proposed dr. boogey layout -- seeking comments

[audioguy]

There are only 5-6 parts in the tone stack. Just leave out those parts.
Leave of everything after Q4 and add a cap to the drain of Q4. 1uf should be fine.
The tone stackless build sounds pretty good still.
Just bit louder and more mids and highs asuming you compare it to the regular version with the knobs set at half way up.

John

OK I'm digging in and want to make sure I do this correctly... where Q4 connects to Q5 (q4's D to q5's S) I would add a 1uf cap. Does that cap go to ground or does it go in-line and continue my path to either a volume pot or whichever EQ I put in place?

Thanks for the help!

[MartyMart]

In line off the drain and onto the next section/output vol pot.

[John Lyons]

I'm going to change my mind slightly and tell you to just go off the Q5 output with a 1uf cap inline into either another alternate tone stack or a volume pot.
Keeping Q5 is a good thing as it will drop the impedance an help keep the output stable when/if you add effects after the DB.
John

[audioguy]

OK... so keep Q5- will that effect the gain any?

[John Lyons]

Q5 doesn't add any gain. It just lowers the impedance. This is good for the tone stack and for the whole impedance of the output section.
Lower is better. When you add another effect the impedance of the next effect will be high (most likely) which is good.
Generally, High impedance in, low impedance out is what you want so you don't load down the input of each succesive pedal.

John

[George Giblet]

> Lower is better. When you add another effect the impedance of the next effect will be high (most likely) which is good.

The output impedance of the dr boogie is pretty high really and some effects will have an effect.   Plugging into a sound card will hard a large effect on the tone.

If you ask me, given the Q5 buffer is there, the whole circuit following Q5 should be scaled down in impedance.  A factor of 10 should sill be ok but I suspect you don't need to go that far.  There might be some reluctance to do this since it results in different tone control part values that the standard circuits but that's only a human thing - electronics says scaling works fine.

Another option is to add  another buffer after the tone control/volume control but you probably don't need to go that far.

[John Lyons]

Interesting George. So you mean scaling down the cap and pot values in the tone stack?
I use a 100K for the volume pot now.

John

[George Giblet]

> So you mean scaling down the cap and pot values in the tone stack?

Yes.  A scaling down by a factor of 10 would be:

Slope Resistor:        was 47k    now 4k7
Treble Cap:             was 680p  now 6n8
Mid Cap:                 was 22n    now 220n
Bass Cap                 was 22n    now 220n
Treble Pot:              was 250k  now 25k
Mid Pot:                  was 25k    now 2k5
Bass Pot:                was  1M   now 100k
Volume Pot             was  1M   now 100k
Presence Resistor    was 22k   now 2k2
Presence Pot           was 100K now 10k
Presence Cap          was 3n   now 30n

A 2k5 pot might be harder to find but you could go for say a scaling factor of 5 instead of 10 in the example.

Quite a few commecial amps use scaled versions of the tone stack - mainly to keep noise down.

> I use a 100K for the volume pot now.

If you *only* change the volume pot, then the volume pot significantly loads the tone control.  When this is done there is quite a change in the frequency response and the "gain"of that circuit is 4 to 10dB lower.  As a rough comment it pushes all the frequencies up because the R in f=1/(2piRC) is smaller.  By changing the pot to 100k it essentially loads down the circuit from the start.  What the scaling tries to achieve is to keep the original response and make it immune to loads placed on the output.  You don't want to scale to too lower impedance  either because then it will start loading the buffer stage.

[Ardric]

It's Q2 thats is giving me biasing problems. It's behaving strangely.

Pushtone, do you have the gain pot in circuit?  Because the Boogey uses the DC path through the pot to bias Q2.  No gain pot, no bias.

Betcha your drain problem on Q2 is because the gate is floating up.  You could also try adding an additional 1M or 2M2 from gate to ground, just to be sure.

[John Lyons]

George
Thanks for the info. More evolution going on!
For the odd pots it would be simple to put a resistor across the outside lugs of a pot to get the correct value. All the odd value pots are linear anyway except for the treble pot. Maybe a 50k linear pot with a reisistor across the outer lugs to get the rough resistance and a reisistor from wiper to lug 1 to get the right taper.

Ardric/pushtone
That make sence about the gate floating without the pot in circuit.
I bet if you hook up the gain control it's bias right up!

John

[Pushtone]

It's Q2 thats is giving me biasing problems. It's behaving strangely.

Pushtone, do you have the gain pot in circuit?  Because the Boogey uses the DC path through the pot to bias Q2.  No gain pot, no bias.

Betcha your drain problem on Q2 is because the gate is floating up.  You could also try adding an additional 1M or 2M2 from gate to ground, just to be sure.

Yep, right on. Biases fine with the GAIN pot soldered in. Silly me.
I see the DC path through the GAIN pot now. C4 was throwing me.

Thanks Ardric.

So the PCB build is seems to be working fine. I've put a 1k tone thru it.
Think I'll skip the test rig and just box it up.

The layout is a pleasure in the order of the pots and the I/O pads on one side.
Thank Gussmarkov, can't wait to compare this to my finished, un-modded Bucksears layout version.

[Victor]

Pushtone,

Any squeals coming out from the new "beast"?

I'm just waiting to see some comparative review of both layouts on this, so I can start my 2° Boogey assured that will work just fine.. 

[Pushtone]

I've decaled the box and I'm just waiting for the clear coat to dry enough to handle.

No squeals, but I only listened with the audio probe at a few random points around Q1 and 2 with only the gain pot connected.



