JM Overdrive Special

[fred.violleau]

I have this old chassis I bought many moons ago.

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It's been sitting around for a while and I came up with this idea and thought it would be cool to challenge it with you valuable experiences and insights.

I have 6 preamp tubes available

Topology of the amp could be as follow :
- Channel1 : clean ala JM Wonderland
- Channel2 : #102 (clean with OD on switch)

I would use a musicman PT from a 212 HD 130

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And I would use a Hammond 1650R Output transformer
I thought I could use EL34 and/or KT88 to support the 700v on the plates

Questions :
Is the Wonderland reverb a keeper? I read it was super cool, but I am wondering about the tuning necessary with the ODS side ... Does anyone has knowledge around this?
I could just add the drive side of the ODS onto the JM clean channel (Did anyone attempt this ?)

Any suggestions ?

Fred.

[10thTx]

Are you saying you want the JM clean and the 102 OD ……………… AND the reverb from the JM? IF you're wanting the reverb also, I think the challenge
will be the insertion points for the reverb once you include the OD channel?

IF you could post a schematic, it would be easier to make a comment.

I think you could do the JM clean, OD from 102 and a Fender style reverb prior going into the LTPI and that would likely work.

My Carolina Overdrive Special Reverb has 3 gain stages in the clean like the JM. Has 2 gain stages in the OD like the 102. And uses a one tube reverb just prior to the LTPI.

You could use a 12AT7 paralleled for the reverb driver similar to a Fender reverb.

Obviously, the clean and OD topology and values are different than the JM or the concept you're suggesting. I am simply posting a schematic as a reference point of the type of thing
I was suggesting to consider.

Look carefully at the reverb on the JM schematic at how the insertion points are set up and you'll see the challenge. Just some food for thought.

with respect, 10thtx

[10thTx]

Maybe something like this ?? Wouldn't have the JM lush reverb but could still have a great reverb sound.

With respect, 10thtx

Attached a SCH editable version that you could use to illustrate what you're thinking?

[fred.violleau]

@10thtx,

Thanks for the schematics, it really helps in order to have an educated discussion.
I am sometimes (often?) excited about ideas and make connections before studying thoroughly the available documentation.

I now realize that the reverb circuit in the John Mayer amp is being fed very early in the preamp, and is then mixed back in right before P.I.
It makes the insertion point for the OD, or other channels, harder to organize and get right.

I might then just re-use the JM clean channel and go for a one tube reverb. I need to review your Carolina 3 schematic.

Thanks again for input, much appreciated!

Fred.

[rootz]

Hi Fres,

Interesting ideas you're trying to accomplish here. You are right, I think, by recognising the insertion point between 10thx's design and the JM amp are different. Off course the JM is just a clean preamp, so it is okay to take the signal from the gain pot wiper to the reverb entrance. Were you to this with an amp with an OD stage, the wet side of the reverb would be always clean. Depending on the mixing point you'd get either a reverbed (for the lack of a better word) OD and clean dry sound mixed OR a mixed clean sound that then goes through the OD stage. I'm pretty sure those designs already exist.
I'm not a big fan of pre-OD time based effects, so for one of my latest builds, I opted for something reminiscent of the JM amp with an added OD, while keeping the same number of preamp tubes. Essentially I dropped the idea of taking the reverb input from the gain pot wiper but instead took it from where normally the master volume pot would be, so right after the clean/OD relay. Also I left out one triode from the parallel driver, while keeping the 12AT7 there. The first reverb recovery is now also half a 12at7. Because it is followed by yet another gain stage for the wet/dry mixer, you can still get all the reverb you wish for. At the same time the dry mixer stage forms the third gain stage like in the JM amp. Actually, there is the infamous 1meg/68k voltage divider right in front of it (I opted for 1meg/82k to get a bit more clean gain). Not to sound overly arrogant, but this is still the most elegant design I saw to incorporate reverb in an OD amp. Why? All added stages feature relatively wide frequency response at relatively low distortion due to local feedback loops, the reverb stages and mixer are very reminiscent of designs Dumble uses (and Two Rock for that matter), reverb is done after OD, frequency response with the reverb stages added is the same as (or at least very close to) as an amp without reverb (not withstanding the effects of parasitics and lead dress).

The attached screenshot is from LTSpice. It compares my design (#124 based), the JM amp and a #124. All on the clean channel. The JM has a very different frequency response. This is due to differences in presence vs. contour and different global negative feedback resistor values mostly. The Miller capacitance from the extra grid on the gain pot wiper on the JM amp steals a tiny bit of high end in that design. Also high end loss due to the voltage divider on the dry mixer stage is not compensated on the JM amp. So it should be a bit darker everything else being equal when compared to a #124.

[fred.violleau]

Hey Rootz,

Thanks for sharing all this info!
It looks very close to what I am after!
Did you record anything by chance?
Your build looks stunning, and must sound stellar!
I will dig deeper on the subject on your LTE Spice simulation. Very interesting to read that frequency range was very similar to an amp without reverb.

I read very mixed reviews about #124 (Marcus did not like it) and others are raving about it. What had you choose this one for OD? And if you have built other ODS, how does compare to the others?

Thanks again for your post, much appreciated!

Fred.

[rootz]

Totally missed your reply Fred! Didn't subscribe

Anyway, there are some files on my YouTube channel and in soundcloud. These are in my post about these amps I built and should be easy to find.

Currently my amp that used to be a #124 preamp, is updated to a #102 overdrive channel, but with a classic tone stack. So it's not a reference point to any real Dumble any more. I never quite bonded with the Skyliner stack for single coils. And I mostly play singles or Filtertrons. I liked low plate classics the best with single coils. There is more life to the classic stack with single coils I think, but OD is much rawer with low plates too. I really liked low plate classic topology for more rock stuff.

I'm not sure how I like the hybrid amp it is right now. It's a bit smoother, but I also changed the speaker from a Neo Creamback to an ET65. I like what I hear, but this combo is nowhere as clear and bright as my 100 Watter that is based on #94, formerly Gregor Hildens amp. That amp is a high plate classic and is paired with a EVM12L. So it's a different setup altogether.

[rootz]

Oh, and off course thanks for the kind words about my build. Here's the low plate in a head shell. Currently my 100 Watter is in the top and the other amp is in a combo. Both have faceplates now.