Blackface Pro Reverb Mid control

[Randall]

Customer has a Pro Reverb that he says is a bit too midrange for him. He wants a mid control. All the mid control mods I am seeing are to add mids, usually by adding a 25K pot in series with the 6.8K resistor in the tone stack, so with the pot down full it is stock, and turning it up adds mids. But I want to go the other way. Would replacing that fixed resistor with a 10K pot do the trick?

https://cdn.shopify.com/s/files/1/23...97210715640943

[pdf64]

Absolutely
Its worth checking that the 6k8 mid resistor value hasn’t drifted a bit high.
Or that the 47nF mid cap drifted low etc.

[booj]

Just replace the 6.8k with a 10k pot. He can control the mids the way he wants then.

[Gaz]

A 10KA pot would be even better

[jack briggs]

All the mid control mods I am seeing are to add mids, usually by adding a 25K pot in series with the 6.8K resistor in the tone stack, so with the pot
down full it is stock, and turning it up adds mids.

Passive controls remove frequencies, not add them - unless you're looking at active circuits.

[Helmholtz]

There is no general reason (apart from circuit restrictions) why a modification to an existing passive filter should not allow to boost frequencies.

Increasing the mid resistance in the tone stack increases the (available) overall gain at mid frequencies. I'd call that a boost. You may call it less attenuation.

[Chuck H]

Just replace the 6.8k with a 10k pot. He can control the mids the way he wants then.

Still best to check for drift in the 6.8k fixed resistor and the value of the mid cap. That may indicate an actual repair is appropriate. Modding to solve problems rather than diagnosing them can often lead to confusion later.

[Chuck H]

Passive controls remove frequencies, not add them - unless you're looking at active circuits.

What Helmholtz said. The mod discussed actually does add mids. At least with respect to the stock circuit. But as a concession to what you wrote I will note that it add mids over the stock configuration by reducing signal loss through the passive tone stack.

[Helmholtz]

... by reducing signal loss through the passive tone stack.

If you use Duncan's Tone Stack Calculator, you will see that it actually doesn't.

Increasing the mid resistance works like increasing the lower (shunt) part of a (frequency dependent) voltage divider.

[Chuck H]

Using Duncan TSC I used the Fender tone stack. I set the mid control full and plugged in a value of 6.8k. That is the green trace. Then I changed the mid value to 31.8k. That is the orange trace.

[Helmholtz]

Using Duncan TSC I used the Fender tone stack. I set the mid control full and plugged in a value of 6.8k. That is the green trace. Then I changed the mid value to 31.8k. That is the orange trace.

Sorry, I misread your post above being irritated by the term "concession". In fact, increasing the mid resistance reduces insertion loss at all frequencies. This means a total boost.

[Chuck H]

If you're the only person I irritate with my choice of words today I'll be surprised

[Leo_Gnardo]

Customer has a Pro Reverb that he says is a bit too midrange for him. He wants a mid control. All the mid control mods I am seeing are to add mids, usually by adding a 25K pot in series with the 6.8K resistor in the tone stack, so with the pot down full it is stock, and turning it up adds mids. But I want to go the other way. Would replacing that fixed resistor with a 10K pot do the trick?

Somehow I missed this 10 days ago. Either that or I thought gaz had the good answer, and I still do. Fender's mid control in the classic mid 60's & 70's amps that have it (Twin Reverb, Bassman 100 & 135, Showman, Dual Showman etc), is exactly that, a 10K pot wired as a rheostat aka variable resistor. As you dial it down from 7 towards 1, you develop a bigger "ditch" in the midrange. I found it handy for use on bass guitar when a friendly chap lent me his 100W Bassman head in the mid 70's - I got the sound I wanted by dipping that mid control all the way.

Chuck, hows about a graph of the mid pot set all the way down to zero ohms compared to stock 6K8 resistor? TIA!

Some guitarists, especially blues players ask me for a mid control that only adds mids, and doesn't cut them. And lots of mids at that. A 25K or even 50K mid pot with the stock 6K8 resistor in series makes 'em happy. I have to warn them, when you have the mids cranked up a lot, your treble & bass pots won't do much anymore. Note Mesa/Boogie's boost circuit in their original Mk I amps switched a 47K resistor in series with the mid pot, effectively bypassing the lossy tone control circuit entirely. Some players loved it, others complained " it sounds honky, and my tone controls don't do anything" whilst in the boost mode. Well, that's why. Then the next iteration of amps from Mesa and their competitors notably Sundown had separate channels with an extra gain stage for the boost plus tone controls for each. That's how progress is made. Mesa's business thrived while Sundown withered on the vine, such is life.

[tboy]

If you're the only person I irritate with my choice of words today I'll be surprised

Well, there's my new tag line.

[Chuck H]