JTM45 Bluesbreaker Build

Cjsinla

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So, I really didn’t need another amp but I’ve had a bunch of time on my hands. I thought that Ceriatone wasn’t doing kits anymore. Apparently that’s not true. I’ve been looking for a 30 watt JTM45 almost since I’ve been building kits. I did a search last month for one and this came up. I’ve been wanting a 30 watt Marshall for some time as my 50 watters can be damn loud, the 1987x Plexi just isn’t tolerated by most people. But I really like the way the 4 holers cut through. I think 35 watts will be just about right. I can crank it and use a boost if I need more gain. I’m also finding that having less gain makes for a more responsive amp. This model includes tremolo but they also have a kit without trem.

https://flic.kr/p/KnmCRu
I emailed Nik and he confirmed that they were selling this kit. So, I ordered one minus the cab and tubes, $700. I paid for it by Paypal before I read the fine print where Nik said that I might want to wait to pay as they are all locked down over there. So much for another quarantine project. It was ordered almost a month ago and finally arrived on Monday. I ordered a cab from Sourmash, they were open for business and the cab arrived the same day as the kit.

The transformers were already bolted in and all the components were neatly packaged in the chassis.

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Cjsinla

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I forgot to say that the kit also has a half power feature which I will wire up. I am omitting the effects loop for now As I rarely use one. This is what the wiring diagram looks like.

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Gtarzan81

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That was another excuse to build an amp. I’ve never had an amp that runs KT66 tubes.
I supposedly can run them in my 6100, but I have to get the right size ones so they would fit. And with 100 watts, I doubt I could drive them hard enough to see a difference over the 6L6s in there now.

Allegedly the KT66 is the perfect median between a EL34 and 6L6.
 

The Ballzz

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Well there ya go! :h5: Have fun with that one. With the skills you've been gaining and demonstrating, this should be a stellar build and an even better/more fun amp to play. I would NOT however expect much less loudness that your 50 watt 1987, especially once you get it really singin'!

Might I suggest investing the time and $$$ into building a great attenuator to allow you to crank it into it's "glory" zone? Here's a link to a great attenuator design that won't mess with tone and response. Its the best of the multitude of passive attenuators I've tried since the late '70s! I've already built one and have been using it in multiple situations to get a handle on what layout and features will be most suitable for my uses. I will build at least two or three more to leave permanently installed in several of my amps. Very liberating, to say the least!


While the thread is quite long, with lots of detailed testing results, evolution, etc, the main designs are in post #1.

Enjoy The Build!
Gene
 

The Ballzz

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@Cjsinla

One great advantage to including the passive effects loop is that it provides a preamp out/power amp in access point for testing and experimentation. For example: You can run the send output to the return input of a known amplifier to test the preamp circuit or the send of another amp into the return input to the power section for testing just that power section. Given that there is no circuitry except for a couple of jacks and wires, it is electronically invisible/inert when not in use! It can work out to be a pretty handy feature! It's also a nice place to add a good reverb and/or delay, should such ever be desired! Not often a good spot for foot/pedal/stomp type/thingies though!

Just My $.02,
Gene
 
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Cjsinla

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Built up the tremolo board, put in the tube sockets and wired up the rectifier. Also Installed the pots and worked on the grounding scheme for them.

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Cjsinla

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It’s been a minute but Did a little more work today. Checking the value of all components and finding that a number of the carbon comp resistors are reading high so I’ve been pulling stuff out of my stockpile. It’s coming along. What’s the deal with the short leads on most resistors? You can see the splice job I had to do.

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coolidge

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Congrats on the kit, following as I may build one of these next. CC's are noisy and over rated imo and will continue to drift in value. I used Takman Rey 1 watt and TDK 2 watt metal films in my recent JCM800 build. Also kicked all pf value silver mica and ceramic caps to the curb in favor of WIMA film/foil caps. The WIMA's are rated 300vac, 630vdc with a 300,000 hour life span.

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Cjsinla

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Finally got off my butt to do some more work on the amp. I was struggling with the heater wiring as the connections from the PT, the pilot light and rest of the heater wires all join at the tube that runs the tremolo circuit. Finally figured out how I was going to do it.

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After that I was feeling a little inspired so wired up most of the AC. It looks like I might be short the T 0.5a fuse for the HT. The T means slow-blow, right, I have some slo-blo 500ma fuses in my stash.

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Cjsinla

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got around to finishing the board today, it’s getting there. There is a few things that look different due to the power scaling, specifically that 5w resistor hanging off the board.

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madhermit

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Just a suggestion, sorry if you mentioned it, I skimmed the thread to see if it was mentioned.

I had a 59 Bassman made, and had the guy put in a variable negative feedback switch so it can switch to a JTM45 sound along with swapping the V1 to a 12AX7. It also has a no negative feedback setting. It’s a simple mod from what I understand and it gives you a couple of options.
 

Cjsinla

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Just a suggestion, sorry if you mentioned it, I skimmed the thread to see if it was mentioned.

I had a 59 Bassman made, and had the guy put in a variable negative feedback switch so it can switch to a JTM45 sound along with swapping the V1 to a 12AX7. It also has a no negative feedback setting. It’s a simple mod from what I understand and it gives you a couple of options.
Not a bad idea.
 

Cjsinla

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Got some more work done this weekend, wired in the trem board and main power connections. I have to finish wiring the power tubes, the 1/2 power switch and impedance selector. Then I can drop in the main board.

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5F6-A

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Nothing but vintage Mullard AX7's and 1958 GEC KT66's in my JTM45. KT66 is the tube for a JTM45.
I totally agree. I love my GECs as well although mine are a little bit more recent
 

Cjsinla

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We’ll see, I’ve never run KT66 tubes.
 

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