My 18w TMB Kit Build

joulupukki

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If it were me, I would just use a 500k and not worry about adding 7.5MOhms. I can tell you are a by the book kind of guy though. :p
Hehe. Not knowing anything else, that’s why I as asking. ;)

If the wiper fails, you are going to know something is wrong and investigate before too long. (Hum, loss of volume, distortion, no clean headroom, etc.) The 7.5Mohms is hardly a "safety" compared to the 1M max on the datasheet. It's cathode biased, so you have some safety there.
I guess the 2M resistors usually shown for this mod are safety backups for fixed biased power tubes? Have read that w/o them you have a chance of burning up your power tubes.

Those look like regular grid stoppers to me.
Ok. I was confused but now that makes sense.

Unfortunately, they will have to be relocated for the Type-2. The best place is right on pin 2 of the EL84's. Some people just float them free as they are strong enough to support a wire. Some heat shrink will give it even more support and protection from inadvertent short. Or you can install an isolated lug to solder them to. Not recommending using pin 1 as it's "IC" on the datasheet (internal connection).
Makes sense … and I just read that NOS tubes used to have pins 1 and 2 internally connected. Modern tubes don’t. i.e., don’t use Pin 1 for a solder point.

The green wires will stay in place, but they will have the grid stoppers on the other side. The turrets holding the 10nF caps will wire to the top of the pots. The wipers will come down to the end of the green wires. And the bottom of the pots will go to ground.
👍🏻
 

Len058

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Watched this video. After seeing it I'm not in a hurry to try the type 2. The difference would be in the presence control, according to this guy. I'm not unhappy with it, as is. Maybe I don't know what I'm missing but for now I'm happy with type 3.

 

andrewRneumann

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Watched this video. After seeing it I'm not in a hurry to try the type 2. The difference would be in the presence control, according to this guy. I'm not unhappy with it, as is. Maybe I don't know what I'm missing but for now I'm happy with type 3.



The Type-3 doesn’t affect the presence control?
 

joulupukki

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This amp doesn't have a presence control but I would imagine that it does affect the tone. In fact, I just wired up Type 3 again with a 10K and then a 100K potentiometer plugged into the breadboard. Both of these set to their max bring down the total capabilities of the amp and also are way too loud to operate inside the house.

Not super precise, but I did an experiment with an app on my iPhone to measure decibel level. With the gain knob on the TMB channel cranked and targeting 80dB I first started with no PPIMV in the circuit. I could barely turn the built-in MV knob up at all but the tone was pretty good (full, bassier). With both the 10K and 100K pots, I turned them to just barely up and then was able to turn the built-in MV knob up significantly. At 80dB the sound is still very good but it's definitely lacking some of the bass.

Also to note, I put my multimeter leads across the same terminals of the pot and measured AC volts. The highest I saw it ever reach was about 0.25 VAC.

I think if I could find a 10K pot with a push-pull switch or even a switch where you turn it all the way down to a click (to turn off), that would serve me pretty well. It'd be plenty loud for anything I'll probably ever do with it ... unless I somehow change my acoustic bluegrass ways and join a really loud rock band. ;) In honesty, this amp is overkill for anything I would ever use it for ... but of course it's a lot of fun to play on.

I should do the same 80dB test with the Captor X attenuator and see if there's much of a tone difference. Maybe later.

Yesterday I was playing a bunch of clean tone, jazz type of stuff through both the 18w and the JTM45-Micro and man, they are both great sounding for that stuff too. The micro definitely has more of a tweed sound to it though and I preferred the 18w (through the TMB channel with the gain low). In isolation for this particular type of playing, I wouldn't know that much of a difference with the micro and would think that it's great. But using my A/B switch and switching back and forth to the 18w ... each time I switched to the 18w it seemed so much more clean and full (they were set at the same volume output levels). Anyhow, fun stuff. :)
 

Len058

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The Type-3 doesn’t affect the presence control?
It does. But After reading about @joulupukki's test I'm in doubt. I think I'll test my micro without any MV. When the amp starts to flap my pants, I know what to try.

Maybe the bass frequencies are easier to cancel out, because the waveforms are bigger.

Thanks for all the info, this is great!
 
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joulupukki

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Alright, during lunch break today I compared the PPIMV (type 3) with the small 10K potentiometer on the bread board against the amp with no attenuator, and then also the attenuator.

