2nd Build-Rob's JTM45 Micro

Discussion in 'Shock Brother's DIY Amps' started by willholt92, Jun 1, 2020.

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  1. willholt92

    willholt92 Tele-Meister

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    So.

    It seems I have been bitten by the amp building bug.
    After I finished my Princeton 5F2A Frontman conversion I have been itching for another project to get into.

    Also since I've had the Princeton finished I discovered that I don't really like pedals anymore, there is something that guitar straight into amp does that I love which my pedal board doesn't give me.

    My main overdrive usually is a Bluesbreaker mk1 clone I built, so I've decided I'm going to build an amp to do this instead. Also I like the idea of a mini JTM45 style head.

    I'll be building everything from scratch, chassis and cab.

    I've started collecting parts already, transformers are here Hammond 370AX PT, 155H Choke and 125B OT.
    Mainly I got these first so I can start planning the dimensions on the chassis and head cab.

    I'm sourcing the rest of the components from Modulus in the UK, although I have laid the wiring diagram out in Illustrator and added a few mods/changes so I want to make sure I have enough of everything before placing the order.

    I originally wanted to stay true to the look of a JTM 45 with 4 inputs and planned to use the FX loop to do this.
    3.PNG
    I know Rob moved the FX loop after the volume pot due to noise issues from other's builds.
    I was wondering if using shielded cable to V2A grid stopper might help this? See the grey wire in the mock up below.

    2.PNG

    Admittedly I probably won't use the effects loop that much, if at all. So I'll probably not include it. But still undecided.

    The switchable bright cap seems a much better route to go for me. So I made a version with this instead of the loop. Again I'm a sucker for aesthetics, I will be using a full size toggle switch for this.

    4.PNG

    1.PNG

    So I have added 1 more input over Rob's layout and another V1A grid stopper.

    I'm also going to do the tone bypass Raw switch with a switched Bass pot, this cleans the wiring up and does away with the need for another toggle switch.
    I've added a bleed resistor to B+ as these make a ton of sense to me.

    If anyone see's anything out of place please let me know, I have literally copied Rob's layout and twekaed it ever so slightly.
    I'll be using a kettle lead to go along with Marshall styling, also makes it easier to wire.
    I'll be reducing the bypass cap as I'll be using humbuckers with this a lot.

    Size wise it will be 450mm wide by 180/190mm tall and the same again deep, (Approx 18 x 7 x 7 inches) this shoud give me more than enoguh room.
    I added transformer and tube dimensions to the mock up to sense check it. I'm going to make the chassis out of aluminium this time I think, much simpler this time all being 90 degree bends. I'll be drawing up the chassis soon.

    5.PNG

    And I've not yet decided on the logo, maybe I'll do the script??

    6.PNG

    Thanks for reading, hoping to get properly started in the coming month or so once I've got my current little projects done, namely refinishing my tele in nitro.
     
  2. Bitsleftover

    Bitsleftover Tele-Meister

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    Hey Will.
    After having a little bit of time to mull over my recent similar build, I’ll just drop a couple of suggestions.
    1/ I love having the 4 inputs to jump the two channels together and blend them in. It’s one of my favourite features of the amp. Since you’ve got the space, it might be worth considering.
    2/ if you go the single channel route, make your bright switch a DPDT and use the other half to switch in a 270p, 470p or 500p treble peaker cap across your mix resistor at the same time. It really adds to the Marshall flavour. Most if not all Marshall lead channels had them.
    3/ A really simple mod is to add a 3PDT on/on switch to add an 82k resistor across the 56k slope resistor to bring that down to about 33k and a second 250pf cap across the treble cap to raise it to 500pf total. It’ll give you the ability to switch to a more 1959/SLP type sound. The switch can be hidden round the back. It’s not something I adjust on the fly. Of all the switches and options I experimented with, the tone stack switch and the V1 shared/split cathode switches are the ones that get flipped. The other are either never changed or have been hard wired in.
    Just a couple of ideas. Good luck


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  3. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    I use an A/B/Y box to do this. You only need two inputs and you have the ability to change channels on the fly.
     
  4. willholt92

    willholt92 Tele-Meister

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    Hey

    Ah that's great thank you for your suggestions. It was your posts about your project that made me decide to go for this one.
    Is your build based on Rob's bassman?

