JTM45 Issue

Johnnyslim · Jul 27, 2025

My JTM45 is experiencing a strange issue. It has been in and out of shops for three months. There is a long story to this and I will spare anyone who can respond to this the details. It is a rather hairy history and this is/was a Mojotone JTM45 kit initially assembled by a custom amp builder. But...that is another story.

I was playing the amp this morning and there was an arcing noise, drop in volume, another arcing noise then dead. The arcing noise was not loud but definitely audible. I checked and the mains 3amp fuse blew but the B+ fuse did not. I replaced the mains fuse and no power at all. Amp tech suggested I pull the rectifier tube but still no power and no blown fuse. I put the rectifier tube back in place and the amp jewel lit for a couple of seconds then dead. Still, the B+ and mains fuses are not blown. All tubes are well seated in their respective sockets. The Mullard rectifier tube was bad so an amp repair guy suggested throwing it out and new Sovtek GZ34 rectifier tube is in now. Going back to the amp repair guy Tuesday morning. Anyone have any clue what might be the issue?

dc007 · Jul 27, 2025

I would try a new rectifier tube. If they short fuse will blow. But a pro looking is a good idea.

Johnnyslim · Jul 27, 2025

But the fuses are not blowing currently. Just a dead amp. It played fine yesterday but a different story today. I did read a past thread about a very similar issue and replacing the rectifier tube resolved that issue. I don't have a spare rectifier tube on hand. I just got this out of the shop Friday and all was good. I am sure a different rectifier tube will be the first troubleshooting step when back in his shop.

Johnnyslim · Jul 27, 2025

dc007...I am thinking the rectifier tube may be the culprit. Though it is a new Sovtek rectifier there is no guarantee it cannot be failing. I will go grab a Mesa Boogie GZ34 tomorrow and give that a try. Worst case scenario...I have a confirmed appointment with the amp shop Tuesday morning. All will be resolved in a day or two. My other JTM45 is alive and well...kicking in the stall!

searswashere · Jul 27, 2025

If its that bad a lemon, sell it cheap and get a proper working jtm-45 build.

LPTDMSV · Jul 27, 2025

searswashere said:
If its that bad a lemon, sell it cheap and get a proper working jtm-45 build.

It does sound sketchy - 3 months in and out of repair shops??

Now it sounds like maybe it’s blowing rectifier tubes as soon as you put them in, or perhaps you haven’t got your 5V AC on pins 2 and 8 of your rectifier for some reason?

But could be A LOT of things … or several things. Tech time to fully troubleshoot could quickly exceed the value of the amp.

Marshall make a good JTM45 re-issue!

Torren61 · Jul 27, 2025

I'd start with the cheapest option first. Replace the rectifier tube. If you still have the problem, take it to the shop.

Johnnyslim · Jul 28, 2025

The thing is, this amp was built in 2012 and performed flawlessly on many gigs. For some odd reason it is now becoming a problem child. The guy who had for five or six weeks said he rebuilt it from scratch with a "proper" board. It played great until it died. The most recent amp shop informed me he did not hook up the B+ fuse correctly and that my presence was not working. When I took the amp I just set my knobs where I like them including the presence and it is fine. I did not spin the presence from zero to ten to test it. If anyone ever needs an amp worked on in Philadelphia I know exactly where not to go...just saying. I think the rectifier tube may be the solution.

Another note...I have both of my JTM45's without bright caps. This most recent amp shop did a demo of my amp with a different value bright cap. That was a very nice tone. He also did a demo with the same value bright cap on the Normal input. Since I never had any bright caps I never really got the input jumping thing. It just did not do much for my tone taste. Now, with these bright caps I get it! Thinking I will be an input jumper going forward.

