Dr boogey with Gaussmarkov´s layout build report and issues/questions

[dschwartz]

Hi all,
I finished my last Dr boogey using gaussmarkov´s layout, i used all j201, and taped pots to get 2.5k.
It sounded rightaway!! i set the bias trimpots for best response by ear and got a very nice distortion.

Mods i did:
- Dual switchable volume pot
- presence cap up to 100n (to cut more highs)

results:
- Not noisy at all!!!
- no oscilations
- background hiss totally acceptable, in fact really good!!!
- great sound, fat crunch, silky distortion and screaming harmonics.

Issues:

- even with 5 j201 i felt it had about the half of the gain than the olcircuits layout.  I tried different j201, but the gain was never as high as the olcircuits (old) one.do this layout have a gain reducer?? how do i increase the gain?

- the mid control is barely effective...no mid scoop available i´m sure i have the pot and caps right, and the old version have the same thing. how can i make this control more apparent???

- the treble knob seems to boost the low mids when turned down..at full treble feels like low mids are gone (they are definetly cut)..is the pot allright or this is how it works?

bottomline:
GREAT build!!!! it sounds and feels like a real amp, but it´s improvable, specially the tonestack.

[Bucksears]

I'd like to know as well since I'm about to populate my Gaussmarkov board.

I'm assuming you did the (10%) scaled-down tonestack values?

- Buck

[dschwartz]

yes i did..i modded the pots with parallel resistors to get the values...(BTW this changes the taper of the pot, but works).

i´m thinking about an output buffer anyway, the scaled down tonestack its still high impedance..

but i still wonder why with the treble knob below 12 o´clock there´s a big low-mid boost.

[ubersam]

Congrats on the build!! Just a couple of thoughts: When you adjusted the trimpots "by ear," you might have affected the gain as well. In my build (my own problematic layout), I only adjusted the input stage by ear, and only to help control the oscillation, ending up w/ about 7.5V drain voltage. I made the drain voltage of the third stage switchable between 4.5V and 7V, the rest of the FETS are set at 4.5V.

As far as what you are experiencing with the tone control, it sounds exactly like how Mesa Boogie tone controls behave. That is how the tone controls in my Road King II behave. They interact with each other, not so much like a boost/cut kind of control. The mid control doesn't really scoop out the mids, it just sets how 'present' or 'pushed back' the midrange is, and is affected by how the treble control is set. According to the RKII manual, the treble control also acts as another 'gain' control, as in, the higher it is set, more of the overall signal is passed on to the output*. If you download the Dual Rectifier or even the RKII manual from Mesa Boogie, and read their explanation of how the tone controls on their amps work, you'll have a better understanding of what is going on with the DRB's tone controls. IMO, there isn't much one can do to improve it other than implementing a completely different tone control.

*= I just had an idea about why you are getting lower gain. It may be because of the larger treble bleed cap that you put in. Try changing it back to the original value and see if that increases your gain.

[Bucksears]

I have to disagree regarding the tone stack, but YMMV. On the original (I still have my original from my layout boxed up), the bass/mid/treble/presence have a lot of range. I've often said that the DB has the most usable eq of any that I've used; moreso than the Boogeyman, Umble, or any Marshall-based pedal. My DB Ultra that I've got in the works has a footswitch that bypasses the mid control to a fixed scoop mode.

- Buck

[dschwartz]

i don´t think that the treble bleed i used on the presence control affected the gain, since it is located after every active amplificaction, so all the clipping has already been made (i dont intend to overdrive the input of the amp), it simply made the presence control extremely effective (i use it as a secondary treble control).

I first tested it with a 10n bleed cap and the sound was too bright to my ears and it had the same lack of really high gain, and the presence control only seemed to do something when the treble was at full.. BUT at that setting i loose all bottom end (the treble seems to cut the mid-lows above 12 o´clock.

After changing that cap, i noticed the lack of gain so i changed r4 to 470k and r7 to 220k expecting more gain..it helped a little, but is nothing like the beast i build with bucksears layout..

I REMEMBERED something.. i tested the 1 M gain pot wired...it yielded 600k at max....is my pot right or is just that  2.2M resistor parallel with ground?

[John Lyons]

The tone stack is not that high of an impedance. Typical padals are around 100K ...A buffer won't make too much difference I don't think.
You need to adjust the drain trim pots with a meter. You can adjust them by ear but it's not accurate if you are going to compare two different builds. Jfets are always different gains. This is why the Dr boogie is a hard pedal to get right consistently as with all jfet cascaded high gain circuits.

Clarification: The OLC circuit is not a Dr boogie, it's close but more based on the Soldano amp circuit.
Buck sears layout is electrically the same as gaussmarkov's layout except for the tone stack and power filtering, there should be no gain differences other than the differences inherent in individual FETs.

The tone stack is scaled by 10. There should be no difference other than the output impedance (which is why the TS was scaled ion the first place)

With the marshall/fwender/vox Treble Mid Bass tone stack moving one knob will affect another. To see how this works you can download Duncan's Tone Stack Calculator and enter the values for the DR Boogie's tone stack. Then slide around the controls and see what the wave shape does.
The only pot that needs to be made 2.5K is the mid pot, and this is linear. Putting a 5K resistor across a 5K pot will give you 2.5K and still give a linear taper.
If you tapered some pots other than the mid pot that may be the problem with your EQ settings. The taper of the pot will alter the EQ shape at "flat" setting with knobs straight up.

John

[dschwartz]

i tapered the treble knob with two 3.3k and a 10k linear pot, for the mid i used 33K resistors and 100k linear pot, that gave me a 2.4k pot and 24k pot respectively.
i can live with the strange eq..the previous DB i did have the same thing.. it´s only that it doesnt work like a standard tonestack..anyway is VERY versatile.

Now...the gain issue.....my multimeter has the Hfe function.. can i use it with fets to read the current gain?
i measured a bunch of them and read between 60 and 70 of Hfe... is that a correct reading??? it´s normal that low Hfe?

[ubersam]

i don´t think that the treble bleed i used on the presence control affected the gain...

Oops... my mistake... I was thinking about the treble bypass cap on the gain pot.

I have to disagree regarding the tone stack, but YMMV. On the original (I still have my original from my layout boxed up), the bass/mid/treble/presence have a lot of range. I've often said that the DB has the most usable eq of any that I've used...

I don't think that the DRB's tone stack needs any improvement. I personally like its behavior just the way that it is, just like the tone controls on my Boogie amp and preamp. If one desires a different behavior or response from the tone stack, where the controls do not interact with or affect each other, an active baxandal (sp?) type of tone controlling scheme might be better suited for them.

[dschwartz]

last night i selected the fets with most Hfe, (between 200-250), and biased by ear to get the max output from each one (biasing at 4.5 doesnt always give the max output from each stage)..it didnt help too much..

what if i change (lower) the 470K series resistor after the gain pot??
It looks like a LP filter so i´ll be boosting higher freqs, maybe that will give me a gainier feeling...

[Victor]

fets don't have hfe
fets have gm

in a common emitter fet config, emitter bypassed by a cap (for AC analysis), gain on stage should be = -gm.Rdrain

[dschwartz]

hmm..
then what did i measured with the MM??

how can i measure fet´s "gm"??
there´s no info on the net about it...