JCM 900 SLX Feedback

[jamesmafyew]

Amazing. I've been seeing this an awful lot. Aside from these JCM900s (always the 100W ones), I have MAYBE seen 2 or 3 blown OTs in the last 4 years. With the 900s, it seems like every fifth one has this specific issue.

Basically any 100W OT with a 2K primary will do, but you'll want a laydown type. Mercury is what everyone asks for when I tell them they need a new one, until I tell them the price. If you can find a used JCM2000 100W OT, it's exactly the same. It'll drop right in and all the taps are identical, though for some reason the JCM2000 OTs don't seem to blow up... although maybe when they reach the 900s' age, I'll see more of it. Seriously though, ANYTHING with the right primary Z is fine and will likely be a big improvement.

Also, any "Marshall replacement" 100W OT with a 1.7K primary will work fine too. The Mercury OTs for the 900s and 2000s have 1.7K primaries.

EDIT:
Just want to check --- you measured the two halves of the primary, right? Not the primary and then the secondary? Because the secondary SHOULD be 1 ohm or less. Realized this could have been confusing.

[jmaf]

Never mind, new tubes, got rocking and zap, back to crappy sound and no volume. measured the output transformer imediatly, 17 ohms one side, 1 ohm on the other... there we have it.

SO classic tone or is there a better source for an OT?

Your OT is probably fine.

There is a triode/tetrode switch(hi lo power) on the back. That switch flicks between hooking the screen grid to the plate or back to +B. If that switch is bad or intermitent, you are losing the voltage on the screens which would also cause what you describe.

[Tage]

I'm not jumping to conclusions calling this output transformer, it is definitely shorted between the centre tap and the one side, IOW the brown wire to the red wire, brown is centre tap. also I have confirmed the voltages every were in the amp seven times, I kept track just to make sure, writing them down each time. I even monitored screen and anode voltages on the power tubes and they never drop out or go intermittently away. also if I touch the grids of the output tubes that drive the "shorted" half of the OT there is hardly any pop, as opposed to the other two grids which give a decently loud pop. I know a lot of guys throw parts at amps, but I'm an auto technician and we are trained not to throw parts at a vehicle, we are diagnosticians and I like to know my problem for sure, that is why I depend on the expertise of my peers here.

Thanks jmaf and jamesmafyew for all the tips and help, I probably would still be trying to clean/replace the return jack if it wasn't for your inputs

[jmaf]

That was the last thing I'd have hoped it was. Did you unsolder all OT leads from the board in order to test it? Does the OT get really hot when you play? A bad tube socket will read good screen voltage but will not send that voltage to the tube itself. Well I guess I'm just trying to get it not to be the OT....

As to your previous question: Hammond 1750X are excellent replacements. When money allows, I use Mercury Magnetics on my amps, you just don't go wrong with them, though some here don't agree with their prices, I've never heard anyone say "this transformer sounds bad" with a MM.

[jamesmafyew]

I went through the same process of "UGH, anything but the OT" the first couple of times I saw this in a 900. In none of those cases did the OT get hot, but in all cases definitely measured the same way in or out of circuit. The definite test is to hook up an AC voltage to the secondary and see what comes out each side of the primary.

I also think Mercury stuff is nice -- in fact, I install a lot of Mercury iron for people who are just upgrading, mostly JCM2000s. They never want to keep the old OT afterward... thus I usually have a few JCM2000 OTs laying around, and they're the ones that have gone into the 900s with this very problem, to save the customers some money. Mercury pricing is sort of redistributionist -- if you are in a discount tier, it's not that bad. Essentially, single-unit hobbyists are subsidizing actual techs/shops. And they do sound very good, and there are interesting creative options available on the basic drop-in replacements.

[Tage]

Yeah I desoldered it and checked again, still only 1.5 ohms on the one half of the primary. Also it never did get hot and that is why I have torn this amp apart over the past week and a half and found absolutly nothing untill that one tube seemed out, and then now I have a hard fault in the OT. I really hate intermittent issues like this, I get them enough with my day job

But I have a transformer en route and my customer seemed kinda excited to put a new one in, he doesn't have the cashflow for a mercury transformer but I have used them in my builds and I liked the results. I'll let you know how the classic tone works out.