As long as I am living...I am still learning.

redking · Jul 28, 2025

WWPTD ("What Would Pete Traynor Do?") - convert it to a solid state rectifier :rofl:

LPTDMSV · Jul 28, 2025

Johnnyslim said:
The thing is, this amp was built in 2012 and performed flawlessly on many gigs. For some odd reason it is now becoming a problem child. The guy who had for five or six weeks said he rebuilt it from scratch with a "proper" board. It played great until it died. The most recent amp shop informed me he did not hook up the B+ fuse correctly and that my presence was not working. When I took the amp I just set my knobs where I like them including the presence and it is fine. I did not spin the presence from zero to ten to test it. If anyone ever needs an amp worked on in Philadelphia I know exactly where not to go...just saying. I think the rectifier tube may be the solution.

Another note...I have both of my JTM45's without bright caps. This most recent amp shop did a demo of my amp with a different value bright cap. That was a very nice tone. He also did a demo with the same value bright cap on the Normal input. Since I never had any bright caps I never really got the input jumping thing. It just did not do much for my tone taste. Now, with these bright caps I get it! Thinking I will be an input jumper going forward.

As long as I am living...I am still learning.

Even though a Marshall is a Marshall is a Marshall, there’s enough differences between models and individual amps that it’s always worth experimenting with the tone controls and channel balance to find the sweet spot.

Most Marshalls need the channels jumpered, perhaps that means most have a bright cap?

Anyhow, please let us know how you get on with the JTM45-alike - enquiring minds need to know and all that. Hoping for a happy ending

LPTDMSV · Jul 28, 2025

redking said:
WWPTD ("What Would Pete Traynor Do?") - convert it to a solid state rectifier

I guess you could say Marshall came to the same conclusion …

searswashere · Jul 28, 2025

redking said:
WWPTD ("What Would Pete Traynor Do?") - convert it to a solid state rectifier

All ya gotta do is buy and install the module, my jtm-45 build had one. Twas a great amp.

Johnnyslim · Jul 29, 2025

Just for the record, I tried a solid state rectifier years ago and it was not to my liking. Hard put a finger on why it did not appeal to me but it was pulled in a day or two. That said, now that I have a bright cap on the bright input and input jumping is appealing to me...maybe a solid state rectifier is worth another look. It will be a long shot, though.

LPTDMSV · Jul 29, 2025

Johnnyslim said:
Just for the record, I tried a solid state rectifier years ago and it was not to my liking. Hard put a finger on why it did not appeal to me but it was pulled in a day or two. That said, now that I have a bright cap on the bright input and input jumping is appealing to me...maybe a solid state rectifier is worth another look. It will be a long shot, though.

You shouldn’t really have to change to a silicon rectifier - some of the GZ34-type valves/tubes currently available must be usable quality, surely??

Groove Tubes used to supply plug-in silicon rectifiers that worked off the original transformer and fitted in the original socket … many years ago.

It does affect the sound quite dramatically, especially at high volume where the tube rectifiers sag massively and so act as a kind of compressor/limiter, which people tend to either love or hate depending on playing style, etc.

Johnnyslim · Jul 29, 2025

Latest update. Took the amp to the shop for another look. Previously the power tube sockets were replaced and all was good but still blowing the mains fuse. He checked the rectifier tube socket and thought that should also have been replaced so he did it while I waited. We checked the KT66 tubes on his tester and all is good. I gave me a JJ rectifier tube since the Sovtek seemed to be faulty. Powered up in the shop and all is good. Biased at around 65%.

Once home I put the chassis back in the combo cab and fired it up. The Strat sounds great and I adjusted the EQ. Whew...finally got it back in the working column. Shut the amp off and I no longer use the stand-by. Then I thought I should really plug the '52 conversion in to be sure the EQ for the Strat is good for the Gibson. Again, not using the stand-by switch, flipped the power switch...BOOM...mains fuse blew. I just don't get it. I called the guy back and he said to bring it back and leave it with him because he needs to do some deeper forensic dive into this issue.

