Problem with tone controls. Please advice.

FWIW, that was my experience also with my Maz Jr and Maz 38 (I no longer have either amp). Most of the useful range of the bass control was in the first 30% of the pot's range. I found the mid and cut control to be the most useful with those amps. Bass and treble had limited useful range IMHO. That's not too uncommon with other amps either.

 

It should be possible to get a more useful sweep by replacing the Linear (that's what the 'B' means) pot with a Log(arithmic) or 'audio taper' (A) pot. It won't affect the total amount of bass available, or anything else about the amp's sound - if you stick with a 250K - just the sweep of the bass control.

With a Linear pot, about 1/3 of the total track resistance is between 0 and 3 (9 o'clock, roughly). With a Log pot, 1/3 of the resistance is between 0 and about 6 or 7.

(With a Linear pot, half the resistance is at half-turn; with a Log, 1/10th is at half-turn.)

NB - this is assuming conventional wiring of the control, not a 'backwards' configuration, where a Log pot will actually make things worse.

Hope that makes sense!

 

Jim- Before you change the pot, why don't you run the question by Mike (Dr Z) via email? Offhand, I don't know why he would choose a linear pot over an audio taper pot for this application. He may have a good reason or he may offer to change it for you.

As for selling the amps, I just found other EL84 amps I liked much better. It's just personal preference. Mike builds some very fine and good sounding amps.

 

The behavior you're seeing may also be implicit in the tone stack's design. Don't know what values/setup Dr Z has used, but a good example is the "classic" Fender tweed tone stack (56K slope, 22nF mid & bass caps, etc).

If you sweep that setup you'll notice that bass boost beyond about '5' on the bass control occurs primarily below 100Hz. So for guitar you don't really hear that much difference.

 

I'd say you're reading the response correctly.

Personally, I believe a smart/caring builder understands the importance to all concerned of maintaining the performance and value of his products throughout their lifecycle.

 

When one analyzes the tone stack you can readily see the problem, The Fender style uses a 100k resistor from the input of the tone controls down to the bass and mid controls. The bass control is often 1 meg. This means that once the bass control reaches 100k the rest of the travel is less than 6 dB. Depending upon the pot, that can be in the center or less, particularly if the mid control is up too.

One way of dealing with that problem is to replace the bass control with a smaller value and potentially replace the bass cap with a larger one... You could probably use the extra treble anyway. If not, put a compensating resistor inseries with the treble pot...

Have a great day, Eric