Dr. Boogie Detective [Why some boogie and others sqeal?]

[mojotron]

I added the miller caps at the other gain stages, and the pedal is very amp like.

I have seen this mentioned a few times, but I am unable to find a reference -
1. Where do I get them ?
2. What are the benefits?
3. Why in the gain stages?

Here's a good reference:
http://www.aikenamps.com/MillerCapacitance.html

And, here's a reference as it relates to this sort of application - in the fetzer valve design by ROG:
http://www.runoffgroove.com/fetzervalve.html

Keep in mind that the ROG method models a 1-3pF source-drain parasitic loss by multiplying the stage's gain by this capacitive loss and shunting this to ground via a gate to ground capacitor which we are referring to as a 'Miller cap'. In a lot of cases a JFET in a gain stage has a rather large relative capacitor as a source bypass. So in the case where you have a source bypass cap, I suggest that placing a Miller cap to bridge the source and gate is far easier to route in a layout and effectively achieves the same result as bridging gate to ground.

In the Dr B. - I used 220pF ceramic caps and just installed them on the back of the board with one lead on the source and one lead on the gate for the jfets that have source bypass caps (from left to right this would be Q1, Q2, and Q4). So to answer your questions:

1) any 150pF to 220pF (my preference) ceramic cap will do - I get these from Radio Shack or Mouser
2) the Miller cap idea changes the frequency responce of the JFET to be closer to what triodes do - the cap size can be very exactly calculated, but I think you get a good result from just using a 150pF to 220pF cap; given the variance in tone from these tubes the amount of 'slack' in this approach is OK - IMO - but the ROG method in the Fetzer valve article is a very good approach for those that would rather 'engineer' the cap choice. I recommend people to do this both ways and see which works better for them.
3) see above - the Miller cap models an effect

(gain) x (JFET Gate-Drain parasitic loss)

if you have lower gain - the Miller cap would be then smaller and have negligible effect on frequency response - though I like to add these into the lower gain stages as well sometimes to control non-audio-range noise sometimes.

So, the result that I hear as being 'more amp-like' is that it sounds a bit more like a tube amp's tone/response and the pedal does not have an abundance of hi-end nor waists gain on non-audio range signals.

[boogietube]

I'm embarrassed  :icon_redface:I thought that it was a brand name like Sprague or something.
And I'm very grateful for your detailed answer. Great reading. I'll try the 220's when I get to populating the boogie board.

[John Lyons]

Just to clarify in this thread: Tube triodes have a capacitance inherently called "miller capacitance". The DR boogie uses caps in these same places but since the DR boogie is not tube the capacitance isn't there as in the tube build. Adding the "miller caps" makes the responce similar to the tube version. They aren't "miller caps" as much as caps simulating miller capacitance. Just though I would keep the terms straight...

John

[aloupos]

An Update to this thread, I've been using Dr. boogie as a pre in my main rig, running it into a tube pre (presonus blue box), then to a crate power block, and finally into a 2X12 cab.   I really love the sound I get from it. 

I've been playing with this circuit trying to develop a clean tone from it over the holidays -- some go to an island, I muck with circuits.  By lowering the gain pot to around 300 ohm (I loaded a pot with a 300 ohm resister between the wiper and lug), I get a nice range of really low gain sounds, that sound to me like the low gain channel on a rocket 44 or something (minus the bottom end).  I was thinking I could just switch a similar resister in or out with a foot switch, but the tone and volume are different at different gain levels.

So I built two of these circuits, which I'll put in one box with only a switch between low gain and high gain.  I'll update this thread when I've finished and post some samples as well. 

[Barcode80]

also, regarding the flux thing, i got the same out of my over-modded and over-re-modded sd-1. crackle, pop, fizz, constant and excruciating to hear. applied the soldering iron to the flux to let it burn off, pedal sounded great! this little tip needs to go in the debugging thread.

[Paul Marossy]

also, regarding the flux thing, i got the same out of my over-modded and over-re-modded sd-1. crackle, pop, fizz, constant and excruciating to hear. applied the soldering iron to the flux to let it burn off, pedal sounded great! this little tip needs to go in the debugging thread.

This is kind of inexplicable to me, but excess flux appears to have been the root of several strange problems I have had with some circuits. I don't know how this can be, but experience has proved that it can do something weird. I have heard one suggestion about flux being hydrophyllic, maybe it's true. 

[Barcode80]

actually, i would think it would have to be conductive, or else how would the joint be conductive if you use flux on the board first?

[John Lyons]

Flux is Hydroscopic (think that's the word...) Meaning that it attracts water.
Water is conductive or course. If you have a flux bridging between traces and components on the board and the flux attracts water out of the air...then you effectively have small capacitors between components, if not small small resistances and possibly shorts (after time that is).

Moral of the story: Clean off the flux with Rubbing alcohol and an old toothbrush or take something small and sharp (a meter probe works) and scrape off the flux in all the "nooks and crannies" of your board.

[Paul Marossy]

Flux is Hydroscopic (think that's the word...) Meaning that it attracts water.
Water is conductive or course. If you have a flux bridging between traces and components on the board and the flux attracts water out of the air...then you effectively have small capacitors between components, if not small small resistances and possibly shorts (after time that is).

Here's a weird case in point: My friend's Mesa Boogie Nomad would act erractically whenever it would be on "channel 1" - it would randomly try to switch to "channel 2" and stuff. I took the footswitch apart and saw huge amounts of solder flux around the LED indicators. I simply cleaned off the flux and the amp worked normally again. Weird, but true!