    I may well still go for 4 inputs but I don't know for sure. I like the idea of it being a miniture JTM and therefore less knobs and less inputs.

    I love the idea of the Marshall lead channel however thanks for the tip I'll definitely look at that. And the 1959 SLP switch is interesting too, especially if I can hide it at the back and have the option there.

    Did you get yours all finished up and working? I saw one of your threads with some trouble shooting issues?
    Being my seocnd build I'm trying to learn more about the process and especially modifications.

    Thanks again
     
  5. Bitsleftover

    Bitsleftover Tele-Meister

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    Well I copied Robs design and layout for the power amp and PI then tried to morph that into a traditional Marshall preamp layout but it was a bit of a squeeze.
    I’m still having problems with it. There is a high pitch squeal/oscillation which I can’t track down. I managed to get rid of it with some really heavy stability caps on both channels. The normal channel took it well, but the high treble channel stability cap has taken all the top end off and it sounds almost identical to the normal channel now.
    I’ve been really busy doing some training for work so haven’t had time to look into it further. Take home message is.....don’t follow my layout! Hahaha.
    In my defence, removing the mods I recommended improved nothing so I don’t think I can blame them.
    The thing I find about the classic 2 channel 4 hole Marshall’s, is that the normal channel is very fat, and the high treble channel is very very bright. Hence loving blending the 2.
    If I wanted to save space, Id go with one “high” 33k input, split the cathode 820k/330uf and 2k7/.68uf, - 470k/470p treble peak - 0.022/0.0022 coupling caps with a vol for each channel.
    If I wanted to save even more space and simplify I’d go with one “high” 33k input, shared cathode 820k/330uf, - 470k/470p treble peak - 0.022 coupling cap with one vol.
    Arnt you just down the road from me? if I get time to get it working properly you can have it for a week or so to play around with if you like. See which configuration you like before you commit to a design.
     
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  6. willholt92

    willholt92 Tele-Meister

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    Oh right cool.
    I went back through your thread and there's some great ideas in there. Reverb and Trem would be great in my 5F2A.

    So with me not building the LTP version of Rob's design would that mean some of your suggestions wouldn't work in the Cathodyne Phase inverter version?

    The cathode on the layout I have from Rob's site 1.5K/250uF/ What is the effect of reducing these values?

    Also the slope resistor is shown as 100K, you said you used 56k? Increase in bass with a smaller value if I understand correctly.

    I'm not far, just over the hills in Sheffield. Where abouts are you in Cheshire? My old man lives in Hyde.
     
  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The Bassman/JTM45 Micro is a single channel amp so you'd have to add the second channel in with an additional triode to get 4 inputs and channel jumpering.
     
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  8. Bitsleftover

    Bitsleftover Tele-Meister

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    Ah, I didn’t notice you were doing the cathodyne PI version. That changes things a bit regarding 4 inputs/2 channels doesn’t it. (And the split cathode suggestions for that matter)

    Regarding the tonestack,its hard to pinpoint because the 3 controls are all so interactive, but it kind of adds more of everything. More Bass and more low Mids, then the 470pf treble cap adds more treble and upper mids leaving a less lossy and less scooped tonestack. More gain and more of that Marshall midrange krang.
    Download the Duncan tonestack calculator if you haven’t already. It’s really simple to use and you can easily see the effect of each component. It’s a brilliant tool.

    The values Rob has on the tonestack of his older 5F6A-M cathodyne schematic (56k slope, 250pf treble, 0.022 Bass and Middle) are the same as used on the Marshall JTM45 and Bluesbreaker reissue. The only real difference is the Marshalls have the 220pf treble peaker across that 270k grid stopper (mix resistor) of the bright channel before the second gain stage.

    If it were mine, I’d put that 220pf, or maybe 470pf treble peaker cap on a switch along with the switchable vol pot bright cap. The 2 belong together really.

    Since you don’t get a presence control on the cathodyne version, I think Robs “5E3 3-Way Negative Feedback Switch” would make a really nice addition here if you use a 0.68uf bypass cap. It might come in handy to inject even more Marshall brightness in lieu of being able to turn the presence up.