This is really wearing on me...crap. It was sounding great then poof...dead. Becoming quite the saga with this Mojotone JTM45. Somewhere, somehow there is someone who can make the necessary correction and get this amp back in a reliable state.

LPTDMSV · Jul 29, 2025

Johnnyslim said:
Latest update. Took the amp to the shop for another look. Previously the power tube sockets were replaced and all was good but still blowing the mains fuse. He checked the rectifier tube socket and thought that should also have been replaced so he did it while I waited. We checked the KT66 tubes on his tester and all is good. I gave me a JJ rectifier tube since the Sovtek seemed to be faulty. Powered up in the shop and all is good. Biased at around 65%.

Once home I put the chassis back in the combo cab and fired it up. The Strat sounds great and I adjusted the EQ. Whew...finally got it back in the working column. Shut the amp off and I no longer use the stand-by. Then I thought I should really plug the '52 conversion in to be sure the EQ for the Strat is good for the Gibson. Again, not using the stand-by switch, flipped the power switch...BOOM...mains fuse blew. I just don't get it. I called the guy back and he said to bring it back and leave it with him because he needs to do some deeper forensic dive into this issue.

This is really wearing on me...crap. It was sounding great then poof...dead. Becoming quite the saga with this Mojotone JTM45. Somewhere, somehow there is someone who can make the necessary correction and get this amp back in a reliable state.

Oh man. I feel for you!

I also feel a little bit for the techs too, multi-symptom problems on homebrew amplifiers, that could be truly exasperating.

Speaking of someone, somewhere, somehow, I follow Patrick Kauffman in Brooklyn on YouTube - never done business with him and not exactly round the corner from you (or me!) but he seems to know his stuff, old school.

CB91710 · Jul 29, 2025

The mains fuse blew before, or after the rectifier had a chance to warm up?
If the rectifier was still cold, then there was no HVDC to pull current... smells like either a problem in the transformer, or possibly in the heater circuit.

Intermittent problems suck.

Here's the 2005 Mojotone schematic for that amp, plus a crop of the power supply.

Johnnyslim · Jul 29, 2025

Yes, the amp tech has this very schematic in his shop. I no longer use the stand-by switch as indicated in a previous post. The amp was fine in the shop. Once home I plugged my Fender in and played and adjusted the EQ now that I jump inputs. All is fine. I flip the power switch off and within a minute or less I decide to try the Gibson. Flip the power switch on and dead...blown mains fuse. So, within a minute of turning the amp off then back on the mains fuse blew. Literally, I am a stones throw away from putting it in a dumpster and giving up. It pains me to think what amp I could have bought with the funds sunk in this amp over the past three months. Surprising amp techs for some reason cannot figure this "run of the mill" JTM45 out.

FYI...the guy in Philadelphia put a new power transformer in...and so has the current tech working on it. So, two power transformers in less than three months.

LPTDMSV · Jul 29, 2025

Johnnyslim said:
So, two power transformers in less than three months.

One is unfortunate - two looks like carelessness. Aside from the rectifier, I would be very inclined to investigate all the passive components around like the bias circuit, smoothing caps and by-pass caps.

As for a stand-by switch, I tend to use them if they’re there. Traditional. I’ve read protracted debates online with some people saying YOU MUST use the s/b switch and others saying there’s NO POINT.

I can’t remember which side I came out on …

searswashere · Jul 30, 2025

Dont throw good money after bad. I wouldnt put more into it, go take it from the tech today. Sell it for parts. Buy a reissue jtm or origin 50 or vintage modern and go.

JTM45 Issue

Johnnyslim

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dc007

Johnnyslim

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Johnnyslim

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searswashere

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LPTDMSV

Torren61

Worst Guitar Player On MLP

Johnnyslim

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redking

LPTDMSV

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searswashere

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Johnnyslim

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LPTDMSV

Johnnyslim

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LPTDMSV

CB91710

Not Michael Sankar

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searswashere

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