JTM30 amp - help it's quiet! lol

[Kirk UK]

Hi All,

I have a JTM30 amp I bought cheap about a year ago. I've known all along that there's been a problem with it as it's quite quiet (on 10 I can happily sit infront of it play guitar and sing and be heard lol). Also - the reverb doesn't appear to do anything - although I'm not sure if this is because the amp is quite quiet - or if it's bust! lol

I've been trying to trouble shoot for a while, reading various posts on here about problems with the JTM30. So far I've worked out that

1) the fx loop seems fine. used contact cleaner and a patch cable and there is no change in volume.
2) All of the tubes are glowing and there is no apparent wiggle - in fact they all fit in pretty tightly.

I got a multimeter off a mate and downloaded the schematics and wanted to see if the treble pot is bust since I know all the sound goes through this. However, I'm not sure how to do this - can anyone point me in the right direction?

I took the amp apart last night though and have two questions (sorry I'm a noob to this sort of stuff).

1)Does anyone know if the amp has a resistor to help the capacitors discharge or should I be trying to do this manually (didn't even think about this until I'd already had my hands in there and was looking around touching stuff!! )
2) I noticed that behind the tube sockets on the back of the pcb board - there was a lot of corrosion around the solder joints - and also on various other solder joints on the board. Could these be the cause of the mysteriously low volume? Or is this normal and should I try to re-flow them all?

Any help grateful as I'm yet to fully appreciate this amp and it's my first tube amp.

P.S it's running 5881/6l6wgc sovteks which I believe are the stock tubes....

Cheers

 

[Kirk UK]

Seriously - no-one on this forum has any knowledge about the JTM30 - or has had a similar issue to me?!

Surely some-one out there must be able to help me!

Even if it's just in relation to one of the points!

 

[Guitar-Rocker]

I went on the web to look at a JTM30 schematic, as I am not familiar with one. I could not find any evidence of a bleed resistor to discharge your caps. Perhaps someone on here could assist you if you can take a photo of the corrosion that you have described. Have you tried swapping tubes at all, either preamp or power?

 

[Gene Ballzz]

Do you have or have access to an o-scope? If so, simply inject a signal to the input and start probing the signal path in order from the input. When you get to a point where the level drops (as opposed to staying the same or increasing), you've found the issue. There are some backyard/shade tree ways to probe, but I won't put them up here, as an inexperienced person could misconstrue and get them self into SERIOUS trouble real quick!
HTH,
Gene

 

[Kirk UK]

hi Gene & G-R,

Thanks for the replies - unfortunately no I don't have access to an o-scope and I also don't have another set of tubes to swap in. I've asked around a few people and no-one I know have any I could 'borrow' either. I'll ask one more option and failing that may have to bite the bullet and buy a couple of test them - i can always have spares I suppose if they're ok.....

I've attached (hoping it's worked) a pic of the corrosion on the back of the tube socket pins. It's the same story on both tube sockets - the pre-amps are fine (you can see part of one on the right).

I used my multimeter set on DC Volts to test the capacitors - hoping I'm doing it right by connecting neg to the chasis and pos to the pos side and was getting 0 - assume this means that they have drained?

Thanks for your help thus far.........

 

[Guitar-Rocker]

Cleaning off that that corrosion between pins is a definite place to start. You do not want an electrical trace path for electricity to flow between those pins. It shouldn't take much to break those paths.

 

[HumanJHawkins]

hi Gene & G-R,

Thanks for the replies - unfortunately no I don't have access to an o-scope and I also don't have another set of tubes to swap in. I've asked around a few people and no-one I know have any I could 'borrow' either. I'll ask one more option and failing that may have to bite the bullet and buy a couple of test them - i can always have spares I suppose if they're ok.....

I've attached (hoping it's worked) a pic of the corrosion on the back of the tube socket pins. It's the same story on both tube sockets - the pre-amps are fine (you can see part of one on the right).

I used my multimeter set on DC Volts to test the capacitors - hoping I'm doing it right by connecting neg to the chasis and pos to the pos side and was getting 0 - assume this means that they have drained?

Thanks for your help thus far.........

Yikes... Unplug the tubes and test for conductivity between socket pins. Although it could be a lot of other things, that picture makes me tempted to say <Voice of Larry the Cable guy> "Well, there's yer problem, right thare."

Good luck.

