6 power tubes, they have 3 12AX7 tubes. For what? These are not heads, they're power amps, right? So why the preamp tubes without the preamp? (Obviously I'm missing something here.)

Michael Brebes · Post by **Michael Brebes** » 17 Apr 2003 7:17 am

It uses one tube for each channel to drive the power tubes (phase inverter circuit), and uses one half of another tube for interfacing the input to that tube and adding needed gain from approximate line level. Hope that answers your question.

Jon Light (deceased) · Post by **Jon Light (deceased)** » 17 Apr 2003 10:12 am

Thanks Michael. I need far more of an education to fully get it but I'm ok for now---I simply always associated the big bottles with *power amp* and the little ones with *preamp*. Now I know there's more to it.

Gino Iorfida · Post by **Gino Iorfida** » 17 Apr 2003 11:05 am

Another way to look at it..
the 2 12ax7s as phase splitters (i.e. one per side), they are not 'amplifying' the signal, but rather splitting it, and sending a 180deg out of phase signal to oen side, while keeping the other side in phase, this is how a push-pull power amp works-- one side pushes, while the other pulls -- in essense it is a 'small buffer preamp, with inverting and non inverting outputs'

The other 12ax7 that is split (1 half per channel), is in essence a small preamp there to ensure that there is enough signal to drive the power amp. Granted you MAY not need it if the 'preamp' you connect has enough signal (most do), however some DO NOT, so they build in the 'compatibility'.... just like on a Fender Guitar amp -- the amps were designed to run clean, and the max power was rated at clipping, yet for most players of most guitars out there, the 'clipping' or max power is reached aroudn the halfway point of the volume knob... so what are 6-10 there for? if you have a weaker guitar signal, there is that 'little extra' built in to make sure you'll get the full rated power available to you (people would get a little testy if they had a 100watt amp, that they coudnt only get 50 watts out of