JCM900 tube troubles

[LeeT4321]

The tubes in my Marshall JCM900 (Groove Tubes) are about 8 years old and HEAVILY used. Just the other day I realized I left the amp on by accident for a few hours (D'OH!). Today, when I clicked it on from standby the tubes started humming like crazy through the speaker. I got scared so I shut the amp off. One of the tubes was lighting up really freakin bright, too. I was afraid something bad would happen. What's the problem? Do I need new tubes?

 

[karpathion]

Re: JCM900 tube troubles

dude if it's been 8 years replace 'em anyway. most of the time 1 year of heavy use is enough for a change.

 

[LeeT4321]

Re: JCM900 tube troubles

don't have the money , i wanna know if there's an actual problem or not.

 

[CaughtLikeFire]

Re: JCM900 tube troubles

The tubes in my Marshall JCM900 (Groove Tubes) are about 8 years old and HEAVILY used. Just the other day I realized I left the amp on by accident for a few hours (D'OH!). Today, when I clicked it on from standby the tubes started humming like crazy through the speaker. I got scared so I shut the amp off. One of the tubes was lighting up really freakin bright, too. I was afraid something bad would happen. What's the problem? Do I need new tubes?

when one of your power tubes starts to glow like that and hum really loud, it's done! you did good turning it off. the chances of you hurting your amp are slim to none though - the tube just shorted and gave up the ghost.

if you had the amp biased with those old GTs, just buy a new set with the same rating number. if you never had it biased by an amp tech, now is the time - pick out a nice set of tubes and take the amp in to be serviced.

also make sure that you purchase power tubes in matched pairs (or quads)

by the way, is it an early EL34-powered JCM900 or later 5881/6L6?

 

[karpathion]

Re: JCM900 tube troubles

doesnt sound like a problem. if there was a problem it probably wouldnt work. it does sound like the tubes are shot. el-34's are not that high in price. unless it's got 5881's
but you should take it to a tech. actualy does anyone know if jcm900's are self biasing or not. i'm not sure. if it's self biasing you can save money swaping tubes yourself.

 

[LeeT4321]

Re: JCM900 tube troubles

by the way, is it an early EL34-powered JCM900 or later 5881/6L6?

two matched pairs of EL34s. I'm looking for a less harsh set of tubes the next time around. Something bright; something with a good balance of clean head room and raunchy breakup; A good set to make my single coil P-90's shine.... suggestions?

 

[CaughtLikeFire]

Re: JCM900 tube troubles

actualy does anyone know if jcm900's are self biasing or not

hell no. wish they were

some of the experts like Myles Rose will even tell you that using a simple bias probe/multimeter is not enough to set the bias properly in most Marshall amps.


as far as tubes go, since you need to buy 4 of them, NOS Siemens may be out of the question. I'd go for SEDs - best affordable current production tube. GT Mullard reissue is nice too.

BUT . . . keep in mind that it takes a lot to really get the power tube character to come out. with a 100W head, you'd have to crank it to ear-splitting levels to really get the power tubes going. if your tone is too harsh/bright (and assuming your guitar and speakers are not the problem), you need to change preamp tubes - that's what you're hearing, not the power tubes (I'm talking about your normal tone, not the hum from the blown power tube)

I've swapped tubes in a lot of Marshalls for bandmates and friends and here's what I'd recommend.

preamp section:

http://www.dougstubes.com/12ax7.html

Tung-Sol reissue in 1st position, Tung Sol or JJ ECC83 in second position, and Suguang 9th gen or Ruby for the 3rd position (phase inverter). Select the balanced triode option for the phase inverter.

power section:

http://www.dougstubes.com/el34.htm

SED or Groove Tubes EL34-M matched quad

 

[karpathion]

Re: JCM900 tube troubles

hell no. wish they were

i didnt think they where. the only self biasing amp i've personly ever seen is my peavy ranger.

 

[LeeT4321]

Re: JCM900 tube troubles

I've heard lots of good things about KT77s. I may order a matched quad of JJ KT77s for my JCM 900... Also, why are my knobs all crackily when turned? Is this the tubes or something else?

 

[CaughtLikeFire]

Re: JCM900 tube troubles

I've heard lots of good things about KT77s. I may order a matched quad of JJ KT77s for my JCM 900... Also, why are my knobs all crackily when turned? Is this the tubes or something else?

http://www.amazon.com/Hosa-Contact-...4?ie=UTF8&s=electronics&qid=1230948677&sr=8-4

get some of this, pull the amp chassis out of the cabinet, and go to town

that should fix your problem

KT-77s are a bit warmer and softer but did you get any JJ preamp tubes to go with them? try a few JJ ECC83s, or at least try one in V1 (1st position) and see what difference it makes on your tone.

 

[Bludave]

Re: JCM900 tube troubles

two matched pairs of EL34s. I'm looking for a less harsh set of tubes the next time around. Something bright; something with a good balance of clean head room and raunchy breakup; A good set to make my single coil P-90's shine.... suggestions?