[J M Fahey]

I´, TIRED of replacing JCM900 OTs.
As in *dozens*.
Specially because the Marshall Importer in Argentina (Import Music) got fed up with replacing them repeatedly (as in a guy blowing them 3 times in 3 months) so they now routinely send them to me.
*BIG* problem lies in ultra-thin "self-soldering" wire enamel, the kind you just wrap around a terminal or piece of wire, just apply your soldering iron and it *evaporates* in 5 seconds, leaving shiny clean copper behind.
I guess they save 3 minutes on each OT by not having to burn and scratch strong Epoxy enamel first.
Oh well.
I rewind them using "Class F" Electric Motor specified 170ºC Epoxy enameled wire.
What the older Marshall OTs used.
Probably they improved the newer ones.
This "self soldering" wires usually comes in special colors as not to be meitakem with the good" one, usually bright red or green , lately comes in a "gold" hue, which can be misleading.
Fine in your SEGA PSU, where wires must go straight to PCB holes or switches and soldered as fast as possible ... but they do not belong in a Tube OT and its associated thousands of volts peaks possible with poor connections.
@ JMAF: estou usando Fio Esmaltado Brasileiro Classe F da São Marco: ótima qualidade

[jamesmafyew]

JM, that is fascinating. I've attached a couple pictures of different OTs. In the picture of the two 100W OTs, the one on the left is a JCM900/4100 100W OT, unlabeled but presumably made by Dagnall. The one on the right in that picture is a Dagnall from a JCM2000 TSL 100. Both have all the same taps. Both are blown with the same fault, but now that you've mentioned this I see that the 900 OT has red enamel and the 2000 OT looks more gold. I suppose it's possible that the 2000 OT is the self-soldering stuff as well, though I haven't seen nearly as many blown OTs in JCM2000 series stuff (maybe 1 or 2).

The other picture is of a 50W Drake OT from a JCM900 2500 head. It's not blown, it just got replaced by a Mercury. But I notice that it also has the red enameled wire.

[jmaf]

I´, TIRED of replacing JCM900 OTs.
As in *dozens*.
Specially because the Marshall Importer in Argentina (Import Music) got fed up with replacing them repeatedly (as in a guy blowing them 3 times in 3 months) so they now routinely send them to me.
*BIG* problem lies in ultra-thin "self-soldering" wire enamel, the kind you just wrap around a terminal or piece of wire, just apply your soldering iron and it *evaporates* in 5 seconds, leaving shiny clean copper behind.
I guess they save 3 minutes on each OT by not having to burn and scratch strong Epoxy enamel first.
Oh well.
I rewind them using "Class F" Electric Motor specified 170ºC Epoxy enameled wire.
What the older Marshall OTs used.
Probably they improved the newer ones.
This "self soldering" wires usually comes in special colors as not to be meitakem with the good" one, usually bright red or green , lately comes in a "gold" hue, which can be misleading.
Fine in your SEGA PSU, where wires must go straight to PCB holes or switches and soldered as fast as possible ... but they do not belong in a Tube OT and its associated thousands of volts peaks possible with poor connections.
@ JMAF: estou usando Fio Esmaltado Brasileiro Classe F da São Marco: ótima qualidade

That is absolutely incredible. It was the only thing I praised about the JCM 900's, the two transformers are big heavy iron, it's unbelievable that Marshall would also save money on that. Thanks for sharing that bit of experience, JM. (Had never seen one with a bad OT, curiously enough....)

Maybe a mod should be standard on these amps, to protect the OT, there are so many tricks that would do....

About the wire: valeu mesmo a dica! Vou procurar por aqui.

[jmaf]

[ATTACH=CONFIG]17139[/ATTACH]

The label on the Dagnall C3070 says 50/60Hz, do you know why?

[Tage]

Yeah this transformer definitely has that red enamel wire too. this is gonna be a standard check for me from now on with these amps. To bad an OT is one of the last things I ever expect to fail. I'm curious as to why it has the 50/60 Hertz label as well, I've only ever seen that on power transformers and only one that I can recall really.

[g1]

I'm guessing all their PT's are good for 50/60hz and all their stickers say that, whether they are PT's or OT's.

[jamesmafyew]

The label on the Dagnall C3070 says 50/60Hz, do you know why?

No idea. They all seem to say that, power and output alike, from many different years and batches.

That one is from a late-90s TSL 100. I had a good one from a 2010 DSL 100 ("reissue"), which I sold last week. Now I wish I had taken pictures of it, because I want to know what the enamel looked like on that one. I do know that the laminations were a different color than either of these and so was the plastic bobbin.

[Tage]

SO this is many years too late but I hate threads that end without a confirmed fix so here I am to say Yes it was the OT and that amp was fixed and honestly a lot healthier sounding than it had ever been.

I have the old OT and some day I will re-wind it with some proper enamel transformer wire to make a nice beefy 50 watt OT...

Tage