ok tone control gods... riddle me this!

[runmikeyrun]

I've got a tone control section on my amp that has a boost button (the dpdt switch shown in the layout below) which boosts the signal but mostly in the upper mids.  This is undesirable to me as a bass player.  I've been trying to figure out how to change the frequency response to boost it at a lower frequency but so far i can't figure out this rotten circuit.  The kicker is (what i believe to be) a relay in the mix, the red box next to the boost switch.  It only has a couple of components connected to it but i can't figure out how to shift the frequency it boosts.  The only markings on the suspected relay are two different voltage/amperage ratings which is why i suspect it is a relay. 

The only thing i have been able to change with the boost button in the effected position is by jumping a wire across the 150k resistor.  This gives even more output and more upper mids.  So reason says that increasing this resistor will lower the boosted frequency and/or lower the output, but I'm afraid to pull up an end of the resistor to try it because this circuit is on a PCB and i'm afraid of lifting up a trace. If this makes sense to someone else i'll give it a shot.  otherwise any ideas that you think will shift the frequency lower would be appreciated.

The connections marked "v+" by the bottom of the relay go to somewhere else on the board that i haven't traced yet.  If you think that might be relevant i'll go trace them and see what it looks like.

As far as the tone controls go, i'm ok with how they shape the sound.  a way to shift the bass control's frequencies lower would be cool but in this setup i don't think it's possible.

[Meanderthal]

 Hmmm.... it's a little odd, but I think I get the drift. It looks like that cap marked 103(0.01uf) might be the key. Try 0.047uf or 0.1uf... I'm not quite sure what's up with the switching though...

[runmikeyrun]

yeah i tried a 473 but no audible difference.  I should try a 1.0 or 2.2 uF i have in the basement to see how that works.  Sometimes the values that would work with guitar don't work for a bass.  For instance another board told me to increase the 473 coupling caps to 0.1uf for bass guitar, but i didn't hear a difference until i changed them all to 1.0uF.  They thought i was nuts!  But i needed that to get the right low end.  I'll do some more experimenting... until then any more suggestions are much appreciated.  Thanks man!

[Meanderthal]

They thought i was nuts!

Naah, yer on the right track!  Some circuits just need to let more bottom end thru, others are ok with smaller value caps... But, you got the idea, so I'm preachin' to the choir here...

[runmikeyrun]

yeah i was a little fishy when they said 0.1uF because i know from building bass pedals that that doesn't cut it, you need to go to at least 1.0.  ha!

[edster]

what amp is this?

[runmikeyrun]

the new peavey windsor... supposed to be a JCM 800 clone and compared to our guitarist's JCM 800 it does a really accurate job, with a little more gain and the addition of presence/resonance controls and a weird class a/class ab pot to blend between the two.  For $400 plus $25 in bigger bypass caps i am getting a smokin amp for bass that sounds really really good on guitar even with the stock Ruby EL34s and JJ preamp tubes.  I swapped out the 2nd preamp tube for a NOS rca 12au7 because with all the added low end things were getting a little unstable with all that gain.

So... back to the boost circuit... with further poking and prodding i bypassed the 150k resistor with a jumper effectively making the 103 cap and 470k resistor into a high pass filter.  By paralleling a 1.0uF cap with the 103 i have let a ton more bass and signal through but it has made the mid hump somewhat more pronounced.  Since this thing is tacked on at the end of the tone controls, the midrange control has no effect if the boost switch is in.  the treble still works though but that doesn't help me any. 

So go the next step I am thinking of getting my boss ge-7 and throwing into the fx loop (which directly follows the boost control) and seeing which frequency i need to notch out, then use the duncan tone stack calculator to modify a BMP tone control to do the work on the right frequency.  I was really trying to avoid adding another circuit board and pot but there is a spot on the back for a footswitch jack that i could pop out and put a pot in there for the BMP control.  The footswitch controls the boost circuit so if i'm modding it to be on all the time then i don't need a footswitch.  I'm thinking that there's no other way to notch out the offending mid frequency by changing components in the boost or eq circuit. 

I've also learned quite a few things about tube amps which i have never seriously gotten into... removing the negative feedback circuit really opens things up for one.  On this amp it is sent to the presence and resonance controls which affect the high end and low end response of the speakers.  Setting both controls to max basically bypasses the feedback loop all together.   They are neat controls to have on an amp.  If you have a tube amp and know how to work on it without getting killed, i recommend disconnecting that loop if you're looking for more punch and gain.

[rockgardenlove]

You could always just stick in the standard Fender tone control.  I'm happy with mine on my Bassman. 

[edster]

the new peavey windsor...

Ok, that makes sense. I was gonna say, it looks a bit like the Classic 30 mid boost.

Here'e a schematic of the Windsor tone stack



The Classic 30 boost function bypasses the treble cap w/a much larger (.047µf I think) cap - allowing more mids into the the treble pot which, as you probably know, is actually a little mixer. The Windsor otoh, bypasses the cap and part of the treb pot. R11 and R57 become more involved in voicing than the treb pot at this point.

There's a couple things you could try here. If you want more of a mid only boost, you're gonna have to corner it. Perhaps changing C38 to a .047µf or .1µf and addind a .001µf to ground between C38 and R11. Or.... you could change the switch to work like the Classic 30 boost. That might suit your application a little better.

Also, I think I'd try changing C32 and C33 to .047µf or .1µf

[runmikeyrun]

wow, where did you get that?  I've been trying to find a schematic for that amp since i got it about a month or so ago! 

I want to give the boost button a mid CUT... I increased c30 to 1.0uF and jumpered R11 and got a huge low end boost (which i like) but also a large mid boost as well.  I think i turned it into a high pass RC filter that just passes everything now  

I tried varying the R57 value after the aforementioned mods w/ a 1M pot in place of r57 with no audible difference in tone.  I got your mods about increasing the mids, how could i decrease them?  When i decreased the value of c38 to remove the mids i lost all the bass too.  If i added a resistor to ground between r11 and c38 would that affect the sound the opposite of putting the .001uf cap in there?   

I was reading about LC filters today, forgot about how i could make a mid cut filter out of one of those, but with the variables involved still might be easier to make a BMP tone control.  Not really sure at this point how to attack it but once i find the center frequency i want to cut i can do some better calculations w/ the duncan TSC.  Thanks for the help so far!

[edster]

There's a coupla ways to do a simple mid cut using the existing parts. Move pin 2 of S1 to the ground lug of VR5 - essentially placing it across R57. R11 can be changed to whatever gives you the cut you're looking for - prolly somehwere between 0 and 47k. You'll prolly want to add a series resistor upstream of C38 to keep the highs out (maybe 10k-ish?) You can mess with C38 to adjust the freq. When you kick in the boost it'll bypass R57 and bleed some mids off to ground. LC filters work well for this, but it's not really neccesary here.

The other way to do a simple mid cut would be to add a resistor in series w/C33 - maybe try 10k - 27k. You put one side of the relay across it wired normally closed for normal mids. Since one side of the relay is available, you could do both of these at the same time.

If you pm me you're email I'll send you the schemo.

[runmikeyrun]

awesome, i will give those all a try!  I am going to try to get a pot in there somewhere to make the mids adjustable somehow.  I use both a 2x15 and 8x10 cab at separate times so i will need to change it between the two. 

PM sent, thanks!!  And welcome to the forum.