Tone Stack Defeat On A Plexi?

[Toneblenders]

I have heard of doing this on an 18/36 watt, switchable via a push pull bass pot. But has anyone out there done or heard of doing this mod on a plexi?

Any advice here would be greatly appreciated.

Thanks!

[jellewelagen]

Hi Toneblenders,

The tone stack defeat works well after an anode coupled gainstage, and for instance Dumble and Boogie use this trick. In a plexi, the tonestack is connected to the cathodefollower. The main difference is the output impedance of the gainstage it is coupled to. The cathode follower has an output impedance of ca. 1k and if the tone stack is lifted then the impedance that the CF sees is that of the phase inverter, which is from the top of my head is close to 15 - 20 M. This is a lot higher that the impedance it would have seen due to the tonestack and as a result the character of the sound will change (and the volume will go up :-) ). I personally do not like the sound change too much.

Also the plexi could get Really loud, which may or may not be a good thing.

An alternative is to switch a 47K resistor between the mid potentiometer and ground instead of lifting it completely. I like this a lot better for plexis. If you make it footswitchable it will make the amp really useful on stage as well.

Hope this helps,

Jelle,

Manhattan

[Toneblenders]

Jelle,
Thanks for the help.:)

So is that how the tone stack defeat is done, by lifting the ground on one of the TMB pots? Does it matter which one?

On the 47k resister between the middle pot and ground. What kind of effect does this have on tone, loudness etc. and why?

Thanks, Bob

[jellewelagen]

Toneblenders,

The only EQ potentiometer in a Plexi that is directly connected to ground is the Mid pot. Yes, the typical boost disconnects this ground connection, also in fender BF/SF style tonestacks.

The effect without the 47k resistor is quite drastic, with the resistor you can tweak the effect between no boost and 100% boost. You can even put a temporary trimmer instead of that 47k resistor to find the sweet spot that you like best and then wire a resistor on the switch.

Placing a resistor there will keep the impedance down and will prevent 'rattiness' in the sound (I know we are talking guitar amplifiers here and not HiFi stuff but at some point it just doesn't even sound marshally to me anymore as the mids change a lot in character.)

Good luck!

Jelle,

Manhattan

[Toneblenders]

Jelle,

Thanks! That helps alot.:D

Bob

[jellewelagen]

Bob,

You're welcome! If you try this, let us know how you like it.

jelle

[Toneblenders]

Jelle,

I'll let you know if I do.

Bob

[VanR]

I have mine wired to a footswitch (my homebrew High Octanes) and use it to boost my leads. Yes it does change your tone, but I only use it for leads.