JTM45 Build

martin manning · Post by **martin manning** » Wed Jul 01, 2015 5:54 pm

You are pretty sure that the board becoming conductive due to overheating is the problem then? Is this a known issue with this particular material?

Littlewyan · Post by **Littlewyan** » Wed Jul 01, 2015 6:02 pm

Well when I disconnected the treble cap and disconnected the wire from the treble cap turret to the treble pot leaving the turret empty and not connected to anything I measured 60mV on it. Also whenever I touched the turret with my meter probe I could hear an electrical connection through the speaker. Same thing happened with the empty turret I put in near the V1 Cathode (in case I ever went split cathode). However even stranger as I could increase the volume of the noise with the volume control!

I was soldering to this board with the iron set to 400 degrees and it's max working temperature is 120 degrees so chances are I have royally duffed it. I dunno, maybe I've sorted it by scraping away that wet stuff I found connecting the NFB and HT turrets, but its not worth the risk putting this board back in the amp. Also if I redo the board I can redo the wiring and grounding.

Its not a known issue with the material but the manufacturer didn't design it to cope with 400 degrees so who knows what I've done to it.

joeboo88 · Post by **joeboo88** » Wed Jul 01, 2015 6:07 pm

Sooty to hear about the board problems Ryan. Keep us updated...

Littlewyan · Post by **Littlewyan** » Wed Jul 01, 2015 6:13 pm

Ah its ok. Lets hope this fixes it.

guitarmike2107 · Post by **guitarmike2107** » Wed Jul 01, 2015 6:20 pm

No its not a known issue with the material, its the same material that was used by Marshall and Hiwatt and the majority of tag strips etc etc.

It may not be the board, I have well overheated these boards before without any issues like this, but like Ryan said if its replaced then at least we know that wont be the issue again.

Just for info the Phenolic material has a maximum working temp of 120 Celsius, and most modern solder melt around 250 Celsius, so it would be easy to locally overheat the board, but I can't see it over heating from one turret to the next, allowing the solder to move through the board.

I was looking at G10 board temps, they vary a bit, but are slightly higher, still possible to damage them with a 400C iron though.

Littlewyan · Post by **Littlewyan** » Sun Jul 05, 2015 2:36 pm

Just finished rebuilding the amp and it has the same issue

. However I think it has improved somewhat as it seems much more playable now with the tone controls on 5 and I'm not measuring any voltage on the treble cap anymore. I haven't yet adopted a new grounding layout as I just wanted to see if the new board fixed it first.

It seems to start when I have the treble above 6 and I also get a mosquito noise trailing some notes which I think Marshalls are known for. So going by that perhaps its lead dress?

Reeltarded · Post by **Reeltarded** » Sun Jul 05, 2015 4:47 pm

That isn't the noise they are known for. You should hear ghosted sub stuff if you have stock filtering and cranked but that thing will come apart cranked because NOW it might be simply picking up ultra stuff.

What a drag.

Is there any change in that kinky PI? What changed about the response..

Please stop that first grid on whichever channel and turn the other channel off and see if it loses it's shit fancy.

Littlewyan · Post by **Littlewyan** » Sun Jul 05, 2015 5:18 pm

Can't recreate the mosquito sound now, possibly because I put valve covers on the pre amp valves. Forgot those, whoops.

The PI still goes nuts on one side if I have the treble on 10 and everything else at 0. Just tried connecting shielded wire to a jack socket with a 100k grid stopper connected directly to V1B (disconnected jack sockets in amp) and this didn't help.

Its interesting that I can now play the amp as long as either the treble isn't above 5 or if it is I have the middle at 5 or over. Bass control doesnt make a difference to the issue. So it has got better, but why! The only change I made that could have helped was I moved the NFB wire to the other side of V4.

I don't think its a faulty component. I think its either lead dress or grounding causing the issue which seems to be parasitic oscillation.

Littlewyan · Post by **Littlewyan** » Sun Jul 05, 2015 7:06 pm

OK bit weird. If I touch my attenuator when both the treble and presence are up then I get a high pitch pulsing noise through the speaker. Pretty sure my Express doesn't do that.....

Another thing to add is that the issue either completely gone or just greatly reduced if I jumper in a cap and resistor to convert the tonestack to 33k/500pF. Its as if a certain impedance with the tone controls set a certain way squashes the oscillation.

Edit: The Express doesn't squeal if I touch the attenuator but the Marshall does, every time! So...what does that tell us? That there is an issue with the speaker ground on the Marshall perhaps? I just measured it with my meter and its grounded ok.

Reeltarded · Post by **Reeltarded** » Sun Jul 05, 2015 7:45 pm

Have you tried bypassing the tone stack, completely?

I wonder about the pots in the TS being baked.. or one them. It's something like that. It has to be.

It has to be the treble pot. Gotta.

Littlewyan · Post by **Littlewyan** » Sun Jul 05, 2015 7:50 pm

I didn't bypass the tonestack completely but I disconnected the ground for it and it was fine. Just tried my Express again with the attenuator and the most that happens when I touch it is the hiss changes very slightly. Marshall, instant oscillation. I guess it could be because the Marshall has a lot more negative feedback.

I've measured the pots and they're all fine. I've cut the ground buss in half by the way inbetween the treble and volume pots. So now the grounds for the pre amp are seperate to the PI and tonestack grounds. What I may try is disconnecting the speaker ground from the lug on the valve socket and connect it to the ground buss for the PI and Tonestack.

Littlewyan · Post by **Littlewyan** » Sun Jul 05, 2015 8:38 pm

Moved the speaker ground to the PI buss but made no difference.

Perhaps I should try making that audio probe Martin told us about so I can hear where in the amp it goes wrong. Its either that or wait for my scope to come back from being repaired so I can use the spectrum analyzer on the amp.

Edit: The fact that its improved a lot since the rebuild does suggest there is a lead dress issue as all parts are in the same positions, except for a few resistors. I would find it hard to believe its the tonestack wiring as so many Marshalls are a complete mess in that area!

Edit2: New scope is arriving this week so i'll have a spectrum analyzer again! Hopefully I can use it to see what frequency is dominating the rest in the tonestack. Its interesting that the 33k/500pF Tonestack seemed to greatly reduce the issue.

Littlewyan · Post by **Littlewyan** » Tue Jul 07, 2015 6:39 am

Are there any specific tests I can do with the scope?

Reeltarded · Post by **Reeltarded** » Wed Jul 08, 2015 12:02 am

Sine wave and find the point where it all goes to hell. It all goes to hell at the PI if it's not in the tone stack. It's at least on that node.

Littlewyan · Post by **Littlewyan** » Wed Jul 08, 2015 7:22 am

I've been doing tests with Colossal and its definitely the PI that is the issue. Resistors all read the correct values so must be a dodgy cap. Going to run more tests either tonight or tomorrow night. I'm starting to suspect both bias caps are bad, as I tried replacing one at a time and that didn't make any difference. However when rebuilding the amp it turns out I had swapped the caps around. Anyway going to replace both and also test both with my meter.

Tube Amp Archive

JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build

Re: JTM45 Build