I have the same amp in a 50W 2 12" combo. I have Sovteks in my amp. It was just retubed over the summer. The day before a Gig I turned it on to test everything & I started blowing fuses. Fortunately my amp tech was able to get it sorted out for me. It was just tubes. Mine were in there for a long time too. Already stated, you should be changing your tubes about once every couple of years depending on how much you use your amp. I only use my Marshall with rehearsals & Gigs. I have others that I use for home practice.

 

[StillLearnin']

Re: JCM900 tube troubles

If you have one tube glowing bright - It's on the verge of internal breakdown and can do anything from just blow the HT fuse, just go dead or cause other more expensive damage when it finally decides to fail completely. Pull the tubes and toss 'em. KT66's are real nice tubes, handle high plate voltages and sound great - very tight and not nearly as harsh as EL34's. 5881'a are nice too, but may warm it up too much for you. Having 34's in there right now will mean a likely mod to the bias circuit to feed the higher bias voltage required for KT66, KT77, 6550, 6L6 or 5881's. You may be able to squeeze enough out of it though without a mod. You want to be able to get enough bias voltage on the grids to get the idle current between 25 and 40 mA per tube - meaning between 100 & 160 mA for a 100W head. I've never used a bias probe since I stand all the cathodes on a 5W 10 ohm resistor in any amp I build or mod. Then all you have to do to bias is measure the voltage drop across the resistor while adjusting the trimpot.

 

[Northwinds]

Re: JCM900 tube troubles

Svetlana Wing C's and Marshalls go together like peanut butter & jelly and draw quite a bit of plate voltage. 900's run hot anyway

http://thetubestore.com/el341.html

 

[ErikH]

Re: JCM900 tube troubles

I hate to be the bearer of bad news, but the problem may be more than just the tubes. Your power transformer could be on it's way out. Did the chassis get hot? Right before my 900 went down, the tubes got really bright and the chassis was really hot and then "pop", there it went. It needs a new power tranny and a full compliment of tubes. The filter caps will get changed as well.

You need new tubes after 8 years, no doubt about it. If you can't afford to get it taken care of, don't use it until you can and play through something else. Tube amps are not cheap to maintain. That is one reason why I have not gotten mine back up and running. I also cannot afford to get it fixed (number one reason).

 

[theodie]

Re: JCM900 tube troubles

I have the same amp but a 100W version (Model 2100).


Sovtek LPS preamp tubes are the only way to go. Sovtek or EH power tubes are good in it too.

 

[Hellion]

Re: JCM900 tube troubles

I like Sovtek tubes. In any case, if they're 8 years old, it's time to retube.

 

[Myles]

Re: JCM900 tube troubles

The tubes in my Marshall JCM900 (Groove Tubes) are about 8 years old and HEAVILY used. Just the other day I realized I left the amp on by accident for a few hours (D'OH!). Today, when I clicked it on from standby the tubes started humming like crazy through the speaker. I got scared so I shut the amp off. One of the tubes was lighting up really freakin bright, too. I was afraid something bad would happen. What's the problem? Do I need new tubes?

You needed new output tubes years ago

 

[Chickenwings]

Re: JCM900 tube troubles

Yep new tubes - eh or the lps sovteks - actually you can save some money by just putting in 2 power tubes and switching your JC to 50 watts. I had a 900 dual reverb in the 90s and found that a 4x12 and 100watts was too much to use for most gigs so i could never really drive the amp properly. 50 watts = less headroom but a much fatter sound because you can actually run some signal thru the power tubes. You will actually have way more headroom than an 8 year old quartet will give you anyway.

 

[Myles]

Re: JCM900 tube troubles

Yep new tubes - eh or the lps sovteks - actually you can save some money by just putting in 2 power tubes and switching your JC to 50 watts. I had a 900 dual reverb in the 90s and found that a 4x12 and 100watts was too much to use for most gigs so i could never really drive the amp properly. 50 watts = less headroom but a much fatter sound because you can actually run some signal thru the power tubes. You will actually have way more headroom than an 8 year old quartet will give you anyway.

I disagree with the idea of pulling two tubes .... but that is just my personal take on things. Here is a clipped section of something I wrote a number of years ago on the subject ....

Then there are those folks that have the great idea of pulling two tubes out of their 100 watt amp to turn it into a 50 watt amp. To put it bluntly and
get a lot of argument, this is a stupid idea.

A great amp is made up of many components. Power transformers, output transformers, capacitors, and other parts, make up the design. If one takes a Marshall 100 watt Super Lead, and pulls two of the tubes, and properly sets the impedance selector, turning the amp into a 50 watt amp, what actually happens? Well, we have a 100 watt power supply, that is now even less taxed than before. The "50 watt" Marshall will now have less dynamics, less feel, less touch sensitivity. It will be a nice, clean, hi-fi, 50 watt amp. It?s 100 watt power supply will never reach saturation. Its output transformer will never be pushed. It will actually be cleaner than it was as a 100 watt amp.

The only distortion you will get is when the output tubes are at their limit, and this will be an unbalanced sound, although some might think this is
just to their own tastes.