Question on a little more gain in a drive section

[markmalin]

Hello all!


I took a typical D-style Overdrive circuit (the 2 gain stages used in the overdrive) and separated them out into a stand alone circuit.  Guitar signal in, through the OD stages, then out to the amp's input.  I've posted the original standard D OD below, and a version of my modifications to get it to function better as a stand alone overdrive.  Stuff like:


- removing the 220k resistor R1 to allow more guitar signal in
- changing R4 to half the value to give more gain into stage 2
- changing C1 to a Fenderish 25uf to fatten it up (i.e. allow more of the guitar frequency range through)
- added a voltage divider (R10 and R11) to lower the signal size going into the second gain stage so it wasn't so loud.
- added a "bright" bypass cap on the voltage divider (C7)
- added a simple tweed style tone control (this is an idea I got from Tubenit a long time ago)


It's sounding pretty cool as an overdrive as is, but I wish it had a bit more overdrive.  A little more extreme, or more distortion/sustain when the "Drive" is cranked.  Can increase that without increasing overall volume - is there an easy way to have the first stage overdrive more?  Honestly, it's PLENTY loud even with the voltage divider. 


Thanks guys!

Humbly,
Mark

[SILVERGUN]

The ODS OD section works in conjunction with the clean channel and actually does a great job of knocking down the excessive gain late in the circuit. You can't really snip out the OD section and get the D tone.

With that said, there is no reason you can't use 2 triodes to OD the front of an amp. Just remove some of the things that are holding it back and you should have plenty of gain for the job.

The first thing I think you should fix is the network of R9, R10, R11. You have created an approx. 20:1 voltage divider and I don't think that's what you intended to do.

[markmalin]

The ODS OD section works in conjunction with the clean channel and actually does a great job of knocking down the excessive gain late in the circuit. You can't really snip out the OD section and get the D tone.

With that said, there is no reason you can't use 2 triodes to OD the front of an amp. Just remove some of the things that are holding it back and you should have plenty of gain for the job.

The first thing I think you should fix is the network of R9, R10, R11. You have created an approx. 20:1 voltage divider and I don't think that's what you intended to do.

Thanks, Silvergun.  It's been an interesting experiment.


The 20:1 resistor network was to limit the output level.  It was crazy loud and it was hard to turn up the Drive to the point where it would overdrive without it being super loud.  So I stuck a 1MA pot in there as a voltage divider.  As you can see, it was nearly at 10 before it quieted down w/out changing the tone too much.


When you say "remove some of the things that are holding it back" do you mean like R9/10/11 and R4?


Mark

[SILVERGUN]

When you say "remove some of the things that are holding it back" do you mean like R9/10/11 and R4?

The effect of R4 is negligible, so I'm not too concerned.

I would think that your 100K input pot would usually be maxxed out to get max gain out of the first stage. I'd be more inclined to change that to a Fixed 1M resistor and set the input impedance, since you know your trying to max out the first stage anyway, the pot seems questionable. (and maybe just cut and pasted out of the original circuit?)

I'd go for less of a voltage divider at R9,10,11 and maybe make that the output pot with a lower value, getting rid of the 1M and 70K  setup.

Either way, there is something wonky about the way you have the 250K pot and the tone stack drawn. If you were going for the tweed setup, something is amiss, but I'm looking at my phone and my brain hurts.

I'd also be tempted to set the output impedance lower and maybe just use a 50K pot as an output control.

I'm no expert and these would just be my experiments to see what is what. It's pretty far off of where you've gone and I don't want to steer you wrong, so take it with a grain of salt.

[shooter]

I'm with SG on your 250k volume?
another quicky, clip a 3.3k in parallel with the cathode 3.3k (R5?), If you play hard that should induce some asymmetry, if it's the right direction, take it down to 820 ohms n test.

[markmalin]

The effect of R4 is negligible, so I'm not too concerned.

I would think that your 100K input pot would usually be maxxed out to get max gain out of the first stage. I'd be more inclined to change that to a Fixed 1M resistor and set the input impedance, since you know your trying to max out the first stage anyway, the pot seems questionable. (and maybe just cut and pasted out of the original circuit?)

Correct - the 100k input pot is from the original circuit.  I find if I turn it up too high, I get what I would call a harsh distortion if I have the Drive up too high.  I realize the original circuit was coming from the 2 gain stages (and tone stack) in a Dumble, so the 100k pot is probably not needed.  You typically dial that in to get the right OD, often about 1:00 - 2:00.  I can certainly remove it and put in a 1M fixed resistor.

I'd go for less of a voltage divider at R9,10,11 and maybe make that the output pot with a lower value, getting rid of the 1M and 70K  setup.

Either way, there is something wonky about the way you have the 250K pot and the tone stack drawn. If you were going for the tweed setup, something is amiss, but I'm looking at my phone and my brain hurts.


I'd also be tempted to set the output impedance lower and maybe just use a 50K pot as an output control.

The tone control looks a little strange here, I realize.  In parallel with the volume pot.  I stole this idea from something tubenit had talked to me about for adding a tweed style tone control after a D-style OD section to be able to change the OD tone to match the clean tone more closely.  If I want to be able to adjust tone, does it make more sense to put a real tone stack between the two gain sections?  Granted, it works this way, but I see how this looks confusing.

Also, the Volume pot being 250k, that's essentially the "Level" control from the original D circuit.  I have not played around with different values - my goal is to get it to not be so loud...