 

[Kirk UK]

OK, so I am posting an update. The AMP is still NOT fixed unfortunately and here is a list of everything I have done thus far:



1) Bought new Power tubes - no difference.

2) Bought one new preamp tried it in each pre-amp socket V1, V2, V3 - no difference.

3) Changed the treble pot as I know these have caused issues on other JTM30's since the signal flows through this pot. No difference.

4) Changed the power-tube sockets incase these had melted, plus there was corrosion around the solder joints on the back of the board. No difference.

5) Put a patch cable in between the FX-loops - guess what. No difference.

6) Cleaned the PCB (both) and checked for corrosion (removed) and bad solderjoints (couldn't see any cold ones). Cleaned all of the jacks. No difference!



I'm not a bit lost as to what could be left! I have a multimeter, and would like to test the OT to see if there is an issue there. Only problem is I'm not technical so don't really know what to do - I am however, great at following instructions! lol So if anyone knows of any good (or can make me) instructions for how to do this easily with a MM (if it's possible). That'd be great.



Alternatively if anyone has any ideas as to what else it could be - I'd be grateful! It's driving me mad! lol

 

[Micky]

There is a possibility that more than 1 preamp tube is weak.
I would change all power tubes, re-bias, and then change all preamp tubes.

Plug your guitar directly into the FX return and see if it is incredibly loud...
If it is, then the problem is in the preamp section.
If not then it is in the power tubes (plus V3 the Phase Inverter)

 

[Kirk UK]

Thanks, I'll try that tonight!

 

[X2203xman]

I have a jtm-30,and really like it.Sounds like you're deffinately doing the amp some good.For what it's worth, when you tube it,get a matched set of 6l6's for your power tubes.They will sound,and run better than the 5881 sovteks that come stock.Look hard at that loop again.Micky had a fine suggestion.

 

[Kirk UK]

Right so I did as Micky suggested. No joy though, it was actually slightly quieter than in the input? Hmmm.

Btw xman thanks for suggestion, that's actually what I did when I bought new valves, I got 6l6s

Am lost as to what this be now though. I've even tried an alternative lead to the cab from the amp. Nada it's still quiet.

Although when I say quiet it's loud, just not 30watts of Marshall tube loud. More like 10 watts of park ss amp loud. Lol

 

[scat7s]

you plugged into the serial loop return with your guitar cable? and no improvement?

try swapping the phase inverter with another preamp tube and do the same test.

 

[Micky]

Yes. The problem lies in the power amp section.

 

[chuckharmonjr]

Sounds like its time to start looking at pin voltages with no tubes in the power sockets

 

[X2203xman]

I think most jtm-30 owners will agree,that the amp is not very loud when running correctly.Cranked,it should be louder than your tv will ever get,but not run you out of the room loud.I think my class 5 may have been louder,or as loud as the jtm-30.

 

[Kirk UK]

Thanks, I think I'll have to start looking at the pin voltage then - I'll do a bit of reading up on this. And I'll try the FX loop input trick again tonight with a new valve in the Phase invertor position - although I'm fairly sure that's where I put the new JJ 12ax7....I'll check tonight.

I mean it could just be me then - but I can happily sit infront of the amp with it at 10 and whilst its' loudish I can sing over the top of it. It's not louder than the TV - but then that's running through a SONY 820 amp with Q Acoustic floor standers, stand mounts and a REL sub lol

Either way I have a SS 5210 50Watt combo 1x12 and that thing at 1-2 is probably louder than the JTM30 at 10 - now i know in part that may be due to the Audio taper on the 5210, but that thing is LOUD and makes the other half complain at 1-2 - not the same problem with the 30.

Thanks for all the suggestions thus far, I appreciate them.

 

[strat59]

I had a JTM-30 a long time ago now, from what I remember they weren't that loud an amp. Could be filter caps or blown O/T.

Servicing a Marshall JTM 30 Combo Best Tech in the UK John Chambers. Email or Call him.

 

[scat7s]

30 watts should be pretty loud.

ive never heard a jtm30 in person, but i have a jtm60. the output transformer is tiny, but it is pretty f-ing loud. it could be an OT.

is it biased properly?

 

[dreyn77]

Have a look at your power lead, plug, connections, power board etc...
I've found corrosion on the pins before.

Mine will get to about 3 and a half on the DSL 100's volume dial. I've never had any problems with it. It's still on the 6L6's from when I bought it. They glow orange and blue/purple.