I felt I should report on my findings implementing the 'Bass Volume' circuit into a little Watkins Dominator amp I built a year ago. Very simple amp with only a volume and I changed the original tone control for a Vox cut instead. I've become bored with that amp and decided to get to fiddling.
Ironically, I already had V1 wired up in parallel coming out into a .01 cap. I was going to change that out for something larger, but decided what the hell. In parallel with the .01 I used a 47pf SM, both hitting the bass pot like in the pic of the Songwriter. I didn't use the following low pass filter after the bass into the volume, mostly because I don't know the values and I wanted to hear the full effect of just the bass into volume on its own. Turns out, this is a VERY interesting circuit and is SOOOO simple! When turned all the way down, it reminds me a little of the AC30's bright channel. Up all the way, you get much, much more bass and gain blending the .01 as well as a severe change in the character of the drive. It certainly isn't as impacting as those found in the Fender designs or Vox TB, but it functions very well for what it is. I dig it!
Hope you guys find this as interesting as I do and urge you to give it a try. Ken was the man!
The values in that part of the SW30 circuit have not been identified (at least to my knowledge), so it's great that you found some values that work well practically. I like small coupling caps.
Yeah I have no idea what they are, but I'm now wanting a single SM cap either in place of the low pass filter or between the wiper and right lug, a la bright switch. I just played the thing for the second time a few minutes ago and with the bass dimed, much of the chime of my 47pf cap is lost, even with the cut dimed as well. I'm thinking of a permanent bright cap across the volume in addition to what I have now. Probably 100pf. I like that in my Top Boost. Maybe bumping the 47pf up to 500pf, too. I love this part of amp building!
To possibly recover chime, how 'bout varying the degree of cathode bypass in conjunction with that SM ? So, a dual pot, each side opposing...one side varies the amount of that SM in parallel with the output coupling cap(per the SW circuit), and the other could increase series resistance to ground with Ck (thereby increasing negative feedback and reducing the effect of Ck). It might take an additional resistor in parallel with the one side of the pot (to set a custom value for that half of the pot) controlling Ck so that Rk isn't screwed with too much wrt to AC.
That sounds like a really cool design, though a little extensive for me. I have slight space issues and a stacked pot may protrude onto my board. What about the blended film cap in parallel with a SM in parallel with a smaller film cap, one that's always in the circuit with an additional SM on the volume pot? I'm thinking about changing the SM that's in parallel with the film cap for something like .0047 going to volume with the blend coming from a larger film, like .03, in parallel with oh, maybe 250pf or up to 500pf? In my mind there would be constant chime from that one, although, I don't know if the bass pot's series resistance when rolled back would inhibit some of that chime. I'll fool around with this a little.
Okay, scratch my last one. I vastly prefer having ONLY a SM active always. With a common value film in place of the SM, there's too much bass already present and depletes the function of the bass control unless the it is extremely small, like .002 or something. I'm going back to the original design with a few value tweaks.
Edit
UPDATE: I think I've gotten it ironed out with what I just tried. All but the value of the film cap, which I will raise the value of when building a new amp. 500pf is a must and retains a great amount of chime, especially when in conjunction with a bright cap as well. I used 47pf. REALLY dig it like this. I am curious though, about whether any bright cap or that low pass filter SM is subtracted because of the 500pf, due to them being in series. Is the resistance present changing that?
And poor amp, I've powered her up around four times today.
I have also used the Bass Circuit of the Songwriter in one of my builds. I build a Fender 5f2a like amp with parallel SE power amp. Because i want a Bass Pot but no signal loss with a standard fender TS i implementd the Songwriter Bass Pot. I used 500pF SM and 0.47µF Mallory 150 cap. I like the amp very much. One question i have is, which Bass Poti did you use? Mine is a 2,5M lin Pot. But i am not shur if this is correct.
BR
Mike
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Very interesting schem you got there. You wowed me for a sec when I read 0.47uf in your post regarding the blended bass cap, but I guess it was a simple typo since your schem says .047uf, which sounds more appropriate. I'd say you pretty much have it with your 2.5m pot. I have a 3mRA in mine and find that it functions even better than expected. I played around with the design a few times yesterday and finally came to a point I can live with.
Hope I can see or hear your amp sometime. Pics are always welcome.
Very cool info, Randall. There are two SMs, though. I assume you mean the first is 1Kpf, but what of the one in parallel with the resistor? You wouldn't happen to know that resistor too, would you? I assume the bright switch is 100pf and 500pf. I'd say most of that circuit has been id'd, save for the NF, maybe two 100Ks in series with a 4.7K to ground? Or 100K in total for the two? That cap in your pic looks like a .1uf, but the one from Proguitar.de doesn't look at all that big.
