Tone Stack Subtleties?

[mfreqmaster]

I built 2 amps earlier this year, a JCM 800 copy and a 6MVIB, which is Weber's TMB version of the Marshall 18 watt. Both have similar cathode follower tonestacks. I noticed in both of these the bass is super responsive. The treble & middle not so much. I can get some difference in tone when adjusting the pot, but nothing like the bass. Just curious if this was normal.

I was wondering about this because 2 years ago I built a JTM 45 copy that was fairly responsive on the treble, middle, & bass, also presence. The presence knob on the JCM 800 does change the high frequencies, but not by much. Any thoughts?

[Chuck H]

It does depend on a few things like the slope resistor value, the range and interaction of the cap values, weather your running the amp into a master volume of any kind, pot value/taper, etc...

Especially if you run the amp into a master volume or attenuator for distortion. This will dull the audible effects of the tonestack and make it more of a 'distortion character' adjustment, leaving you at the mercy of your OT and speakers for the final EQ.

The tone stack following a cathode follower does not equal adjustment make. Try downloading Duncan TSC on the Duncan amps website. It's a free tonestack calculator/SPICE kind of program. Some time experimenting will reveal all.

Chuck

[pdf64]

Regards the tone stack, along with the response above, also check the taper of the control pots - Weber audio tapers are often about 20 or 30%, rather than the (modern) normal 10%. The TSC allows you to select various tapers.
Regards the presence control, the JTM45 power amp has a lot of global negative feedback - it has a 16 ohm output but uses the same feedback ratio resistors as the 5F6A bassman with a 2 ohm output. Therefore the presence is very effective, as it removes the negative feedback for higher frequencies.
The JCM800 has less global negative feedback on the power amp than the JTM45, a 100k feedback resistor compared to 27k, so the presence control isn't as effective.