Build Log - John Mayer Amp Clone

[Johnny_Boy]

well I am in idiot!

1. I forgot that when I am turning the bias pot from INSIDE the chassis, I need to turn them backwards so what I thought was the minimum setting was the maximum setting!
2. Also, the reverb is working fine now that I've switched the RCA cables around. I've wired the in and out on the wrong color'd RCA jacks, so I corrected that by just swapping the RCA cables from the amp to the spring reverb box.
3. Used 1K instead of 1 Ohm for the bias resistor and it only went from 60V to 58V. Used 6.7K ohm that was laying around and it dropped it to 48V. Close enough. So maybe 5K ohm might be a good value to drop 60V to 50V on the bias tap.

[10thTx]

I'll bet any experienced builder has done similar and gotten something backwards or reversed. I sure have. Looks like you got it resolved in short order! Hurray! I like stories that have a happy ending.

With respect, 10thtx

[norburybrook]

well I am in idiot!

1. I forgot that when I am turning the bias pot from INSIDE the chassis, I need to turn them backwards so what I thought was the minimum setting was the maximum setting!
2. Also, the reverb is working fine now that I've switched the RCA cables around. I've wired the in and out on the wrong color'd RCA jacks, so I corrected that by just swapping the RCA cables from the amp to the spring reverb box.

great, I've lost count of the silly things I've done, just remember to stay safe


enjoy your amp.


M

[rootz]

Congrats on the build!
I always make one or two mistakes. Mostly small ones. Never had the bias wrong though. It’s one of the most important things to get right from the start. So I dial in the lowest setting and actually measure the voltage on the output of the bias to confirm it is on the lowest setting (most negative voltage possible). This way the new tubes will be in cutoff instead of instant red plating.
I made the exact same mistake on the reverb first time I did it. I was so puzzled why the reverb didn’t work!

[Johnny_Boy]

Finished Clone
Well, here are the shots of my clone build. Looks decent from about 3 feet away
The plexi thing didn't work out that well, as I've already cracked the rear plexi while trying to mount the amp to the cabinet. I think I am going to re-do this sometime in the future with actual screen printing process.

For those who wants to build one in the future, build the cabinet first and then use the empty amp chassis to mount it in place first. It is hard to drill the hole on the top of the amp cabinet and trying to align that to the 4 holes on the chassis with long screws. I think it would be easier to do it before you install all the heavy things all over the chassis.

• 20200111_044105188_iOS.jpg

• 20200111_045855271_iOS.jpg

Work in Progress Inside
I am going to switch out the last two power supply capacitors with Electrolytic capacitors.
I've used Solen capacitors since I like them on hi-fi amps and I've had those laying around, but watching the voltage when I turn on the amp, it gets close to ~480V before it settles down to ~250V, IF I don't use the STAND BY switch. The Solens are only rated for 400V while the JJs next to them are rated for 500V. If I remember to use the Stand By switch, I don't need to do that, but sometime I forget where that setting is on when I turn on the amp. So to be on the safe side I will change it to 500V rated caps.

• 20200101_215244630_iOS.jpg

(Edited later - do not use above photo as the gospel! I mistakenly added the screen connections to pins 4 where they should be at pins 6 on my 6L6s. I caught this later and fixed it)

[Johnny_Boy]

Materials for the Cabinet
For the front and rear "grill" covers I wasn't sure what to use, but I ended up using this mesh fabric from eBay "Nylon Military Mesh Mil Spec Fabric Raschel Tactical Durable 46" Wide Bag Vest". They were pretty thick and stretches enough for me to wrap it tight and staple it down.

• Mesh.jpg

And as previously mentioned the tolex cover material was this faux carbon fiber. Though I wish it was thicker and stronger to resist me accidentally cutting through with a slipped tool.

• Carbon.jpg

[Johnny_Boy]

Correction & Clarifications on the Layout file
It was mostly accurate, but this threw me off for a bit. IT should say V1b and not V2b tube.

• Layout correction.jpg

Also, in case you weren't paying attention, you will need to connect these two capacitors per the red dotted line that I've drawn in. At least that is how I did it since the schematic showed it that way.

• Missing Wires.jpg

Of course, do NOT forget to wire up the heater wires! They are not shown on the layout file to keep things cleaner I think. I almost forgot to wire that up, especially since the original schematic that I used also did not show any heater wires.

• HeaterWires.jpg

For those of you who are confused on where the output transformers, filter chokes and reverb drivers are mounted, here it is. They are hidden under the eyelet boards.

• undertheboard.jpg

[rccolgan]

Nice!!! Well done! As for the faceplate, you could have a vinyl shop print your jpg to proper scale and then wrap it around your chassis as a thought. Congrats!

[M Fowler]

Great job building your amp! It looks great and you can get some panels made later.

I built one of these when the schematic was first posted no layout at that time. Not a pretty wiring amp build my me but has been functioning great all this time.

The customer informs me it is his favorite amp and it is still rocking. I've built him three different types of amps and he enjoys them all but the John Mayer Signature is his and my favorite.