With the gain knob cranked you can tell there’s a difference like I described before (less bassy sound with the PPIMV, maybe a teeny bit less treble - basically less full). With the attenuator, I get pretty close to the original. But, of course, with just the amp, it sounds best.

The big realization I had was when testing out the clean sound. Holy smokes! I am no longer interested in any sort of PPIMV for this amp. The cleans lose so much with it in the circuit. I’ll just deal with the fact that the attenuator has to be plugged in or be ready for a rude awakening if I accidentally move the amp’s TMB MV knob too much.

So the only real major annoyance I have is that the TMB’s MV knob seems non-functional between 0 and 1.5-ish. Same exact thing on my Princeton Reverb. They are 1MA CTS pots. Maybe that’s just how they work.

Also, after this experiment, I think I’m gonna use the MV on the micro amp a lot less. I’ll just keep it cranked most of the time and use the two volume controls on the preamp instead. It’s definitely a fuller sound there too. It is handy though to dial it back just a little bit when needed.
 

Len058

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I've just tested a bit. I couldn't really play because I got some sore cuts in my fingers. Who knows fixing bicycle tyres could be so dangerous.

I cranked the PPIMV and then turned it down while upping the gain. I couldn't hear any loss of bass or treble.

Do you get the same effect with the micro ppimv? Maybe the long wires to the breadboard eat your tone? Or maybe I stay on the good side of the cancellation because the EF80's don't have to be turned down too much for low volume.


If the micro MV has the same issue I must be deaf or the EF80's work better with this kind of MV. But I highly doubt, the EF80's contain fairydust.
On the other hand, if you read into the MV discussion, you see a lot conflicting experiences.
 
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joulupukki

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Oh man, hope your fingers heal up quickly!

I tried a little bit just now with the JTM45-ML and didn’t really notice much difference. The thing with that amp though is that it’s not really ear-splitting loud to begin with, nor does it seem to have the same umph/air-pushing capability, nor as much bass to begin with. So it seems in that amp the PPIMV works great. It also has a nice audible difference as you cycle though the different positions of the MV pot (unlike the 18w where it’s loud right out of the gate).

Edit: True, it could be that it just needs shorter shielded lines and a higher-quality pot in the 18w. I don’t think I’ll go there though.
 

willholt92

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Thanks. Of course ... that's actually how my Bassman Micro project started. I ran out of tolex for finishing my speaker cabs and realized I had to order more ... and there was plenty leftover to build another small amp head. Haha. What's crazy is that the micro amp easily cost me more than the Mojotone 18w TMB kit. I guess I could have bought cheaper components, but figured I would just make it really good (including the custom faceplates).
Haha yeah it's easy done isn't. I must admitt I've been building my micro for so long that I don't know fully how much it's cost me but it's definitely more than the Mojotone Tweed princeton I built first. And I've spent a lot on the detail like face plates as well.

If I could build a Marshall-style amp that's powered half-way between my micro and the 18w that'd probably be about perfect. I love the size of the micro head. I definitely feel most accomplished from that one since it's a from-scratch build and not one from a kit. I'll probably end up trying a few mods on the 18w to make it more usable in my small bedroom studio. ;) The Captor X works pretty well, but sometimes a pain to have to connect it. It'd be nice to have a bit more control of the master volume instead of from 0 to 60 with the slightest turn. I may try that post phase inverter master volume to see how that works.
Yeah I'll bet, a friend of mine has a 1974X 18 watter and it is SO LOUD, and the volume pot has a lot of sensitivity to get it at the right levels for home playing. It sounds AWESOME when he plays live though.
I wonder about a single ended EL84 amp? Maybe 5-10 watts. Another friend of mine had a Cornell Plexi 7 that was a great little amp, might bridge that gap.
Does the master volume work across both channels or just the JCM TMB one?
Excellent looking build so far! Interesting you also saw that there's less hum with the OT slightly tilted. When I installed it, I didn't bother to actually implement the tilt and it's actually super quiet. I mean, I could probably get it more quiet, but it's performing really well.
Thank you, I'm hoping it will be a nice quiet amp. What power valve are you using in yours now?
Not sure. I'll have to ask my friend who printed it for me. I designed the artwork with vector art in Photoshop, exported it to an .ai file so I could open it in Inkscape to convert it to an SVG that I then opened in tinkercad.com to make the .stl file.