    By the way, I’ve moved from Cheshire to Gainsborough a while back for work. (But that’s a whole other story). I must change my profile.


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  9. willholt92

    willholt92 Tele-Meister

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    Thanks Rob
     
  10. willholt92

    willholt92 Tele-Meister

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    Yeah that's what I thought, I'll keep it simple for now I think with the two inputs as I'm conscious I don't want to end up in a nightmare of problems I don't know how to fix haha. But I am learning, it's a really interesting subject.

    I haven't yet but I'll check it out, thanks for the tip.

    Ah okay that makes sense, and sounds like a great suggestion. I'll make some changes to my wiring layout and check it with you if that's okay to make sure I am understanding right.
    I'm back on paternity leave for the next week so I'm hoping I might get time to get the rest of my parts ordered from Modulus, in between all the nappy changes and tantrums.

    Oh right, yeah you are just up the road from me then. I hope you have got sorted after losing your job earlier in the year?
     
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  11. willholt92

    willholt92 Tele-Meister

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    Okay so, in my simple paint by numbers mind this is what I think you mean to add the treble peaker cap to the mix resistor on the same switch as the Bright Cap for the volume.
    DPDT On/On, 220pf Cap in to Mix resistor.

    Bright switch DPDT MOD.PNG

    @Bitsleftover am I right on this? If not please tell me how and why I've been stupid haha.

    Thanks in advance
     
  12. Bitsleftover

    Bitsleftover Tele-Meister

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    Nearly but your treble peaker cap is going to the wrong side of the resistor.
    Id do it this-
    Sorry for crude edit. DPDT on-on
    So Bright (Blue) 100pf across vol pot.
    220pf treble peaker cap (Green) across grid stopper.
    You can mount the caps to the switch contacts.
    With the switch making the lower contacts, there is open circuit and the caps are both shorted out.
    With the switch connecting the upper contacts, both caps are switched into the circuit.
    .[​IMG]


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    Last edited: Jun 15, 2020
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  13. willholt92

    willholt92 Tele-Meister

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    Don't be sorry that's perfect, thank you.
    I thought it would be better to mount the caps on the switch but wasn't 100% sure I was right in my thinking.

    Cool, so I need to add a DPDT switch and another cap to my BOM.

    In your experience is it worth forking out for more expensive mica caps?
    I've gone for F&T filter caps, and Sozo Coupling caps as they weren't a great deal more expensive them no name items. And I want to use god quality parts in this build.

    The mica caps are pretty crazy though on Modulus. Difference between Sozo and others are over £10 more expensive each.
    Worth the additonal money or not do you think?
     
  14. Bitsleftover

    Bitsleftover Tele-Meister

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    I don’t really know on that one. Hopefully one of the more experienced guys round here will head you in the right direction. (I’m only about a build and a half further down this path than you.
    On my current nightmare, erm, I mean project, I went with reasonably priced parts. I’m glad I did because I’ve removed and refitted them all so many times while troubleshooting I’m too embarrassed to post any more pictures when I ask for help.
    When I eventually get to the bottom of what I’ve done wrong, I might consider rebuilding the board with top end components just because I’ve already invested so much time and money already it seems a shame not to have the best in there.
    But if I’m really honest, after 30 years of loud guitars and even louder jet engines I can only just hear the door bell over the high pitch ringing in my ears!
    I don’t think I’d pass a Pepsi challenge on 95% of the things we discuss around here! Haha
    Ooh, actually, I just remembered as I was about to hit reply.... On my troubleshooting thread, someone said the non branded silver mica caps have a reputation for producing squealing if overheated while soldering. Wasn’t the cause in my case. But might sway you’re decision.


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  15. willholt92

    willholt92 Tele-Meister

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    No worries at all I assumed you had a lot under your belt from your knowledge and your help so far.

    Haha, to be honest your project is a hell of a lot more ambitious than mine is so I'd not worry too much.
    I'm thinking I might plum for a Sozo one for the treble cap and maybe slightly more affordable ones for the bright channel.

    £13 each for the Sozo one's just seems crazy to me, that way I can always easily change them down the road.
    Thanks for your insight

    Hahah I guess you need loud amps to make up for that then eh? Maybe a superlead :)
     
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