John Mayer Non Reverb Project

[johngovan]

Good day TAG. Advance happy New year. This coming year, i would like to build a John Mayer Non Reverb Amp which is a stripped down version of Joyful Music but without a reverb section. I will begin my amp building journey as a goal this coming year 2020. Here is the layout and pdf file of what i have mind. I also get this file here in the forum. I just make some modifications to this file and remove the reverb section.

Since this amp uses 1 1/2 12ax7's in preamp section. Can you give me some advice where can i use the other half in the second stage? Can i use that half as cathode follower option like the SSS #004 preamp stage? Or i will just live it alone unused which i think is not practical.

[Bombacaototal]

What you have to keep in mind is that the reverb path leaks signal to ground tamming the high end. Also the final sound is the sum of the dry path, which is brighter, and the reverb path which is fatter. Your amp in the end will be very different to the original. I spent months fighting an amp which I took a similar approach....

[johngovan]

It's okay for me to make it sound a little brighter and not as the exact copy of the JM amp. It will be more expensive to build a JM with reverb and i don't have a lots of money to build the whole amp. I will make this one a small wattage amp. But as for now i just want to know if it will still sound good. Maybe i will try or maybe i will quit the concept and make a Princeton non reverb amp. I'm not sure.

[Bombacaototal]

It's okay for me to make it sound a little brighter and not as the exact copy of the JM amp. It will be more expensive to build a JM with reverb and i don't have a lots of money to build the whole amp. I will make this one a small wattage amp. But as for now i just want to know if it will still sound good. Maybe i will try or maybe i will quit the concept and make a Princeton non reverb amp. I'm not sure.

From my experience it will not be a little brighter, but a lot brighter.

[rootz]

In my experience the grid stopper on the first reverb triode in conjunctions with its Miller capacitance load down the high end early in the preamp. You could simulate that with a resistor and cap to ground. At least that will look like a reverb in 0 at that point.
That bright cap is awfully big. I never used a 470p bright cap in my Wonderland clone.
The loading of V1b is wrong. First the 1meg resistor, then a 68k to ground. This will dump a lot of signal, maybe too much to your liking, but is how it is done in the original amp.
I think with the reverb set to 0 (or no reverb at all, like in your case) the loading of V2 should be different too. Keep in mind the anodes are tied together at that point in the original amp. Thus the plate resistor or the reverb mixer stage forms an AC path to ground via the power supply. Not really sure what the effect is though.

[johngovan]

Thank you Rootz. I admire your builds here in TAG. I see some mistakes in my schematics. I'll post a revision again. I hope i can build a similar amp without reverb with an additional few components just to compensate for the removal of the reverb part. Though i know that reverb section adds a lot in terms of tone into the circuit. But i hope i can made one nice amp that was a stripped down version and stripped down cost.

[10thTx]

Can i use that half as cathode follower option like the SSS #004 preamp stage?

It's okay for me to make it sound a little brighter and not as the exact copy of the JM amp

Just an idea for your consideration given 1) your doing something innovative knowing it won't sound like a JM but could be a great sounding amp in it's own right and 2) you asked about the unused triode.

I've done this mosfet CF (but with a tone stack following) several times as the "send" of an active FX loop. It has worked well and to my liking! One of my amps has this idea and I think it sounds lovely.

Then you could put a really nice reverb and/or delay pedal in the active FX.

With your amp idea, I simply put a 220k in place of where I've used a tone stack. I might also use a 2.2uf with V2-3 cathode bypass cap.

With respect, 10thtx

[johngovan]

Thank you 10thx for the inputs and the generosity of sharing your thoughts and schematics to those who needs one. I'll try consider that one or your kinglet 6v6's. Is fixed bias more dangerous than cathode bias amp?

[rootz]

Both are perfectly safe when you work safe. Both are perfectly safe for your power tubes too, though cathode bias is a bit easier to set up. Fixed bias is a bit more powerful and in most cases needs a output transformer with a slightly lower primary impedance.

What is your planning for the power section? Something like 6V6 push-pull? Or even smaller?

[johngovan]

I'm planning to use 6v6's in push-pull mode. For this amp will be use for practice band set-up like a small room practice setup. I don't want to use 6L6's for it is too loud. I saw that EL84's are much easier and lesser vPT voltage, but as i was reading forums after forums, they say that EL84'S are brighter and has more earlier brakeup than the 6v6's. So i think i will go to 6v6 route. But i hadn't made a fixed bias amp. So this is my first time trying a fixed bias power stage.

[rootz]

I don’t think EL84’s break up earlier. They do need a lot less drive from the PI. Pentodes generally behave this way when compared to beam tetrodes. 6V6’s behave more like the bigger bottle 6L6’s in that respect. JJ 6v6s tubes can also take higher voltage than EL84’s can. This means the power supply can be more like a real Dumble, with +/- 300V on the PI plates. All things considered I think 6V6 tubes are a more logical choice.
What power transformer are you going to use? Something that gets you around 420V on the plates of the power tubes?
If you want, you could make the amp switchable cathode and fixed bias. Have seen the before. Costs you a switch, power resistor and bypass cap, couple of bucks. Cathode bias should be fun. A little more compression and overtones when compressing, a little less power. An 8k primary would be fine I think for both.

[Bombacaototal]

I think with the reverb set to 0 (or no reverb at all, like in your case) the loading of V2 should be different too. Keep in mind the anodes are tied together at that point in the original amp. Thus the plate resistor or the reverb mixer stage forms an AC path to ground via the power supply. Not really sure what the effect is though.

This actually didn't occur to me. Should one place another 100K plate to ground to simulate the loading of the other side (ie reverb mixer) of the anode?

[rootz]

Actually, no idea! It just looks to be an obvious loading point to me. I can have a look in ltspice to see how it loads the mixer stage and if it really affects anything. It probably does, although not clear to what extend.

Edit. Will make a comparison, but have to change a flat tire on my car first. New year started good with a flat tire from some burning wood with nails on the middle of the road.

[rootz]

I figure something like this would work to get more to the original sound without reverb. Loading the volume pot wiper down with the 68k resistor in series with the Miller capacitance of the first reverb triode. Miller capacitance should be Cin = Cgk + Cga(1 + A), or more simple A*Cga (good enough as a close approximation). S0 1.7pF*65=110pF. That excludes stray capacitances, so it would actually be higher. I would test with caps ranging from 120p to 220p.
The loading by R17/C14 provides the other missing part from trimming out the reverb. What I don't get is that the loading should be higher. Plate resistance//plate resistor -> 120k, so 37k5 + 120k = 150k. This does not provide enough loading however. Maybe someone can chime in where my logic fails me.

[johngovan]

This is interesting. Thank you for your suggestions and advices. I really appreciate it. As of now, what i have in hand is a PT with 250-0-250v with 50v bias tap and 6.3v filament tap. And OT with 8k primary impedance and a 4 ohm secondary. Okay. I'm on it. I will use 6v6 power amp stage. And try to learn biasing a fixed bias amp. I will try to search for step by step biasing. I watched a video on the YouTube of TubeDepot biasing a fender hot rod deluxe. Maybe it can help me in understanding how to bias a fixed bias amp