Tone Stack Location

[tyru007]

What is the reasoning behind locating the tone stack after the first preamp versus between the preamp and the power tube?

I'm buidling a simple SE with two preamp stages uing 12ax7 to a SE 6V6 tube with TMB tone stack (typical fender). Several schematics have the tone stack located after the first preamp (princeton) vs between the preamp and the power tube (AX84P1 extreme) with a gain knob.

Advantages/Disadvantages to each?

[Don Moose]

The short answer is that a tone stack, unless dimed, represents a pretty heavy loss of signal ... 20dB, IIRC, and you want a gain stage after that to pump it back up.

Given the insertion loss, and that output tubes need a pretty stiff signal to get them going, the last place you want to stick the stack is between the output's grid and whatever's driving it.

You can overcome that by having a ton of gain before the stack so that the net gain is the same - 3 - 2 + 1 == 3 + 1 - 2, after all - but that means you're really slamming the stage just before the stack.

The long answer is: try it. You may prefer it, and it won't actually hurt anything as long as the stack is AC coupled at both input and output (and the output tube's bias connection goes to its grid - not before/into the stack).

Hope this helps!

[chipaudette]

I've not looked at too many SE amps, but there are tons of PP amps out there that have the tone stack at the end of the preamp and just before the phase inverter. The famous 59 bassman does this and all the famous Marshalls (which are based on the bassman) do this.

For these amps, they still have the problem that Don Moose speaks off...massive insertion loss caused by the tone stack. Tone stacks just kill the signal level. Marshalls take care of this problem by: (1) tweaking the tone stack values so that there is less insertion loss and (2) preceeding the tone stack by a cathode follower, which can supply more current to the tone stack, which further reduces the insertion (voltage) loss.

As a result of the lower overall insertion loss, Marshalls can get by putting the tone stack last and not needing a "make up" gain stage after the tone stack. Seems like a good thing, right? Saved half a pre-amp tube by not having the make-up gain. But, note that they had to add that cathode follower stage. The cathode follower uses half a pre-amp tube. Nothing saved.

So, you don't get something for nothing...you either have to use the gain stage before the tone stack either as gain (Don's suggestion) or as a cathode follower (Marshall), or use it after the tone stack as make-up gain (most Fenders).

Chip

[Gtr_tech]

I think its more application specific. If all you want is a clean tone, or pwr amp overdrive then it won't really matter much where you put the tone stack as long as the loss is accounted for. If you have a cascaded preamp, then things get hairy in a hurry. When the TS is after the first stage, all you can get rid of is harmonics generated in that stage, and thats usually not a lot. Anything that gets generated after that is there for good. If you put the TS at the end of the gain stages then you can have more control of what the final result is. Since cascaded preamps generate alot of harmonics (high end trash) I prefer to have the TS at the end of all this. I don't like a sizzly high end...sounds like pissing on tin foil to me. I attenuate alot of highs to get a more midrangey tone that cuts better.

Anyone who knows about analog synths can relate to this theory. Dial up a sawtooth wave or square wave (lots of harmonics) on a synth with the filter wide open. Pretty obnoxious eh? With no resonance on the filter, sweep it down. Gets more listenable as you bring it down until the point that its starts to sound dead.

The whole point is with a cascaded pre you get more than what you start with and there must be a way of having control over it to get the desired end result.