Help with a James tone stack.

[jonasnoble]

Hey everybody, I spent some time here at the end of 2023, but got caught up with life. I was working on a Bass DI a la REDDI, and built one that sounded great. Then I tried building another, adding a gain stage and some tone controls. Originally, I took it from the b-15n schematic, but I have gone back and forth between James and Baxandall, played with all the values and gotten nowhere. It doesn't really look like the Ampeg flip top anymore, but it doesn't seem to matter what I do, the results are the same.

I can screen cap some freq response graphs in a bit, but here's the problem: I have played with the modelers and they imply a huge sweep between min and max on the controls. My bass control is not very responsive on the boost end. If I wanted to increase the bass response, which components values should I be playing with?

Not sure if this link will work:
Tone Stack Calculator

And here's a photo:



I've subbed in multiple values on the bass controls, and still don't get much boost at all. I'll add some photos of my test later.

Thanks in advance.

 

[jonasnoble]

Here are a few screen shots of of my frequency response.

 

[abbey road d enfer]

View attachment 135886

I've subbed in multiple values on the bass controls, and still don't get much boost at all.

Well, you already have 18dB boost.
Remember, boost is relative. Actually it means that the rest of the frequencies are much more attenuated.

 

[jonasnoble]

Hey, thanks for the reply. Are you getting the 18dB figure from the white picture? If so, that's on online calculator/model. I probably didn't explain this very well, but what I'm trying to show is that my actual results do not match the calculator's predicted curve.

I think I see what you're saying though. It could be that the highs are not attenuated enough, which makes the bass sweep look much less effective.

 

[abbey road d enfer]

Well, the responses are more like a typical Fender/Marshall stack. Weird.

 

[abbey road d enfer]

Have you considered that you are loading the output too much? If you connect directly to the QA input, the 100k load seriiously shifts the LF response.

 

[jonasnoble]

Good thinking. I haven't looked at the spec sheet in quite some time, so I had no idea the hi-z is only 100k. Turns out I must've done something wrong. I pulled the tone stack apart, took out the 500k reverse log pots that schematic called for. Put in 1M audio taper and followed a b-18n schematic. It works great. I changed all the values but I think the potentiometers made the biggest difference.

I'm looking to add a high pass and low pass filter in the chain, per request from my friend who's getting this one when I'm done. I calculated the -3dB points, but the high pass isn't really doing quite what I expected. Anybody know what's the standard way to implement these filters? I'm having trouble with the switching and getting weird phasing and frequency attenuation.

 

[abbey road d enfer]

Good thinking. I haven't looked at the spec sheet in quite some time, so I had no idea the hi-z is only 100k. Turns out I must've done something wrong. I pulled the tone stack apart, took out the 500k reverse log pots that schematic called for. Put in 1M audio taper and followed a b-18n schematic. It works great. I changed all the values but I think the potentiometers made the biggest difference.

Whoever suggested RevLog pots in a James was not fully awake.

I'm looking to add a high pass and low pass filter in the chain, per request from my friend who's getting this one when I'm done. I calculated the -3dB points, but the high pass isn't really doing quite what I expected. Anybody know what's the standard way to implement these filters? I'm having trouble with the switching and getting weird phasing and frequency attenuation.

The problem is that high and low-pass filters react with their environment. Ideally they should be driven by a low impedance source and loaded with a very high impedance. This is not always possible in an existing circuit. One solution is to add one or two active stages (cath-followers are good for that).
In order to answer precisely your question, we would need to see the full schematic.

 

[jonasnoble]

Whoever suggested RevLog pots in a James was not fully awake.

Haha, you know more than I. The old Ampeg schematics don't say, as far as I know, and that calculator I was using acted really funny when I changed to linear or audio from the default RA. Anyways, I had no idea how that stack works until building this, and I've learned a lot.

And regarding the drivers for the filter circuits. Oof and facepalm. I should've seen that but I didn't even consider. I worked on the switching last night and got closer, but they're still not behaving, and that's exactly why. I don't think I can fit another tube in there, so I'll maybe have to pivot and do that on my next run.

 

[jonasnoble]

Whoever suggested RevLog pots in a James was not fully awake.

The problem is that high and low-pass filters react with their environment. Ideally they should be driven by a low impedance source and loaded with a very high impedance. This is not always possible in an existing circuit. One solution is to add one or two active stages (cath-followers are good for that).
In order to answer precisely your question, we would need to see the full schematic.

I'll try to draw a schematic, but I have been hesitant until I get closer to the final working product. I am considering a simple N-channel source follower circuit to run my hi/lo filters. I know they will run off tube voltages, I just have to find the right one from the supplier. Anybody have any experience with this? Hopefully looking for a TO-220 package that can handle around 200Vdd.