I'm no expert and these would just be my experiments to see what is what. It's pretty far off of where you've gone and I don't want to steer you wrong, so take it with a grain of salt.

it's all good - I'm kind of playing around, really, trying to understand how to make the first gain stage drive harder, and the second quieter (so it's not such a loud circuit).


Also - shooter - good thought on clipping in a 3.3k in parallel on R5.  Thanks for that thought. 


Again, thanks guys. 
Mark

[SILVERGUN]

I'll ramble a little bit and maybe say something that means something along the way.
I've heard this before: "use the OD section of a D style amp" and then refer to the 2 triodes that get switched in as the source of the OD tone. Sure they add distortion and gain but I don't see that section of the circuit as the sole OD section. The signal still passes through the first 2 gain stages + tone stack where the TONE initiates. (and plenty of gain)

I remember the first time I switched in the mid boost on the clean channel and turned it up some and I was getting the harmonic bloom effect without the "OD" section switched in.
It was a pleasant cleaner OD that I found to be more controllable. I always wanted to go back and re-visit that as a standalone amp.

So, what I'm saying is....if you look at your setup as a D "clean channel" pre-amp into your existing amplifier as the OD section it will change your perspective of how this can be done.

I think you might be better suited to setup the preamp with the actual D tone stack (maybe fixed switches to your preferences - for me it was Rock/Jazz = Rock, Mid boost = on, Bright = off)

Do you get where I'm going with this?

[markmalin]

I'll ramble a little bit and maybe say something that means something along the way.
I've heard this before: "use the OD section of a D style amp" and then refer to the 2 triodes that get switched in as the source of the OD tone. Sure they add distortion and gain but I don't see that section of the circuit as the sole OD section. The signal still passes through the first 2 gain stages + tone stack where the TONE initiates. (and plenty of gain)

I remember the first time I switched in the mid boost on the clean channel and turned it up some and I was getting the harmonic bloom effect without the "OD" section switched in.
It was a pleasant cleaner OD that I found to be more controllable. I always wanted to go back and re-visit that as a standalone amp.

So, what I'm saying is....if you look at your setup as a D "clean channel" pre-amp into your existing amplifier as the OD section it will change your perspective of how this can be done.

I think you might be better suited to setup the preamp with the actual D tone stack (maybe fixed switches to your preferences - for me it was Rock/Jazz = Rock, Mid boost = on, Bright = off)

Do you get where I'm going with this?

I think so.  I've already built a Dumble preamp section, tone stack and all, and it sounds fantastic going in to the front end of another amp.  I wanted to see what I could do with a single tube (2 triodes) as an overdrive, so I took the Dumble as an example...wondering how much of the Dumble OD tone is in that part of the circuit.  You're right, as a stand alone section (verbatim) the Dumble dual triode OD section separated from the rest of the circuit doesn't really cut it.  Plus, left as designed it's too loud for me - I don't need that much signal going into the front of my amp...so I started messing around with it trying to get more frequency response with a larger cathode bypass in triode one, and started going from there.


There are so many opinions of what "Dumble tone" is, isn't there! :P  Anyhow, the idea was to play around with 2 gain stages.  The hack/changes I made have gotten it to where it's a nice sounding OD pedal for bluesy overdrive, running into a clean amp, but I'd like to try to get more drive and less volume.


I appreciate the input, especially regarding the impedance changes.  And I appreciate your thoughts on considering this a clean gain section to drive the amp.  It's just been fun playing around.  I have a ways to go before it's usable, obviously.


Mark

[SILVERGUN]

There are so many opinions of what "Dumble tone" is, isn't there! :P

These are the some of the ones that stand out to me:
https://www.youtube.com/watch?v=C8V6OUUYgLI

https://www.youtube.com/watch?v=L0eJFATTY7k

For high gain (ignore the mixed in Marshalls)
https://www.youtube.com/watch?v=4Z-EfcQeifc

[markmalin]

There are so many opinions of what "Dumble tone" is, isn't there! :P

These are the some of the ones that stand out to me:
https://www.youtube.com/watch?v=C8V6OUUYgLI

https://www.youtube.com/watch?v=L0eJFATTY7k

For high gain (ignore the mixed in Marshalls)
https://www.youtube.com/watch?v=4Z-EfcQeifc

Yup.  Gotta love Larry :)   And Robben :)


Like I said, I wasn't so much trying to recreate a Dumble just with the 2 gain stage OD section, but wanted to use that as a basis for messing around.  I'd like to be able to get this level of overdrive/distortion from it...it's close, and use whatever amp you run it into to shape the tone.  It's just not quite enough overdrive in the first stage, and way too loud exiting the second stage for a small device to drop into your pedal chain as it is drawn in the picture above.  I like the idea shooter had of trying some smaller cathode resistor values in gain stage 1...and I'll work on the funky voltage divider and pot at the other end...


FWIW, here's a clip from the preamp stand alone D-type preamp.  Excuse the cellphone mic quality sound.   

[pdf64]

To do what you want, another gain stage may be necessary.
To get a smoother overdrive out of a cathode cathode clipping stage (at the cost of losing a few dB of gain), trying removing or reducing the degree of, cathode bypass. ie lift C5 or put a 1k resistor in series with it.
The high values of the output vol / tone controls will create a large change in output impedance according to the control settings. This will then interact with the capacitance of output cable and form a low pass filter, whose corner frequency varies according to the particular cable and control settings used.
It seems an unusual and 'quirky' design choice to do this, when a consistently lowish impedance output (which greatly reduces this normally unintended variable) is an equally simple option.