Bass rolloff control

[zhyla]

You guys may remember I was working on a 5B6 bassman build a while back (schematic). I'm pretty happy with it at the moment but the bass response is really over the top (hmmm, wonder why they call it a "Bassman"...). What I've done is change the C1 preamp cathode cap to a .33 uF in order to shift the bass cutoff of the preamp tube. This helped quite a bit. I put a switch on it so the amp can be used for both bass or guitar.

I was thinking it would be nice to swap out the treble rolloff tone control for a bass rolloff. I couldn't find any examples of a bass rolloff circuit. I know this should be simple (just changing a low-pass filter for a high-pass filter) but I'm actually a little confused here about the tone filter as it is. I would have expected something like this:



Which I could swap for this:



I tried to get a Pspice sim for the existing circuit but I'm really bad at that. Can someone point me at an example circuit or something?

By the way thanks again for the help I've gotten on this forum, I'm really enjoying amp building as a hobby.

[GibsonLover]

Tweed Bass Control

I've found the bass control on the 5C8 Twin rolls off the bass nicely. The 2M pot can be 1M. You can experiment with the Cap value. The 47K resistor is there to prevent muting the signal entirely when the pot is at minimum. This circuit shows up a couple of times on the Fender tweeds, though sometimes drawn incorrectly. (The 5E5-A Pro comes to mind - one side of the bass pot is grounded and should not be.)

[loudthud]

Here's something I did on a 5E3. You can just ground the bottom of the 330K or stick the volume pot there. There are examples where Gibson used the same principle.

[zhyla]

Perfect, thanks so much. I actually drew something nearly identical to this out the other day (but had no idea it would work well).

[loudthud]

Your idea would work but sticking a pot where the R is in the high pass would change the load on the previous stage which would reduce gain at treble frequencies. The control I used is really a shelving network, it doesn't reduce the signal to zero at DC like a real high pass but the blocking cap (0.1uF) does that. The problem is that it requires a big pot. I used 4meg because I had one on hand. You could use a 1meg but you won't get as much bass attenuation. Experiment with the values to find what works best for you.

[zhyla]

Finally got around to trying out the bass rolloff circuit from the 5C8. Works like a charm as near as I can tell. I somehow ordered the wrong pot (it's a 2 meg, but there's no shaft) so I'll play with it some more once I can button up the chassis - it's pretty noisy with the chassis open. Thanks for the idea.