I couldn't find anything in tinkercad that would let me add draft. I imagine some pay-for CAD software could do that, like you suggest. I didn't know what feature to search for, so thank you for mentioning that it's called, "draft."
Well let know know if you are struggling my engineer at work might be able to quickly add some draft to an STL or SVG
 

joulupukki

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Haha yeah it's easy done isn't. I must admitt I've been building my micro for so long that I don't know fully how much it's cost me but it's definitely more than the Mojotone Tweed princeton I built first. And I've spent a lot on the detail like face plates as well.
They are worth it, for sure. I always seem to be so curious how it will sound that it keeps my motivation extremely high to carry me to the end. When I started building the cab for this micro the baltic birch I used had all sorts of tearout when I was routing the finger joints so I switched to 3/4" pine. But part of me wants to glue together that bb wood anyway and just build another micro cab. But then I'd have to order more tolex ... and probably have leftovers and try to start another build. ;)

Yeah I'll bet, a friend of mine has a 1974X 18 watter and it is SO LOUD, and the volume pot has a lot of sensitivity to get it at the right levels for home playing. It sounds AWESOME when he plays live though.
👍🏻 That's awesome. This would be a lot of fun to play live through, for sure. I'll keep practicing and maybe get there. I think I'd be booted out of my bluegrass band if I showed up with it. Although I could probably get some seriously loud banjo through it. New genre: bluegrass rock! ;)

I wonder about a single ended EL84 amp? Maybe 5-10 watts. Another friend of mine had a Cornell Plexi 7 that was a great little amp, might bridge that gap.
Antek makes a 10w output transformer (toroid) that would match up well with a pair of EL84s as far as I can tell. That may be an interesting build: https://www.antekinc.com/content/MP-10W80.pdf

Does the master volume work across both channels or just the JCM TMB one?
MV is only for the TMB channel. The normal channel has a volume and tone control and it's not a high gain channel so it's not quite as drastic when you use the volume knob – though definitely also plenty loud.

Thank you, I'm hoping it will be a nice quiet amp. What power valve are you using in yours now?
I'm running the 12BH7 though it really isn't any different than running a 12AU7 (I've tried both). I'm using the bh7 and will keep the au7 as a backup.

Well let know know if you are struggling my engineer at work might be able to quickly add some draft to an STL or SVG
I'll DM you an STL file. :)
 

joulupukki

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I put on some ear muff hearing protection today and cranked the 18 watter. It was crazy loud. But … I must have damaged one of the power tubes. Now when I flip the standby switch off (after startup) it makes a bunch of noise. Once it’s warmed up for 10-20 seconds or so it doesn’t seem to cause problems. But there’s now a bunch of popping and whooshing at that point. Dang. Pretty sure it’s gotta be one of the EL84 tubes because I swapped all three preamp tubes with the micro amp and it had the same issue.

Also I had them out in the back yard for fun and definitely loud enough to play any gig I’d ever do. It was fun to have my buddy down the street let me know he could hear me. ;)
 

joulupukki

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Bummer! Maybe one tube wasn't right from the start, with the amp eating fuses.
Good luck
Could be. Once it’s warmed up it seems to work fine. I’ll just avoid those cranked volumes from now on. It’s not very practical anyway. Just to double-check I chop-sticked all the solder joints and components to make sure it wasn’t something like that. Nothing found. Next time I need to make an order for something, I’ll pick up an extra set of tubes.
 

joulupukki

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Good news. I finally got a ship notification from Mojotone today that the proper power transformer is on its way to me. It should arrive sometime next week and I'll take out the Antek Toroid (which will free it up for a new amp build ... hmm, what to make ... haha).
 

mountainhick

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joulupukki

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This Antek OT power rating is 10W HiFi watts which go to 20Hz. For guitar 80Hz it must transfer at least 30W possibly 40W?
So maybe just use a regular OT like one that’s used for a Deluxe Reverb or something?
 

joulupukki

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Look what FedEx just delivered! I know how I'll be spending this evening. It'll be nice to have this amp all finished up. Hopefully I won't run into any gotchas with power differences or whatnot.

IMG_1409.jpg
 




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