HELP!!!

[Yeadaki]

I built my JTM 45 in 2009. Today it has no output. I was playing and the sound just dropped. I've got the power light and the tubes glow.

My first inclination is to go back thru the testing points in the instructions when I first fired it up. But, I'm not sure if thats the best route.

Troubleshooting is not my strength. Can anyone point me in the right direction?

I'll start searching for similar threads.

Any help would be much appreciated.

[Yeadaki]

I swapped the output tubes and it works. Looks like it was a tube issue. I hate it when its something simple - I feel like there's more to the story.

No news is good news.

[Yeadaki]

I guess there is more to the story. It did it again. Just playing along and it goes silent.

Something is not working upon load/heat buildup.

So, it's not working and it doesn't appear to be a tube issue after all.

Any help would be appreciated.

Thanks!

[danman]

When you say that you swapped the power tubes, did you put in a new fresh set and properly bias the amp? How about preamp tubes? You may have a failing preamp tube, so if you have another good one from a different amp, I would try subbing it in all three spots and testing the amp each time. It's also a good idea to try the amp through another cab and try out another speaker cable between your cab and head. Any one of these items may be failing and it will take the process of elimination to find it.

[Yeadaki]

Yes - the tubes were properly biased. It's a trusty backup pair that I have. I went back to the old ones and I got the same response. Speaker cab and cables are good. I'm currently playing them thru another head and all is well with those items.

I will go for the pre-amp tubes as you suggested. I guess its a good time to acquire some back ups for those too.

I'll post results when I get some new tubes.

Thanks!

[Yeadaki]

UPDATE: I borrowed another JJ ECC83s from another amp. Starting at the preamp tube furtherest from the power tubes, I swapped tubes. The first one made no difference. Then I swapped the middle one and it fired right up.

Here's the weird part (and it's happened twice now), the amp goes silent when I roll the tone towards 10 on the guitar.

I'm playing a Les Paul with Seymour Duncan antiquities and two vintage Sprague "bumble bee" mylar capacitors (.022uF, 400 Volts).

But I don't suspect the guitar as I'm playing fine through two other amps - all ranges of tone settings.

But twice now this is when the JTM 45 has died - when I crank the tone on one pickup.

I ordered a new set of preamp tubes from tube depot - matched set. I'll try that when they get here.

Any other ideas guys?

Thanks!

Chad

[neikeel]

Odd isn't it!

Turning the treble up and it cuts out sounds like parasitic oscillation. So it would be interesting to know if dialling back the treble brought the sound back (if it does then you need to examine your lead dress, and that you have grid stoppers on pin 5 and your presence wires are correctly routed).

We know that the treble caps in the tone stack have a failure rate (the silver mica) too, consider that.

What preamp can do you have?

Also remember that V2 gets a really hard time in one of these and you should get the most robust version (a NOS or good pull Mullard, Brimar, Phillips etc) would be my choice .

Please keep us updated.

[Yeadaki]

UPDATE:

I installed a new matched set of 12AX7's - nothing.

So I went back through the power up procedures in the build instructions and all voltages were within tolerance according to the chart in the instructions.

I can hear a faint click when standby is engaged but I have no sound.

There's no change when volume is increased.

Can anyone provide me a some test points with normal voltage readings that can possibly help me narrow my search?

Thanks!

[stef]

Post pictures...

[Yeadaki]

Will do when I get home. I may try to pull up old pics from previous posts on the forum.

It worked great for 6 years. I'm thingking something died on the board. All the voltages are correct for tubes, etc. I've got new preamp tubes and I've tried swapping power tubes with known good ones. Rectifier voltages were good.

Given that, is there a chart avaialble, etc., for testing voltages for components on the board? Or are there some common test points to check for basic troubleshooting?

Thanks.

Chad

[Yeadaki]

Heres some pics

[neikeel]

It worked great for 6 years. I'm thingking something died on the board. All the voltages are correct for tubes, etc. I've got new preamp tubes and I've tried swapping power tubes with known good ones.
Chad

I agree, most likely thing as above is a duff treble cap in tone stack.

[Yeadaki]

I'll swap it and try - it's cheaper than an output transformer.

I sure do appreciate your help.

[neikeel]

I'll swap it and try - it's cheaper than an output transformer.

I sure do appreciate your help.

Re reading the thread do not forget to give the output selector a few spins too (they sometimes stick and make a poor contact, check your soldering between the OT and output jacks, silly things but worth exploring.

[bobtec]

you did a nice job looks clean..I know you will find problem...