Amp with active pre-amp controls

MtB

So I saw a thread on here a while ago about how to find the amp's sweet spot. Is it the same procedure on an amp with active pre-amp controls - like a Sound City? 

timmysoft

There is no procedure, it's all internet waffle.

use your ears, they're fundamental in the hobby of guitar

ICBM

timmysoft said:

There is no procedure, it's all internet waffle.

use your ears, they're fundamental in the hobby of guitar

Yes, but it actually works to some extent. At minimum it will let you hear what the controls are really doing.

It will work just as well (or not, depending on who you believe) with active controls.

DJH83004

I'm sure you are right about ultimately using your ears but the Sound City family of active tone control amps such as the 50 Plus or the Lead 120 offer such a huge range of cut and boost compared to a standard passive tone stack, and they do influence the overall gain of the amp particularly mids and treble, plus they are somewhat interactive, all adds up to an uphill task if you are trying to dial in a particular sound, as I am sure MtB is finding. Interestingly Dave Reeves didn't adopt the design in his later Hiwatt designs, which may be tells its own story 

MtB

Hi Dave - this is exactly what I was thinking about. As I turn up the bass or the treble, so the overall gain and volume of the amp increases - so I guess it's harder to hear the sweet spot, as it just sounds like you're turning the volume up.

So from one Sound City source:

Each tone control in the Mark 4 "active" preamp, which was first available with the introduction of the 50 Plus, 120, and 200 Plus amplifiers in about 1973, acts as a sort of gain control for that tone control's specific range: i.e., Bass, Middle, Treble, and Presence. If all "active" tone controls are set to zero (0), no sound will be available from the amp even if the Volume control is set to 10.

According to the Mark 4 preamp's designer, the circuitry of the "active" preamp represents a "ladder circuit" where the signal from the Volume controls is "fed" to each tone control (i.e., each rung) in the ladder, and each tone control is essentially a gain control driven by its own ECC83 tube. Although scorned by some, this feature of the "active" preamp explains why the Sound City Mark 4 amps are very loud amps indeed. 

DJH83004

Yes Mike the description is essentially correct there are a series of RC filters, which are tapped of at different points and fed into the grids of each half of the triodes, so cutting / boosting those selected frequencies. The presence control is actually conventional, feeding back from the o/p tx to the PI.
Appreciate that doesn't help you much, but at least you know how it works!