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Unread 05-04-2008, 04:29 AM   #1 (permalink)
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Changing Tubes In JCM 900

I'm thinking about changing the tubes in the Marshall JCM 900 SLX-100 Model 2100 I recently acquired and wanted to ask the experienced Marshall players for recommendations and suggestions. It's pretty dusty in there and I was going to open the head up and clean it up anyway so I thought while I was in there I might as well do a tube swap.

Nothing's wrong at all with it, she sounds great imo, but I'm not sure when the last time she had a good going over was.

I have several EHX 12AX7's virtually new here that I have stored away and a matched quad of JJ EL-34's I took out of my JSX head when I installed TAD 6L6's instead.

Any pointers would be greatly appreciated.

I've had tube amps many years but never a Marshall until recently. Not sure if I would need to re-bias and if so, how. In my town the techs are pretty unreliable.

Thanks!
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Unread 05-05-2008, 12:51 PM   #2 (permalink)
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Re: Changing Tubes In JCM 900

Hi have a JCM 900 100w amp - the preamp tubes should be the normal recommended ECC 83 - these give a 100% gain and no biasing needed.

EL34s main tubes have a performance tolerance - I have some Russian Army tubes which are supposedly 'matching' tubes yet the tolerance varies a lot. I was told not to remove them and change their sequence. Still, normally you should have them biased to optimise performance, but in most cases you can get away without. So what can go wrong? If you have tubes which are all over with their tolerance and are pushing the limit of the biasing settings then the tubes will not work together and they will perfom badly, soon you will need new ones again - and the sound will suffer.

If you are happy with the way it sounds and performs then ok, you have saved money! Go buy a case of beer instead!

Cheers Brett
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Unread 05-05-2008, 04:44 PM   #3 (permalink)
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Re: Changing Tubes In JCM 900

yea you should get the power tubes rebiased,
but to tell the truth i never do and its fine
but dont listen to me hahah


(probably why i used to blow fuses)
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Last edited by coldsteal2; 05-06-2008 at 12:37 PM.
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Unread 05-06-2008, 04:09 AM   #4 (permalink)
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Re: Changing Tubes In JCM 900

Thanks for the info folks!

Not sure about how the whole re-bias thing goes as Boogie tubes are already biased and with my JSX head I can get in the back of the head and adjust it myself.

I'll search the net about info on that and I plan on calling a tech in another town who's reputable, as the one's here are flakey.
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Unread 05-06-2008, 12:36 PM   #5 (permalink)
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Re: Changing Tubes In JCM 900

The biasing happens to the Amp not the tubes
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Unread 05-06-2008, 01:01 PM   #6 (permalink)
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Re: Changing Tubes In JCM 900

Quote:
Originally Posted by coldsteal2 View Post
The biasing happens to the Amp not the tubes
I understand that coldsteal but is that something a layperson can do, and if so where can I get info to go about doing it?

Getting an amp biased if it must be done by a tech isn't the easiest thing to do in this town as I stated above.

Thanks
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Unread 05-06-2008, 02:30 PM   #7 (permalink)
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Re: Changing Tubes In JCM 900

Yes, you can do it yourself, but you'll need some equipment.

Check this site for good info:
Eurotubes
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Unread 05-06-2008, 03:42 PM   #8 (permalink)
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Wink Re: Changing Tubes In JCM 900

Quote:
Originally Posted by Adwex View Post
Yes, you can do it yourself, but you'll need some equipment.

Check this site for good info:
Eurotubes
Thanks for the info. When I'm ready to do this I'll probably just haul it to the city and have a tech give it a good going over and install the tubes and set it up properly.

I really wanted to avoid this, not so much because of the cost but because I'd have to leave the amp behind and do another 3-4 hour drive (1 way) to pick it up, besides I'm not afraid of doing it so long as I have the correct bias settings and know where the trim pots are etc. info which has eluded me thus far
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Unread 05-06-2008, 03:56 PM   #9 (permalink)
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Re: Changing Tubes In JCM 900

Didya check the video:
euro-q

Dude, for the cost of gas, and your time, driving 3-4 hours.....twice....you should equip yourself to do it. You CAN do it.
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Unread 05-06-2008, 04:07 PM   #10 (permalink)
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Re: Changing Tubes In JCM 900

Yes I checked the video and absolutely I can do it, I've done it on other amps before however I have the settings for those amps and know where the adjustments need to be done whereas, with the JCM 900 I don't!

The video isn't for the JCM 900 but you knew that.

p.s. those drive times are 1-way and I agree for the cost and time for TWO 6-8 hour trips, I'd MUCH rather do it myself!
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Unread 05-06-2008, 04:51 PM   #11 (permalink)
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Re: Changing Tubes In JCM 900

on the videos select jcm 800, and it sez it works the same way for 900s as well
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Unread 05-06-2008, 05:04 PM   #12 (permalink)
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Re: Changing Tubes In JCM 900

Thanks for pointing that out.
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