More Marshall JTM Amp Problems

[benjaturner]

This is really a continuation of a thread title 'Scott_F Please Read'
The topic changed and stopped being about what it initially was. So I thought it best to start a new one.

My Marshall JTM60 is having interminant problems. It's fine for about the first 10 minutes of it being on. Then it starts to drop off in volume and comes in and out from then on. There are no unpleasant sounds (apart from my playing! Ha!) and the drop off is quite linear but still very fast. Sounds like someone is randoming turning the volume knob up and down.

Jeff Seal advised me to remove the heater rectifier and it's corresponding capacitors, which I did but to no avail. The problems still exist as they did before.

This all seems like a valve problem, but all the valves in are less than a year old. They look and smell fine. The amp hasn't worked particularly hard and I think I take pretty good care of it. I've just opened it up to look for any burnt components or any sign that something is wrong. I couldn't see, or smell, anything unusual.

Please Help!
Benja

 

[Pepi]

Re: More Marshall JTM Amp Problems

I would try swapping tubes. Sounds like a bad tube to me. Just because they are newer doesn't mean they can't go bad.

 

[benjaturner]

Re: More Marshall JTM Amp Problems

Ok. Thanks Pepi. Last time I bought valves I bought cheap E-H ones. I think next time I'm going to get some Groove Tubes. As they're quite expensive I think I'll just buy one set (power-amp/pre-amp) and hope the other are workingg fine. Do you think I should try the power-amp valves first? Or is the problem more likely to be with the pre-amp valves?

Thanks,
Benja

 

[ErikH]

Re: More Marshall JTM Amp Problems

Sorry to hear this man. That sucks. Don't get Grooves Tubes. They are just rebranded tubes from other manufacturers. GT does't make any of their own tubes. Try some Svetlana's in the power amp and EH or JJ preamp tubes. My tech recommended them to me and I like them.

Good luck. I hope you can get this taken care of.

 

[benjaturner]

Re: More Marshall JTM Amp Problems

Don't get Grooves Tubes. They are just rebranded tubes from other manufacturers. GT does't make any of their own tubes.

Wow! I always thought Groove Tubes were the industry standard. Just because you see them everywhere and they're usually about as expensive as anything else. Would love to hear on this if anybody knows much about it. I had heard that they rebrand other tubes. I thought the idea was that they test them first and match them for you. I didn't think the people who made tubes actually sold them on the public market. I thought all tube brands were just distributors. Jees! How embarrassing!

Thanks for the info, will definately take that into account when the time comes.

Thanks again,
Benja

 

[wixedmords]

Re: More Marshall JTM Amp Problems

Benja,

I am a big fan of the JJ E34L's, regular JJ EL34's or SED EL34's. Fair priced tubes also. I run the E34L's in my JTM 622.

But, you gotta get the tubes biased. It doesn't take much of a turn on that bias pot to get the bias too high.

At best, pick up a matched quad somewhere, dougstubes is my favorite in the states, and see if swapping the tubes helps. At least you will be able to rule it out.

You seem to be going alot of directions with this amp. Try the easy stuff first.

 

[tone?]

Re: More Marshall JTM Amp Problems

Dude that is EXactly what happens to my JTM60 for years now and it has made me pay alot of money and wasted a bunch of my time.

i always would get solder joints unsoldered and the amp would fade in volume.

screw it and sell the amp as i am.
it isnt that good of an amp. you could find a much better used Marshall out there.
i am personaly thinking of going VHT.

When i first got this amp it was pretty darn good after breaking it in but after i took it a few million time to get serviced it lost in tone and just is plain unreliable like crazy.

i even wrote to Marshall England about this problem and they didnt do much to help at all.

None of Their techs gave me a workaround CAN YOU BELIEVE THAT???!!!

Actually i want to start a thread here on how Marshall Service SUCKS! especially from such a huge company.

anyway sorry Benja that is my two cents about this particular amp.

i want to play and not try and goof around with the amp so much cause it cant stand the heat. Wimpy amp!

