Scaling Bridged-T tone circuit

[Steve A.]

I've been experimenting a bit with this circuit since finding it in some Epiphone EA-50 amps from the mid-60's to give their tweed style amps more of a blackface tonality. If you have plenty of gain you can add this in line, like right before a volume control and it sweetens the sound by cutting the mids a little bit.

One suggestion was to put a pot on the "tail" of the 3300pF cap going to ground- I hadn't tried that yet so I don't know whether to use 25k or 1M...

Ignoring the 3300pF cap to ground, we basically have two 220k resistors in series bypassed with a 270pF cap (the junction of the two 220k resistors goes to a 3300pF cap to ground- the attached picture is not very clear, but was a hand edited copy of a schematic).

The $64 question is this: how would I scale down this bridged T network to use two 100k resistors instead of two 220k resistors? With two resistors and two caps just thinking about the RC values gives me a headache...

On a related note, how did Randall Aiken scale the brownface tone stack to use a 250k tapped treble pot instead of the original 350k pot?

Finally, I believe that there is electronic circuit software that will analyze simple circuits... any suggestions on something cheap that could help me figure out stuff like this? I've always been skeptical of modeling software- IMO you really have to solder the parts together to hear what they will sound like. But I can see how a software program might help get me going in the directions I'd like to go.

Thanks for any help with these questions!

Steve Ahola

http://www.blueguitar.org/

http://www.myspace.com/steveahola

[loudthud]

In general, when you want to scale the impedance of a network downward, you increase the value if capacitors (conversely decreasing the value of inductors). So if you want to go from 220K to 100K, you would roughly double the caps to 6800pf and 560pf or something close.

Most bridged T's use a bridge cap that is 1/10th the value of the shunt cap, but this changes the depth of the frequency dip, so experiment. I find that the component values in these things interact so much that I just start with a dual pot and a couple of capacitor substitution boxes until I find something I like.

The pot in series with the shunt cap gives control over the depth of the dip also. A 1meg log pot will give you a lot more range than a 22K. A 250K might be a better choice, start with something 1x to 2x the value of the bridge resistors.

Modeling software is a lot of work and in the end, you still don't know what it will sound like. There are free versions of SPICE available on the net if you want to go that direction. A few quick searches on this forum and the net should turn up many choices.

[raiken]

On a related note, how did Randall Aiken scale the brownface tone stack to use a 250k tapped treble pot instead of the original 350k pot?

It's all here:

http://www.aikenamps.com/ToneControlScaling.html

Just calculate your ZSF by the ratio of the new/old impedance and scale the R/C/L components accordingly.

Randall Aiken

[Enzo]

If in doubt over whether to use a 25k pot or a 1 meg pot as a variable resistor, just wire in the 1 meg and see how much of its range is useful. Then mount a pot of that amount. or close.

[Rick Erickson]

Steve - I used a circuit like that in a Brown Princeton Amp to sweeten the tone to my liking. I'm pretty sure I sent you a diagram titled "Useful Filter Circuits" or something to that effect, for you to post on your site. Do you still have it, or would you be interested in me resending it, if I can locate it?
Also - some Gibson amps (and probably Epiphone amps too) used these filters two or three times in one preamp circuit. It is my opinion that these amps can be opened up with better tone and gain by removing the extra filter circuits. One is usually enough, after that you are just removing character from the amp. Of course you have to add jumpers to connect the appropriate dots, or you won't get signal flow after removing the redundant filter components from these amps.

RE