Issues with new JTM45 build

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JoeTele
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Issues with new JTM45 build

Post by JoeTele »

Howdy,

A friend of mine built the mojo JTM45. I went down today to help him power it up but there were some issues. In a nutshell only the lo inputs seemed to pass a signal, and even with my cell phone signal generator cranked and the amp dimed, we only got a super quiet signal out the speaker. Absolutely nothing with a guitar plugged in, not even speaker hum. Also, my friend thinks enough heat is being generated to melt wire where the phase inverter grids meet the one meg resistors on either side of the 470ohm bias resistor (i.e. it wasn't his soldering iron that did it). Plate and heater voltages look good throughout, the tone caps are blocking DC, as are the caps feeding the power tube grids.

Any ideas what might be going on?

Thanks!

Joe
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M Fowler
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Re: Issues with new JTM45 build

Post by M Fowler »

A lot of build experience? My first impression is miss wiring of the power section.

If one is not familiar with wiring tube amps solely depending on the layout from Mojotone, which is not very good, one can get into a heap of trouble.

Otherwise, can't advise without clear photos.

Mark
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xtian
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Re: Issues with new JTM45 build

Post by xtian »

Stop! And do basic first-startup procedures:

http://paulrubyamplifiers.com/info.html#FirstPowerUp
I build and repair tube amps. http://amps.monkeymatic.com
JoeTele
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Re: Issues with new JTM45 build

Post by JoeTele »

Thanks for the tips! We did a series of Ruby inspired slow power ups with a Variac starting with no tubes, etc, while monitoring current on a Kill-a-watt and checking temperatures with a thermal gun. Seemed to go very smoothly, and when all of the tubes were in a scope showed AC all the way to the grids to the power tubes, though judging by the volume it's obviously a small one, so I guess you could say we're in Ruby's AC debugging phase.

One thing I did notice was that at full power a meter showed about 30 volts on the phase inverter grids. Once I saw that I initially thought it could be the problem, as a JTM45 voltage chart we were following had those pins blanked out, but then I found a thread on a different forum about the same issue that suggest this was normal, so did not include it in my post yesterday.

Could someone just tell me if it is indeed normal to read a voltage like that on the PI grids?

Thanks again!

Joe
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Re: Issues with new JTM45 build

Post by sluckey »

Could someone just tell me if it is indeed normal to read a voltage like that on the PI grids?
Perfectly normal for that LTP PI.
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MakerDP
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Re: Issues with new JTM45 build

Post by MakerDP »

If you have a scope check each point in the system for the audio signal or use an audio probe and small amp if you don't have a scope.

Start at the input jacks and follow the signal path through the system until you find where you lose the signal. That will narrow down where your problem is and you/we can go from there.

I'm assuming you did the obvious things like changing to known working tubes, your guitar cables are OK, etc.
JoeTele
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Re: Issues with new JTM45 build

Post by JoeTele »

Yep, did run a scope through and saw signal on the PI grids, but only the super weak signal out the speaker. Think I'll need to check the preamp stages more closely, including the tubes. Probing the PI grids also put out a little pop, understandably.

Come to think of it, early in the day there was noise out the speaker, but also a good degree of transformer rattle, which eventually went away. I figured the rattle may have just been due to the way the amp was sitting on the stand or something, but will look into that more. My friend lives a ways away but hopefully I'll be able to look tomorrow.

Thanks!

Joe
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MakerDP
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Re: Issues with new JTM45 build

Post by MakerDP »

What about the signal coming off the PI plates?
JoeTele
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Re: Issues with new JTM45 build

Post by JoeTele »

Should have said grids to power tubes.

I've got the amp and am troubleshooting it now. I'm thinking it could have to do with incorrect wiring of the Marshall style input and speaker jacks. Is it correct to say that:

1. If an input jack is wired properly and I plug a 1/4 inch cable into it, I should see a high impedance when measuring from the tip to the sleeve of the exposed side of the cable (FYI I'm doing this with the amp off)?

2.If the speaker jacks are wired properly, and I plug a speaker, say 8 ohms, into the primary jacks, again with the amp off, I should see that 8 ohms (+ meter lead resistance) across the jacks terminals?

I've seen noting but 1.7 or so ohm readings when I do this on all of the jacks, both speaker and input. On my own Super Reverb build I get about 1 meg and 125k depending on the input jack. From what I can tell, my friend followed the wiring diagram for those jacks exactly, but truth be told they're like black boxes to me and a previous post obviously mentioned issues with some of those diagrams.

Needless to say I'm checking the OT.

Thanks!

Joe
JoeTele
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Re: Issues with new JTM45 build

Post by JoeTele »

Just and addendum to question 2 above.

2.If the speaker jacks are wired properly, and I plug a speaker, say 8 ohms, into the primary (or only) jack, again with the amp off *and the OT secondaries disconnected*, should I see that 8 ohms (+ meter lead resistance) across the jacks terminals?

Joe
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MakerDP
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Re: Issues with new JTM45 build

Post by MakerDP »

So I don't understand... you are saying that you see a signal (I mean you used a signal generator of some sort into the input jack say like a 70 to 100mV @ 1kHz signal) and then you see that signal on a scope at each plate output (post coupling cap)? And they are reasonably amplified at each point? And the waveforms look like you would expect them to? You see the signal at the power tube grids?

Then why do you suspect the input jacks are bad?

What am I missing here? :?: :?: :?:
JoeTele
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Re: Issues with new JTM45 build

Post by JoeTele »

Turns out it was indeed the wiring of the jacks. I pulled all of the cliff input jacks and put in a single fender jack with a 680k input resistor. I then decoupled the primary speaker jack from the second one such that, with the ot disconnected, the jack was actually seeing a resistance beyond that of the meter leads with a speaker plugged in (and pretty close to the rated impedance of the speaker). I re-variac'd up slowly, monitoring current and heat, sent a sine way to the input and saw it beautifully rendered across the speaker output. Got out a real speaker, plugged in a strat and, behold, she's a Marshall!

I also made sure some of the hot spots I mentioned in earlier posts were no longer such with the thermal gun.

Thanks again!

Joe (grateful his friend's OT didn't go poof!)
JoeTele
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Re: Issues with new JTM45 build

Post by JoeTele »

Maker DP, I think I posted at the same time as you. Yes there was a signal all along, even out the speaker, but it was minimal, even with the volume on the amp all the way up. In hindsight, I've measured and it was about a 300mv signal going in from my phone, certainly enough to make a loud sound out the working JTM45. My scope experience isn't super extensive, but I think the ability to zoom in on the signal made it look bigger than it actually was. I failed to actually look at the volts per division to see whether it was really as big as it should have been in the various places.

Joe
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M Fowler
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Re: Issues with new JTM45 build

Post by M Fowler »

A good set of eyes should have solved that problem prior to fire up. wow.
JoeTele
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Re: Issues with new JTM45 build

Post by JoeTele »

Fair enough. For future reference could someone tell me whether the attached diagrams are correct in terms of how to wire the cliff jacks? For better or worse, this is what we followed. I've compared the actual jacks to the diagram several times and the wiring matches. I assume that in the diagram the solder terminals are oriented upwards as the builder would be looking at them/soldering to them, and that the small rectangles conjoined to the larger represent the input side of the jacks.
InputJacks.jpg
SpeakerJacks.jpg

Joe
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