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Old 01-20-2013, 04:52 PM   #1
VeilSide
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JCM900 Tube question.

Hi everyone,

Well, yesterday i turned on my amp when suddenly i heard a squealing noise and the amp lost its power. it still played, however, i could turn the volume all the way up and and hardly hear anything.

The amp is a 94 100W JCM900.
Later i checked and both of the fuses were gone.

From a similar experience with an other amp it was a bad V1 preamp tube and a blown fuse.

Could it be the same case?

If so, ive decide to swap all the tubes this time (both power / preamp ones)
Right now its running 12ax7s and 6L6s.

They sound good to me but i was wondering if the JCM900 can be used with EL34s?
Whats the main difference between them?

Thanks!.
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Old 01-20-2013, 04:57 PM   #2
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Is your the 5881 model then no it is made to only accept 6L6 type tubes.

SED =C= are awesome tubes, but very expensive. I swear by there powertubes now.

JJ are a good budget tube. JJ also make the KT66 which is a little diffrent than the regular 6L6 but it is a drop in replacement. JJ Ecc83 are great in a Marshall and I really like the new Mullard 12ax7

Asfar as your amp, I could not tell you, but it sounds like your powertubes are shot. Are any of the fuses blown?
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Old 01-20-2013, 04:59 PM   #3
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If you've blown the HT fuse you've most likely lost a power tube.

The JCM900 originally came fitted with EL34's. Some American ones had 5881 tubes (basically 6L6's). They have a different screen resistor. If it is one of those you can't just go shoving EL34's in there.

And run JJ tubes in it, especially in the preamp.
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Last edited by Cathbard : 01-20-2013 at 05:01 PM.
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Old 01-20-2013, 05:00 PM   #4
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Quote:
Originally Posted by Cathbard
If you've blown the HT fuse you've most likely lost a power tube.

The JCM900 originally came fitted with EL34's.

And run JJ tubes in it, especially in the preamp.

They also made a JCM900 DR with 5881 (6l6). my buddy has one
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Old 01-20-2013, 05:02 PM   #5
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Yeah, see my edit.
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Old 01-20-2013, 05:04 PM   #6
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Quote:
Originally Posted by Cathbard
Yeah, see my edit.


Yeah I thought that was kinda wierd comming from you
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Old 01-20-2013, 05:06 PM   #7
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Quote:
Originally Posted by Robbgnarly
Is your the 5881 model then no it is made to only accept 6L6 type tubes.

SED =C= are awesome tubes, but very expensive. I swear by there powertubes now.

JJ are a good budget tube. JJ also make the KT66 which is a little diffrent than the regular 6L6 but it is a drop in replacement. JJ Ecc83 are great in a Marshall and I really like the new Mullard 12ax7

Asfar as your amp, I could not tell you, but it sounds like your powertubes are shot. Are any of the fuses blown?



I'll check them out, thanks.

And yes, there are 2 fuses under "Output Valve Fuse" and both are gone.




Quote:
Originally Posted by Cathbard
If you've blown the HT fuse you've most likely lost a power tube.

The JCM900 originally came fitted with EL34's. Some American ones had 5881 tubes (basically 6L6's). They have a different screen resistor. If it is one of those you can't just go shoving EL34's in there.

And run JJ tubes in it, especially in the preamp.



Yeah, it's the 4100 with 5881 tubes.
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Old 01-20-2013, 05:07 PM   #8
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The only time a 4100 sounds good is when you crank the power section into distortion for that classic Marshall crunch and when you're doing that EL34's kick a 6L6's arse. 6L6's in a 900 is just wrong.
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Old 01-20-2013, 05:14 PM   #9
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Quote:
Originally Posted by Cathbard
The only time a 4100 sounds good is when you crank the power section into distortion for that classic Marshall crunch and when you're doing that EL34's kick a 6L6's arse. 6L6's in a 900 is just wrong.


Well, i won't lie, it took me a long time to get the sound i wanted out of that amp...
umm i ended up never even using the B channel on it, just overdriven A channel.


If i use the same 6L6 tubes that are in it right now, does it need to be re biased?
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Old 01-20-2013, 05:19 PM   #10
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You always have to check the bias when changing the power tubes. On the plus side, you can do a quick rough bias with just a multimeter. You just adjust the bias pot until you get -50Vdc at the junction between R26 and R27.
That does mean poking around inside a live amp though so don't do it if you don't know what you are doing.
Here's the schematic:
http://www.drtube.com/schematics/ma...60-02-iss14.pdf
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Old 01-20-2013, 05:19 PM   #11
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Quote:
Originally Posted by VeilSide
Well, i won't lie, it took me a long time to get the sound i wanted out of that amp...
umm i ended up never even using the B channel on it, just overdriven A channel.


If i use the same 6L6 tubes that are in it right now, does it need to be re biased?

if you can get the exact same tubes with the same rating you'll be fine. but rebias every time

Cathbard knows some great mods for the 900 DR that will really open the amp up. I'm sure he'd enlighten you if you ask him.

EDIT: he already did.
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Old 01-21-2013, 01:11 AM   #12
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You can fix channel 2. Refer to the this as I explain it so you can follow what I am saying.

The problem with channel 2 is what I consider to be one of the greatest design flaws of all time.

See BR2 and D10? BR2 is a bridge rectifier and together with the D10 forms a clipper that limits how much signal goes to IC4b by clipping off the top. This results in that ugly fizz that is driving you away from that channel.
Without the clipper you can actually drive V1 harder and can get just as much distortion. So what that bit of circuitry does is replace tube distortion with solid state distortion. just
So what you do is get rid of the clipper and replace it wit a 100K resistor. You can just remove BR2 and put the resistor in the holes left behind so it connects C36 to ground.

Channel 1 has some clippers too (LED 1-4) but they aren't so bad. That's doing the tubescreamer type thing. As you've found yourself, it's ok. Channel 2 is the bitch that needs to be slapped.
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Old 01-21-2013, 04:18 AM   #13
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JJ are the to go tube in this case, but I personally prefer EH for a bit more "rude" sound. I've also put in Tung-Sol and Shuguang in there for different results. The Tung-Sol are a bit more rounded, while the Shuguang were more midrangey with a dip in the lows.

Sounds like power tubes to me.
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Old 01-21-2013, 05:04 AM   #14
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First off, I have to say: Listen to Cathbard.

Second, if there is a blown fuse then don't just replace it and keep chugging along. Fuses are there so when something in the amp malfunctions, the electric signal can be stopped before other components are damaged. Figure out what's wrong with your amp, get it fixed, then replace the fuse.
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Old 01-21-2013, 10:37 PM   #15
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Wouldn't power tube failure trigger a fuse to burn out?
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Old 01-21-2013, 10:39 PM   #16
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Quote:
Originally Posted by diabolical
Wouldn't power tube failure trigger a fuse to burn out?
Usually but not always. It depends on how it fails.
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Old 01-21-2013, 10:49 PM   #17
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So if he were to replace tubes and bias, and the fuse doesn't burn out again, he wouldn't need to visit techs as i_am_metalhead suggested, right?
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Old 01-21-2013, 10:55 PM   #18
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Correct. Unless he wants to change to EL34's of course because that would require a different screen resistor. If he sticks with 6L6's it'd be fine.
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Last edited by Cathbard : 01-21-2013 at 10:56 PM.
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