Odd reaction with modified BMP tonestack

[Les Paul Lover]

I recently built a BMP clone using the multi muffins pcb sold by www.pedalparts.co.uk.

Beside the tonestack, I also changed :

- Swapped Q1 for MPSA18
- Fitted on /off / on switch as the pcb allows, and put 2 1N4148 and 3 1N34A on the other side for the 2nd stage clipping

Now the real mod I want to discuss is that of the tonestack. That's what I used:

- Low pass: 22nf and 10k
- High pass: 33nf and 22k

This combination gives great mids rich tone in the centre position , and still allows for very useable tones on the bass side or treble side of the sweep.

And that works perfectly... Really really well in fact..... Until I put the volume quite high up.

After a certain point, the tonestack doesn't seem to work at all anymore. Or work in mysterious ways!!!! The point at which the tonestack doesn't have the intended effect comes in earlier as I switch in between the setting - the louder it is, the earlier the point at which the volume control seems to confuse the tonestack....

So with the germanium diodes, it comes late on the volume control, with the silicon diodes a bit earlier, and a bit earlier still when the switch is off and there are no diodes anymore.

It's really odd, because until then, that tonestack works beautifully!
Is this something common to BMPs?

Or is it just mine?

[Les Paul Lover]

Here's the lay out and schem I've used - including mods mentioned in the above post.

The version I've built is the ram's head version.
http://pedalparts.co.uk/docs/MultiMuff.pdf

[Les Paul Lover]

Bumpity bump. 


No one has any idea as to what could cause my tonestack to be perfectly fine with the volume not boosted too much, but then becoming rather useless when the pedal volume is turned up quite a bit?
It feels to me almost like the volume stage transistor is overwhelmed by the signal going through and reintroduces lots of harmonics, highs and lows??

I've heard of some old marshall valve amp behaving a bit like that - EQ useable at low volume, useless at high volume, but never heard of that issue with a BMP clone.

[antonis]

Shouldn't you check pot's behavior..??
(continous measurement across wiper's full travel..)

[Les Paul Lover]

Thanks for the suggestion Antonis - I have done that, and I did check them during the build as well, as I was head scratching about linear / log.

The pots are fine - and the funny thing is that it all works fine up to a certain volume level.
Tone works perfectly until the volume gets brought up too high, and then it has little or unexpected effects


FWIW, the pedal sound damn fine at any volume - even when the tone seems to go all over the place.....

It makes me think that I'm gonna try that again on another amp, just to make sure it is the pedal, and that it isn't my Orange AD15 that's being bamboozled by the hot signal.

[antonis]

Is there any (even hardly noticeable) tonal altering at high volume level with bypassed tonestack..??

(just to ensure if there is a "sneaky" interaction between wiper's position and tonestack..)

[Les Paul Lover]

Is there any (even hardly noticeable) tonal altering at high volume level with bypassed tonestack..??

(just to ensure if there is a "sneaky" interaction between wiper's position and tonestack..)

Yes, the tone pot still seems to have some effect when the volume is turned up high, just not what is expected of a traditional BMP tonestack, and it seems to be a bit random too.

Funny thing is the spot at which that tonestack doesn't work anymore isn't the same with the germanium diodes, silicon diodes or without diode. The hotter the signal, the earlier the tonestack seems to "go".

[Digital Larry]

It feels to me almost like the volume stage transistor is overwhelmed by the signal going through and reintroduces lots of harmonics, highs and lows??

For sure, if there is clipping occurring after a tone stage, you'd get harmonics that are out of the tone stage's ability to control.  I'd expect highs rather than lows.  Only way to be sure would be to look at the waveform.

[duck_arse]

check your transistor voltages, the driving and the make-up-ing, see if any change as you vary the tone pot. it may be a dc problem. do you have a cap between the driving collector and the first R/first C of the tonestack? and check your driver transistor resistor values are what you think they are. also measure the resistor values in the make-up stage, they might be loading odd if wrong.

[Les Paul Lover]

Interesting development.....


Finally managed to find the time to try out my BMP clone with my Orange RV50 clean channel.
My tonestack works flawlessly, at any volume, and with any clipping setting (Ge, Si, or none). I'm relieved.

In related news, it appears it's my Orange AD15's preamp to becomes overwhelmed by the signal out of my clone.


For those interested, I used 33nf and 22ko on the high pass, 22nf & 10ko on the low pass.
It works really well.

It hasn't got the range you can find on the standard BMP tone stack, the lowest setting still retain enough mids to be useable, and similarily for the highest settings - it doesn't sound as thin and trebly, so all my tonestack range is very useable to me. And it's beautifuly middy set mid way, with quite a bit a low end. The mids aren't overpowering like a tube screamer is, I think the midrange bump is only +3db or so.

So the sweeping range isn't as dramatic, but who uses their tone at such extremes anyway? Well, no me at any rate!!!!!!   
I'm delighted with that one, great range of very useable tones all the way.

I would highly recommend it!!!!


Thanks again for all the advices - I'm sorry I appeared to have wasted your time - but a least I can give you new values to works with for future BMP type build!!!     

[Les Paul Lover]

Oh, and we like pictures.....

So here is the now certified to be working pedal:

[antonis]

Good job ..!!

Doesn't all these jacks and cables close to foot switch bother you..??
(I shouldn't feel comfortable to stomp on the swich with a No 45 (12) boot..)

[Ben N]

Thanks for the info--your tonestack "sounds" like what I would want in a BMP.

Interesting stompswitch placement vis-a-vis the artwork, BTW.

[Les Paul Lover]

Thanks for the info--your tonestack "sounds" like what I would want in a BMP.

Interesting stompswitch placement vis-a-vis the artwork, BTW.

I heavily recommend this tone stack values. Again, they don't offer as much range, you don't get the extreme trebliness or muffled low end only, but you still get a setting that's too bassy for me and too trebly - just not as much. And in between, you get lots of mids rich tones.


And yes, my green panda does look slightly aroused........     

Good job ..!!

Doesn't all these jacks and cables close to foot switch bother you..??
(I shouldn't feel comfortable to stomp on the swich with a No 45 (12) boot..)

Dunno, the pedal is stable when stomping on it. That issue hasn't even occurred to me!