I'll be finishing it up and I'm worried I won't keep the star ground intact.

To keep the integrity of the star ground Gussmarkov has set up I'll be bringing EIGHT leads to the output jack ground.
Wow.
I can cut that down to five if I solder together all the shields from the I/O cables into one drain wire to output jack ground.
But still, FIVE.

PCB ground
Input jack ground
DC ground
Switch ground
I/O cable shields (x4)

I don't know if I can ward off temptation to bond to the input jack ground too, in addition to the output.
Because of the side I chose to put the footswitch, the input jack is closer to some things.

I'm also concered about George's comment about the 100K VOLUME pot. I may use the 1M for this build. My other one is very - ultra sonic.

[Pushtone]

I'm finished with the build. Fired up great.

100% No squeals at all, even with all unshielded wire, even with all controls maxed.

Congratulations Gussmarkov!


One tiny, itsy bitsy thing. The MID pot works in reverse. 
Full clock wise is max cut and full counter clock wise is max boost. I'll tripple check it again.

All other controls work fine and similar to my Bucksears build.


Here are the specs.
All J201's
I used all the exact values except for:

MID pot - used 25kB
TREBLE pot - used 250kA.

I used trimers to bias Q1 and Q2 . The other transistors I used fixed resistors.
Still playing with the bias but I'm liking 7.8V on Q1 and 4.4V on Q2.

.05uF for an input cap and .1uf for an output cap.
Are these values OK?

I had to use electrolytics for the 1uf caps C1 and C6
How bad is that? Is it worth ordering in some small 1uF film caps?

All grounds go to the output jack sleeve lug.



Controls on both layouts respond basically the same.
Gain control works better with the 1M pot than with the 500k.

SO it's working but it sounds a lot better than my version on Buck's layout with the 100k VOLUME pot.

I think George has a good point about scaling the values for the 100k pot.
The one with the 100K VOLUME pot has more high end content and an almost ultrasonic component.
I think I have to replace the 100K VOLUME pot I used on Bucks layout with the stock 1M.
I bet both layouts will sound the same then. 

Is there another way to deal with the output impedance of a 1M pot? A buffer following?

The layout is great! No squeals, except for the MID pot no debugging, (it IS working, just backwards).
My input wire is on the left. The out from the VOLUME pot to the switch and switch to jack are on the right.




Bigger version of insides picture
http://www3.telus.net/public/david65/pedal-pics/DrBoogie-Guss-insidesB-.jpg
http://www3.telus.net/public/david65/pedal-pics/DrBoogie-Guss-insides2-.jpg
http://www3.telus.net/public/david65/pedal-pics/DrBoogie-Guss-PCB.jpg
http://www3.telus.net/public/david65/pedal-pics/DrBoogie-Guss-topNK.jpg



Thank Electritabs for releasing this over-the-top distortion project.
Thanks Bucksears for the first layout that started the buzz.
Thanks Gussmarkov for your huge refinement effort.

[drizzt]

thanx guys for the improvements on this circuit. I´m looking forward to rebuild mine with this new layout since the other one i've built from gringo's, is quite "trippy"! 

[dschwartz]

What if moving the volume control BEFORE the last FET? all the low impedance we get with this buffer is brought up again with the 1M pot, so i think it will keep the output impedance real low, with little o no change on sound...

[John Lyons]

Looking good pushtone! Thanks for posting your results. The 1M volume and stock tone stack values work fine if you just use the DB alone. The impedance is high which is far less that optimal but it will work. The problem is if you run anything after the DB you run into a good bit of loading and the tone goes downhill, sucks the high end out of the DB. We'll have to try Georges tone stack scaling and 100k volume pot.

Dschwartz
Putting the Volume before the last fet would requitre putting the tone stack before it as well. They are both High impedance circuits.
Adding another buffer would work but then it gets even more complex...
The way I see it Q5 lowers the impedance that aloows the tone stack (which is high impedance) to work better. The after the tone stack the impedance is high which feeds into the 1M volume pot. So if we lower the tone stacks impedance as George outlined we can use a lower volume pot and retain a low impedance....

John

[dschwartz]

but isn´t the tonestack already before Q5??
maybe scaling the tonestack and moving the volume control between the tonestack and q5 could improve the impedence issue....

[John Lyons]

Q5 is before the tonestack and volume.
Scaling the tonestack is the cure it would seem.
If you scale the tonestack then there is no need to move the volume (100K).

Q1-Q4 is high impedance
Q5 lowers the impedance as not to load the tone stack (which is high)

Seems like Q5 (low imp) into a scaled tone stack (low imp) and a 100K volume pot would keep things low imp and not load down pedals or the amp down the line.

John

[Victor]

Pushtone,

Your impression sounds complete, very detailed. Thank you, I will proceed in building mine now. 

I did some simulations on TSC using George's theory about scaling TS components on 100k vol pot's use. Found some interesting values, and a similar curve. I'll try those values on mine, hoping that all works fine as it says on paper (screen).

But I have two new questions:

1) Did somebody measure Vgs e Idss of their J201 used on Dr. Boogey? I'm going to do that, and also try to unveal the "magic" happening on paper caused by their different values......

2) Is there any way to implement a Road King switch mod, like Raw/Vintage/Modern switch? I was thinking of using it to change between Q1's source cap values in order to get those sonic variations......... Raw would be standart cap, Vintage equals no cap, and Modern would be a bigger cap, but I don't have any Mesa Road King sound references, so I'm a bit confused........ can someone enlighten me about this (if it is worth the "pain", how does it sound in the real amp, etc..)?