I posted this in another thread but thought it was worth a new one so people would see this as I feel its the solution to the error in the original layout.
When i built mine I always thought it wasn't quite right gain wise, seemed quiet for a 100W amp and i always thought it was the 68k-1meg divider that was the issue regarding loss of gain, it was in fact the mixer in the resistor circuit. the original AN layout shows a 120k there, I swapped with Ryan's latest version of 220k and boom...it's how I always thought the amp should be level/gain wise , I've no need for the variable gain pot on the back now the amp is lovely and loud.....
thank you guys for finally getting to the bottom of this. I would urge everyone who's built a wonderland in the past to modify their amps ASAP ,especially if you're @50w
I changed the cap on the reverb too and added 300R to the bass pot resistor to bring it in line with the new measurements. 1.82k
check out ryans updated version of the JM wonderland for the correct values to transform your amp into what it should have been
I noticed too some variation in the resistors exiting the mixer tube plates (to Master).
Some schems had the 68k on the reverb side, some the other way around and Ryan layout have a 68k on the Dry side and 220k on the reverb side (120k on previous layouts).
Edit: I now understand your point. The 1M/68k is good but that 120k on the reverb mixer should be 220k and that swap resolved your issue. Will try that in a few hours.
Marcus do you like higher R values on the power supply better than stock ?
Same with Cut control, did you kept it or switch it off ?
Last edited by mojotom on Mon Apr 26, 2021 3:46 pm, edited 1 time in total.
definitely better with 220K
on my JM I totally removed the cap from Return pot, much better and clearer sound IMHO
I put switch on Master volume so I can choose between no cap,47pF and 68pF, also LNFB switch on the back
Just to be clear, I've circled the resistor in question on Taylor's JM Wonderland layout (January 2017) and for comparison on Aaron's JM Sig Photo schematic (2012 09 14) showing 205k. Do I understand correctly?
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@Martin , yes exactly that, makes a huge difference and the whole 68k/1M divider now makes sense.
@Tom, I have the contour on a switch and depending on what I'm doing I flip it in or out . For that classic SSS SRV type tone I have it out I like the higher R values in the power supply yes .
@neskor, so you have NO cap on the reverb return but bright caps on the Master volume?
Many thanks to the guys who finally sorted this conundrum out for me.
For me that’s not only volume/gain wise but overall balance is improved. I was fighting the amp for a while, never fully bond with it, trying to find some fat and less articulation and that did help a lot !
norburybrook wrote: ↑Mon Apr 26, 2021 5:50 pm
@neskor, so you have NO cap on the reverb return but bright caps on the Master volume?
that is correct
NO cap on the reverb return will gain more top end on a wet signal, bigger value of this cap will cut more top frequencies
also cap on the master volume will bright this amp
at the end I get similar bright amp as my #102
norburybrook wrote: ↑Mon Apr 26, 2021 5:50 pm
@neskor, so you have NO cap on the reverb return but bright caps on the Master volume?
that is correct
NO cap on the reverb return will gain more top end on a wet signal, bigger value of this cap will cut more top frequencies
also cap on the master volume will bright this amp
at the end I get similar bright amp as my #102
similar bright to #102..wow that's interesting my #102 isn't a bright amp compared to my JM/SSS , the JM with the contour out of the circuit and the bright switch is really bright, in a good way, a different level to the #102 all together, I wouldn't put them in the same ball park even. I have a 300pf bright cap on the Jm like the AN layout, perhaps that's why?
After playing the amp yesterday the swap in resistors/caps means the reverb level is now generally lower, or in fact the dry level is a LOT louder making the reverb seem quieter in comparison. This means you have to turn the send and return levels up now on the reverb a lot more. I have 750pf ceramics on the reverb return which darkens the verb but blended with the bright of the dry signal it sounds nice. I'm not sure what changing the bass 1.5 to 1.8k resistor has done but the amp is sounding very well balanced all round now into my 2x12 vintage G12-65 dumble cab. I have the bright on and the deep switch on, contour out of circuit for that SRV tone. For a perfect smoother fender type clean I turn off the bright or use the contour, or roll off the guitar tone.
Great to hear about the new discoveries with your JM Marcus! Thanks for posting
I am going to experiment with a larger Send pot for the reverb on an ODSR build. It has a similar mixer as the JM. I will use a 250KA instead of the usual 100Ka. i also switched my mixer to the new values mentioned here.
Maybe switching your send from 100 to 250KA will give you more reverb if needed.
I believe this has a resistor for the reverb and none for the clean. You can see it coming over horizontally over the plate resistors for the Tweed Mixer. Tube beside the phase inverter tube. From there it connects underneath the board to the orange drop cap above it. This is the orange cap that has the large blue wire connected on top.
Guy77 wrote: ↑Tue Apr 27, 2021 2:49 pm
Great to hear about the new discoveries with your JM Marcus! Thanks for posting
I am going to experiment with a larger Send pot for the reverb on an ODSR build. It has a similar mixer as the JM. I will use a 250KA instead of the usual 100Ka. i also switched my mixer to the new values mentioned here.
Maybe switching your send from 100 to 250KA will give you more reverb if needed.
Guy
thanks Guy. I think I'll be ok as I can turn up the levels to give quite a lot of reverb still, perhaps the 250 would mean the levels would be lower for general use and have more extreme if needed.
mojotom wrote: ↑Wed Apr 28, 2021 4:35 pm
Marcus 250k Send pot and 25u on the cathode of the reverb tube can help.
A long three spring reverb tank too (9AB2C1B).
Tom, I've got a vintage USA Acusonics Tank I pulled from the donor peavey amp along with the transformers it's a sweet reverb tank, better than any new one I've heard .
I'll give those tweaks a try though.
I presume you mean the send Cathode with the 2k2 resistor? I would imagine that will let a whole lot more bass through, yes?
I can put the 68k (1M) resistor back in now as well and remove the pot now it's all as it should be.
I'm thinking of adding a FET as well as I have a JFET and a board here in a box of stuff. Would I need to use a relay to get this on a Foot switch?
