Dr Boogey - Squeel Remedy?

[rtill]

I think that will help me out a lot.....
tonight I was destined to find the cause of this squealing....so, I took out the 1st stage altogether, put my input right before the 470k resistor before stage2 (Q2).  No squeal at all...just got a dirty old fender type overdrive sound (could be a good bluesy sound).....so made up a quick lpb boost circuit and wired it before the 470k resistor to act as the 1st stage.....guess what......no squeal at all.... sounded like a marshall stack at half gain, so I'm thinking (at least in my 3 builds of this circuit)  the squeal is from the Q1 stage....I'll experment witth different values on the caps and resistors using the basic idea of stage 1 ..... to get the meaty high gain sound like a dual rect.   I think your idea of a 500k pot for gain may do it....I really appreciate everyone's input on this!

[John Lyons]

The first stage has a few things going on. the 2M2,.002 and parallel 680K is a tone shaping network which is mid/treble peaking which sets a high freq tone. I'm not sure if it throws off the eq curve but possibly lowering the 2M2 would get you less gain through that stage.
The .001 cap on the gain pot is a treble bypass cap that lets through more treble as the gain pot is lowered. At high gain settings it is out of the circuit effectively.

I'm going to have to take a look at my Mesa schematics and see which circuit the Dr follows...Anyone know if it's the orange or red channel?

[ubersam]

...This soundclip was recorded a little dark but there is plenty high end on tap.
http://www.mrdwab.com/john/Drboogie.mp3...

That settles it... I'm building the Dr next..

[MetalGuy]

When looking at the original DR schematic and the switching matrix one can see that the 680k/2n combo is either paralleled with the 2M2/82p one or out of the sound chain. In other words there's no such mode where the sounds passes only through the  680k/2n combo. When both are paralleled you get  519k/2.082n.

[John Lyons]

Ahh!! Pretty sharp there Metalguy

I pulled out the scheanmtic for the DR and you are right. I'll have to try swapping those out. Doesn't seem like a lot of difference between 519K/2.08n and 680K/2n2 but maybe a slight eq shift. Although it must be enough for Mesa to set it up that way.

The 2M2 reisitor and the 519k (or 680K...) form a voltage divider. If you lower the 2M2 you'll get lower gain which may bring down the squeal and give you more gain range.
Even with a 500K pot I get a pretty small gain range. It's pretty much full gain and when rolled back almost to zero a nice rock sound still with a lot of gain. With the stock 1M it's got lots of gain and then when dimed way too much gain and overload oscillation city!

Another thing to try out is swapping out the 680PF cap in the tone stack for a 500PF which is a common marshall value and will give you less mids (or more treble and bass).

John

[rtill]

The first stage has a few things going on. the 2M2,.002 and parallel 680K is a tone shaping network which is mid/treble peaking which sets a high freq tone. I'm not sure if it throws off the eq curve but possibly lowering the 2M2 would get you less gain through that stage.
The .001 cap on the gain pot is a treble bypass cap that lets through more treble as the gain pot is lowered. At high gain settings it is out of the circuit effectively.

I'm going to have to take a look at my Mesa schematics and see which circuit the Dr follows...Anyone know if it's the orange or red channel?

This explains a lot about what is happening in this circuit....thank you soooo much, I would have never guessed what these values do....

[rtill]

In regards to the 1 uf cap to ground in the first stage, what does this cap do? if lowered to a .47, what would happen? if a higer, like a 2.2 was in place what would happen?

[John Lyons]

In tube amps they are called cathode bypass caps. There affect gain and frequency. A larger cap will let more gain through and more bass.
A 1uf cap lets through mids and highs while a 25 uf will let through most everything. A 2.2 uf will let through slightly more bass that a 1uf.
Keep in mind that in the first stages of a high gain circuit you want to control the bass quite a bit so the sound remains articulate and sensitive to picking and touch. If you has 25uf caps on all the bypass locations the sound would be muddy and pretty gross and not sensitive to dynamics. COntroling bass is a good thing in these multy stage amps and er...emulations.  A .47 or.68 would be pretty close to a 1uf and would cut off a little more mids while passing highs more that mids and bass. Since there are a few stages here, one bypass cap isn't going to be very noticeable in the whole of the circuit. All the byapss caps add up the the sound you get. There are other factors for tone shaping as well...

[rtill]

used new values for the 1st stage on all the components...added the 220pf cap  as mentioned, .....etc.  squeal is gone!!!!!!!!!!!! ONLY if the volume is around 1/2  or lower...so...
Another question....I realized I used a 100k pot for the volume. this circuit calls for a 1M....could this be the cause of squealing with volume over 50%?

[John Lyons]

rtill
Actually the opposite is true. The 1 meg pot means 1 megaohm resistance from ground which is 10 times more than 100K. The more resistance to ground = more gain. I started with 1M, then changed to 500K. I was thinking of using a 250K to see what the gain range would be but I'll stick with 500K for now. Make sure to use an Audio/Log pot, otherwise you will get too much gain too soon in the pots rotation.
With the Things I've done mentioned above I can get about 3/4 or more gain setting, and not squeal, which is plenty of gain for any metal playing!

john

[rtill]

Finally got the sqeal to go away......shortened the output wires to the jack to 1 inch whereas I had them down to 2 inches.  Squeal is gone even when cranked to 10!!!!!!  This pedal rocks!!!!  Thanks for all your help!!!!!