Mark

John Mayer Signature clone 001.JPG

Fowler 2Roc JM Signature 03.jpg

[jszair]

Bill of Materials

Ugh, I had to order like 10 different orders from varying places to get all the parts ordered over multiple weeks! I hate that. Keeping the placeholder here, so I can list out the parts for those who want to order it in one shot in the future.

Transformers
Basically I've ordered the whole ClassicTone Twin Reverb set.

• Transformers.jpg

Orange Drop Capacitors

• OrangeDrops.jpg

I've built a few Hi-Fi amps before but never a guitar amp, so I was totally against using Orange Drops for any kind of signal path coupling capacitors, but after doing some research it looks for guitar purpose you do want the coloration that comes from the Orange Drop capacitors.
[Side note - polypropylene vs polyester orange drop. Of course when I was building hi-fi amps, I would pick polypropylene over polyester any days, as it give more "hi-fi" sound. I didn't realize for guitar amp builds most people prefer the polyester orange drops instead. By the time I've learnt this I've ordered and soldered half of them in, Ugh. I might have to switch them out later if the amp sounds too pristine.]

Sprague Atom Capacitors

• c-sa_group_0.png

These for the power supply section. Yikes, since when these become so expensive? Mouser.com had crazy expensive prices for these. I ended up going with a mixture of Sprague Atom, F&T (which are popular among Hi-Fi builders) and JJs.

[To be further edited as I go along here...]

Newbie question: what should be the wattage for resistors?

[pompeiisneaks]

I was going to say 'check the schematic and/or layout', but just checked them and didn't note them mentioning.

Generally most of the main signal path resistors need to be 1/2 watt. The power supply dropping resistors somewhat vary depending, between 2, 3 or 5 watt. I would look at one of the other dumbles like a 183 to see what value resistors were used in almost identical locations etc. Maybe someone else can see what I missed, but this one definitely didn't seem to have the wattage listed.

(Finally went and found it...) Oh there it is, went back and looked there's an updated schematic on this thread:

https://ampgarage.com/forum/download/fi ... &mode=view

that one's got the 3W or 5W mentioned on the specific resistors you'd want.

Otherwise 1/2W are fine.

~Phil

[justin_t_]

Fantastic post! A lot of constructive feedback and material here.

Would you happen to have a spreadsheet of the bill of materials by any chance?

Thank you

[bam]

I'd like to build this amp and would like to use a Fender "The Twin" as a donor (the maligned Red Knob Twin.) I have a question about how I might modify the design to use the power transformer from that amp for this build.

It seems this amp calls for a 660 VCT power transformer. If I understand correctly, the "The Twin" uses a power transformer with a 337 VCT secondary.

If I were to follow the schematic from this thread and tie the center tap RED/YEL to ground I would end up with around 235-236 VDC coming from the rectifier. Since I would need nearly double that for the plate voltage I realize I need to modify the wiring a bit but I'm not clear on how to accomplish that. 

secondary wiring CT grounded.png

Looking at the schematic for the "The Twin" I note it uses full wave rectification and that the center tap RED/YEL is tied between the two 220k Ohm / 220 uF in the filter section rather than to ground. There is also a HI/LOW switch which it appears would share the B+ between the choke and the 22uF filters when it is in the HI position. I see there is also two 10W resistors which I presume serve to drop the voltage further before reaching A/B/C. 

Red Knob Twin power section isolated.png

My question is how might I wire the secondary from the PT so that I can get in the ballpark for the B+ voltage? 

Do i need to use full wave rectification, and tie the center tap RED/YEL in between the two 220k Ohm / 220 uF filters as pictured in the "The Twin" schematic? (it's not entirely clear to me how wiring the center tap RED/YEL into this position accomplishes the higher B+ voltage) 

Could I then keep the choke inline with the 20uF filter sections rather than feeding the choke's output to the 220k Ohm / 47uF filters pictured in the "The Twin" schematic? (I presume the 220k Ohm / 47uF section is based on the HI/LOW functionality which would omitted for this build)

[pompeiisneaks]

I think you're misunderstanding what that transformer puts out.

If you wire it up like the schematic shows (Full Wave or FW) it shows the amp at +469VDC at Hi Power and at half power +236 The Lo power switch basically takes the transformer and only uses half of the winding if I'm looking at it correctly. Halving the power.

The preamp tubes after some voltage dropper resistors work at +397, +369 and +310 VDC.

Those are much higher than the 235 you mention? Not sure if that came from seeing the half power switch in action?

At any rate, if you wanted higher voltage output, you could use a full wave bridged rectifier (FWB) (with the center tap NOT connected at all, just shrink wrap it off and you're good) and you should get a lot higher voltage output If I recall it's the difference between 0.9 X VIN for FW to 1.4X VIN for FWB

now I may be mathing this wrong, but :

VIN * 0.9 = 469

OR

VIN = 469 / 0.9 = 521 VAC on that winding.

So for FWB

521 * 1.4 = 730 VAC... I'm not 100% confident on the math, but I'd guess that's pretty close, since the schematic shows those values coming out on HI power mode.

~Phil

[martin manning]

Assuming the choke has adequate current capacity for the screens and the rest of the preamp, it'd go like this: