JCM 2000 Volume Drop

Vig

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I have a JCM 2000 that out of the blue the volume will drop to completely quiet then about one minute will go by and it comes back up. Relatively random as to when it happens mostly in the first 15 minutes but not all the time. Getting more frequent. Where do I start? Thanks in advance.
 

Marcomel79

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Im no expert but i would start by checking the preamp tubes...
 

Tatzmann

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Start: Effectsloop Patchcable.

Volume back to normal? Then clean
effectsloop-jacks. Maybe effectsloop
button need cleaning too.

Volume not back to normal?
Maybe dirty preampsockets like guy
before me suggested.

Still have problems?
 

Vig

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Is it a slow smooth fade, out and back in?
Yes, slowly goes to barely any amplified sound and slowly rises back up after about 1-2 mins max. It seems to be starting to happen more frequently to where I am anticipating something more to start happening and do not want to make it worse either.
 

Vig

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Start: Effectsloop Patchcable.

Volume back to normal? Then clean
effectsloop-jacks. Maybe effectsloop
button need cleaning too.

Volume not back to normal?
Maybe dirty preampsockets like guy
before me suggested.

Still have problems?
This started happening a couple years back and I only recently in the last 2 months started using the effects loop and that said, it does seem to be happening more frequently since I started using the effects loop.
I can play for days weeks and nothing happens then out of the blue, it will slowly fade to nothing and anywhere from 30 seconds to a minute or 2 max it will slowly fade back in. I have never had a repeat while playing. One and done.
 

Handel85

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Hey guys, I'm having a similar issue with my JCM2000 DSL100. to catch you up on what i've done so far. Volume started going in and out and then would simply die. switched to the 16 ohm input and it corrected it, then that also died. did some research and applied the speaker ground fix that evidently plagues this amp something awful. that at least allowed me to hear my amp but now it's at what sounds like half volume. this happened before but I just swapped out some of the preamp tubes and that seemed to take care of it temporarily. I bought this off reverb and it's been a great amp but just recently started giving me grief. it's a 2002 model. The two preamp tubes, #s 3 and 4 seem very dim and i've swapped out several different tubes for those sockets and all seem to produce the same results. then again, these tubes are not brand new. I have the stock originals that came with the JCM2000 and the player's preferred preamp tubes when they sold the amp. so 8 previously used preamp tubes in total, and i imagine right now it's like im trading out 4 well used AA batteries for another 4 well used AA batteries.

biased at 75 for each pair of EL34 poweramp tubes btw. got any thoughts? I'm a newbie when it comes to amp repair and just know enough about soldering to do that simple ground fix on the speaker inputs.
IMG_3757.jpg IMG_3758.jpg
IMG_3759.jpg IMG_3760.jpg
 

neikeel

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Of course it’s your filter caps you need to replace them🤣 that is the answer to everything in valve amps (that and 42) - sorry - could not resist, highly unlikely!
The fact that swapping preamp tubes helped - albeit temporarily suggests a poor contact on one of the preamp sockets. Either where the tube pins go into the socket or more likely a dry/failed joint on the board. Particularly if a lot of tube rolling has occurred.
You will need to open the amp up and carefully inspect each preamp mount on the board. Reflowing the joints with fine iron, dab of flux and good solder is likely all that is needed.
 

Pete Farrington

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Are the duller looking valves, in V3 and V4 sockets, getting as hot as those in V1 and V2?

How bright a valve glows is an uncontrolled characteristic, totally immaterial to its operation. It’s just a byproduct.
How about if the valves in V1&2 are swapped around with those in V3&4; does the degree of glow follow the valve or stay with the socket?
 

Handel85

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Are the duller looking valves, in V3 and V4 sockets, getting as hot as those in V1 and V2?

How bright a valve glows is an uncontrolled characteristic, totally immaterial to its operation. It’s just a byproduct.
How about if the valves in V1&2 are swapped around with those in V3&4; does the degree of glow follow the valve or stay with the socket?
i will test this today and see what results i get back. these are good troubleshooting steps i have not tried up to this point. thanks!
 

Handel85

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i forgot to upload this video showing the powertubes with the amp dimed. one pair seems to possibly be matching what i've seen in red plating tubes, but again, this is all relatively new to me. if need be I'll be sure to open the amp up and check for dry/failed joints and reflow. thanks guys
 

neikeel

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Those tubes are redplating and will be toast shortly!
Back off the bias on that pair - see if that stops it.
It maybe that the dodgy joint is on the PI tube (in your amp V4) and the non-redplating pair are idling and the redplating pair working too hard.
Either way do not run the amp with those tubes set up like that.
As Pete said the glow of pre-amp tubes is not indicative of activity or goodness (most Mullards glow brightly on fire up and then settle down to virtually no glow) obviously if no glow at all then the heater might have failed!
 

Handel85

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Those tubes are redplating and will be toast shortly!
Back off the bias on that pair - see if that stops it.
It maybe that the dodgy joint is on the PI tube (in your amp V4) and the non-redplating pair are idling and the redplating pair working too hard.
Either way do not run the amp with those tubes set up like that.
As Pete said the glow of pre-amp tubes is not indicative of activity or goodness (most Mullards glow brightly on fire up and then settle down to virtually no glow) obviously if no glow at all then the heater might have failed!
thanks again neikeel and pete! so is it safe to assume next steps would be to lower the bias on that pair and also buy another pair of EL34s (If I can muster up the cash to afford them these days)? and of course not use the amp if lowering the bias does not help with the red plating

also as far as the dodgy joint, do you mean a solder joint that needs reflowing?
 

b-spot

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Some jcm2000's have a fault that can develop with the switching jacks for the speaker outs, resulting in the amp seeing no load and the audio cutting out. The grounds should be hardwired.
 

Handel85

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Some jcm2000's have a fault that can develop with the switching jacks for the speaker outs, resulting in the amp seeing no load and the audio cutting out. The grounds should be hardwired.
yeah that was the first thing i did apart from checking the bias. im really glad i applied this mod. it's been long overdue
 

Handel85

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I'd honestly like to try these as my replacement tubes. what do you guys think? for the JCM2000 DSL100
 

Handel85

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update: board is now detached from chassis and inspecting solder joints, etc. thoughts? i do plan to order the jcm2000 bias kit soon to get that out of the way. i checked bias before I took the amp apart and it was around 80 for each pair. no positive change in symptom. Volume was still significantly lower than it should be. power tubes that previously seemed to be demonstrating red plating weren't doing so today.

IMG_3774 (1).jpg
IMG_3775 (2).jpg
 
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neikeel

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For some reason I cannot mag up your pictures, even so I suggest you reflow the preamp tube sockets.
We may be getting close to your problem.
I do not think you need the bias board mod, it is more likely to be one of your bias pots being faulty. Clean them both with contact cleaner and apply some Deoxit gold to lube and clean them, then retry. If it does not work they may need replacing.
 

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