 

[The Golden Boy]

Re: More Marshall JTM Amp Problems

Sorry to hear this man. That sucks. Don't get Grooves Tubes. They are just rebranded tubes from other manufacturers. GT does't make any of their own tubes.

From what I've heard, this isn't entirely true any longer.

I've heard that GT bought some equipment and is actually making some tubes in the US. Those tubes are even more expensive than the already outrageous prices they charge for their matching and rebranding service.

 

[Stratman]

Re: More Marshall JTM Amp Problems

There seems to be a problems witrh the 100K resisitors that heat up intermittenly.
Open up the amp so that the tubes are facing down and the bottom side of the chassis is facing up

You should see a light gray resistor (Square in shape) and it will start to turrn red when it heats up, hit itwitha spray of CO2 to cool it down (Don;t over do it)
ANd if the problem goes away, you now know thesource.

 

[benjaturner]

Re: More Marshall JTM Amp Problems

T

 

[benjaturner]

Re: More Marshall JTM Amp Problems

Sorry about my last post. Pressed enter too early, don't you hate that?

Was going to say... Thanks for the tip Stratman. Are you saying I should do this while the amp is powered up? Because I figure that's got to be really dangerous. But for the resistor to heat up like you described the powere would have to be on. Does anybody else have any opinions on this technique for trouble-shooting? I'd like to hear a second opinion before trying it. I don't want to die, obviously!

Thanks
Benja

 

[tone?]

Re: More Marshall JTM Amp Problems

hey benjaturner,

do you really like the sound of that amp so much?
sorry i dont want to discourage you, but it has so so many problems thats all.

i am looking to get rid of mine so bad.

i tried to mod it but people would run once they saw the inside of the amp.

 

[benjaturner]

Re: More Marshall JTM Amp Problems

I definately know what you mean Tone? I am gonna get a new amp. maybe today. there's a Marshall Country Club 4140 2x12 on Ebay ending today. Looks tempting. Can't find any info on it though. Have checked Harmony Central and it doesn't seem to have the reliability issues the JTM has. Infact, it's got a really high score for reliability and it looks like it's gonna go for less than £500. Please don't bid on it because I can't afford to go any higher than that! lol. If anybody has any inof on the amp it'd be greatly appreciated.

Thanks,
Benja

 

[tone?]

Re: More Marshall JTM Amp Problems

do you live in England??

No dont worry i wont bid on the amp. LOL

i think you should definately listen to an amp that you are gonna buy and then get it from ebay.
tone is a personal thing so just because a bunch of people might like it doesnt mean you will.

if you live in england you have a bunch of stuff to look for.

i saw a few older Marshalls on the UK ebay that i would go for instead.

 

[benjaturner]

Re: More Marshall JTM Amp Problems

£500 is my limit though. If an older Marshall goes for less than that then it means there's probably something wrong with it, I'd guess. I'd love to try things out before I buy them but the fact remains that I can get things that I otherwise wouldn't be able to afford. Stuff goes on Ebay for £500 that would cost £900 in a shop. I simply can't afford that. Ebay means I can get much higher grade stuff. I think as long as you do your research properly and trust your sources then Ebay can be a brilliant way for people like me (poor people, lol) to shop.

-Benja

 

[benjaturner]

Re: More Marshall JTM Amp Problems

Oh yeah, sorry Tone? Forgot. I do live in the U.K. but I live in the sticks. No good guitar shops anyway near me.

 

[Bludave]

Re: More Marshall JTM Amp Problems

I had a similar problem with my 900. WHat it turned out to be is the jack plugs in the effects loop were real dirty and had some corrosion. I cleaned them up real good and sprayed them out with contact cleaner and have not had a problem since.

IU would also recommend switching tubes though. One year does not mean that they are not bad. ALL tubes in effect can go at any time. Replace the tubes and you should be good to go.