The 5E3 Tone Stack

[Platefire]

Could someone explain to me how the 5E3 tone stack works? I know the bright channel works like a the normal 5F2 Princeton by sending the signal through the 500pf cap with the amount fed back into the main signal grid according to your adjustment of the tone control. I think the lows are also shunted to ground trough the .0047 cap according to tone adjustment. If I got it all wrong please correct me.

I'm a little fuzzy on how the tone control effects the normal channel. I can tell the tone control has some effect on increasing treble to some degree on this channel but not sure what going on here. I would like to understand it well enough that I could expain it to someone else. Thanks! Platefire

[sluckey]

Here's the practical way I think of that circuit. Refer to the pic...

Turn the wiper down and you have a .005 cap tied directly to the grid, shunting the signal to ground. That cap has more shunting effect on the highs and mids than the lows, so the signal that reaches the grid will be mostly lows and sound muddy or boomy.

Now turn the wiper up to the top. You now have a .0005 (500pF) connected to the volume pot just like a bright cap in an AB763 amp. High signals pass thru the 500pF going around the volume pot without being attenuated and appear at the grid. Note, the 500pF bright cap has no effect if the volume is maxed. Also, the 1M resistance of the tone pot minimizes the effects of that .005 shunt cap. So, you will have a lot of highs reaching the grid.

Now you have a muddy tone at the bottom end of the pot and you have a brassy tone at the top end of the pot, and the pot allows you to vary anywhere in between those two extremes.

Since the normal channel ties to the same grid as the bright channel, that .005 shunt cap has the same effect on either channel, but the normal channel won't really feel the effects of the 500pF (bright) cap.

[tubeswell]

And just to complicate things, you have 2 lots of (variable) grid load resistors in parallel at V2 grid

[Platefire]

It's easy for me to see how the bright channel works but on the normal channel is hard for me to see how it would be effected at all. So when the preamped signal of the normal channel reaches the grid intersection where both channels are joined together---at that point what is the routing of normal channel signal to be effected by the tone control---would it be that depending on the tone control setting would be acting like a water valve shunting some of the signal to ground through the .005 cap and the remainder going directly to PI input grid?? This is confusing to me!

[sluckey]

Assume no signal on the bright channel. The 'blue' indicates the normal signal path and the 'gold' indicates tone filters.

Look at Figure 1. The tone pot and .005µF cap form a variable series shunt filter connected directly between the normal signal path and ground.

Look at Figure 2. The bright channel volume pot and the 500pF cap interact somewhat with the normal signal also. Imagine the tone pot wiper at the top end of the pot and the bright volume pot at it's bottom end. The 500pF cap is shorted out in this case and it's effect on tone is minimal. Now crank the bright volume pot to the top and you will see that the 500pF is now connected directly between the normal signal path and ground, shunting some high frequency signals to ground. Moving the bright volume pot between extremes varies the amount of attenuation to the high frequencies, acting like a tone control for the normal channel. The effect is not nearly as noticable as the real tone pot, but you can hear the effect.

[Platefire]

Wow Sluckey! I'm very impressed  :shocked: I've been researching this for a while before I posted the question here. IMHO this is the best exlination out there I've seen and for the first time can understand how the interaction of the 5E3 tone stack works. Great job! and thanks for taking the time to map it all out for me. This definintely needs to go in the Hoffman information bank!

Now I'm wondering if Leo intended this interaction or was it just an accident? We may never know for sure. Platefire

[FYL]

Now I'm wondering if Leo intended this interation or was it just an accident?

Other Fender amps from the 5B, 5C and 5D series used a similar arrangement.