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i need a bit of help concerning my amp...

 
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LoveRocksRule
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Joined: 19 Oct 2005
Posts: 325

PostPosted: Sat Jan 28, 2006 7:15 am    Post subject: i need a bit of help concerning my amp... Reply with quote

ok guys and gals...

i have a marshall jcm2000tsl 3 channel 100watt head... i have had it for 6 years and in those years i have replaced all the valves on 2 different occasions... yeah i changed the enitre set of preamp and power amp valves at the same time, i just used marshalls own brand of matched valves which by the way wasnt very cheap to do!!!!! i dunno if u are supposed to replace the entire set like i done but anyway i have done this 2 times in its 6 years of life.... i just thought of it like guitar strings... like if u only change the string that snapped it sounds all bright and the other strings have that dead sound... i thought that valves worked in that same principal.... please excuse me if this is the wrong way as i am only 26 im still a young gun , and i never seeked any advice on it... so i just thought that replacing them all would be the best way to do the valve change....

for another 2 years it sat in a cuboard in my sisters house getting no use at all... i got it back a while ago and it was fine....

but just recently i have noticed that i will be playing away and then my volume all of a sudden goes to about half of what it was and the amp starts making a weird hissing sound...

this only happens once in a blue moon its not a frequent problem but when it does happen when i take a look down into the vents on the top of the head sometimes 2 of the EL34s are glowing like a lightbulb its usually just 1 of them but sometimes 2.... so i immedialty turn the amp off.....

leave it off for an hour or so... then fire her back on and shes fine... her sound is back to normal and the valves aren't glowing like light bulbs....

when it is happening and im standing infront of the amp looking down into the vents its always the 2 tubes at the right hand side that are the ones that are glowing......


does anyone know what the hell could be causing my head to be doing this....

is it just a simple case of replacing all the Poweramp Tubes. or is it a bias problem.... my flatmate is a qualified electrician so he knows about setting the BIAS on valve amps and we have a voltmeter to check the bias also......

as far as i understand so long as u understand electrics biasing a tube amp is relatively easy... not that i would perform it... it would be left for my Flatmate to do!!!!!


anyway any help on this problem would be gladly appreciated guys and girls.....


as i want my amp to be how she was when i first got her no problems at all.....





Dave
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LoveRocksRule
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Joined: 19 Oct 2005
Posts: 325

PostPosted: Sat Jan 28, 2006 7:16 am    Post subject: Reply with quote

oh sorry i forgot to ask as well.... is there any other brands of tubes out there that i can just fire into my marshall that are gonna give me a kinda higher gain sound like zakk wylde tone????
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brad347
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Joined: 03 Oct 2005
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PostPosted: Sat Jan 28, 2006 8:11 am    Post subject: Reply with quote

talk to mike at www.kcanostubes.com and get some quality New Old Stock tubes. Don't know who makes Marshall tubes but if they're made in China then there's your problem. Chinese tubes are "^&*( in a glass." If you can't afford good NOS tubes then get JJ/Teslas or the Mullard copies that they're selling under the Mullard name.

Get it re-biased, and for heavens sake DON'T change perfectly good preamp tubes. Preamp tubes should only be changed if failing OR if you want a different sound, it's a waste of a perfectly good tube otherwise.

Get it biased any time you change tubes. If you didn't do this before then that's probably a part of your problem.
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LoveRocksRule
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Joined: 19 Oct 2005
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PostPosted: Sat Jan 28, 2006 8:27 am    Post subject: Reply with quote

hi brad,

thanx for the quick reply.... i wanted to also know right because the tubes i have in my amp at present have only had about 2 months pretty light use.... could i rebiad my amp with these existing valves???

or should i go out and get a decent set of power amp valves then rebias her??????
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brad347
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Joined: 03 Oct 2005
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PostPosted: Sat Jan 28, 2006 8:36 am    Post subject: Reply with quote

go get decent "valves" as you britons call them...

It's likely that one of your valves was bad from the beginning and is now failing... in fact this is the most likely scenario. This is especially likely with Chinese valves.

What color does it glow when it glows? If it's a deep cherry red then it was overbiased. If it has lots of blue or pinkish-blue in it beyond the normal slight amount of blue flourescence on the glass envelope, then the tube is gassy and definitely needs to be replaced.

In order of preference:

1. NOS valves, virtually any brand is better than any current production tube.

2. JJ/Tesla vales are the best new production valves in my opinion

3. The new Mullard reissue is supposed to be pretty good but I haven't tried it.

DO NOT get Groove Tubes, Mesa Boogie, Ruby, or any other "high end" boutique tube from the United States. These are just re-branded Soviet and Chinese tubes and are no better than a good matched pair of Sovteks... in fact this is what they usually are (or worse.... chinese tubes).... for about twice or three times the cost. RIP OFF... don't believe the hype.

PS my prejudice against Chinese valves is almost universal... every amp tech I know immediately removes all of them from amps and throws them away.

*Also... does your amp have a valve rectifier?
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LoveRocksRule
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PostPosted: Sun Jan 29, 2006 2:15 pm    Post subject: Reply with quote

no valve rectifier.... just 4 power amp tubes and 4 pre amp tubes

i think im gonn just get a decent set and fire em in and ceck the bias...

that shoudl do it....

when u say what colour the valve is when its glowing its like red like its about to blow up!!!!!
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brad347
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Joined: 03 Oct 2005
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PostPosted: Sun Jan 29, 2006 4:04 pm    Post subject: Reply with quote

that means it was biased WAY too hot.

You are stripping the tube... heating it up too hot causing all the electrons to boil off the plate instead of being heated up and releasing them naturally.

Is there an external bias adjustment? These are always trouble... my friend had an old red knob Twin and his would get bumped occasionally causing this to happen.

OR something could be wrong inside the amp... not familiar with Marshalls... Fenders I know inside and out

BUT it's most likely that it was just overbiased, or never biased last time you put your tubes in.

Good luck! Let us know how you make out.

When you get your new tubes biased up, watch them carefully the first few times you play. If they act up like they are now then turn the amp off immediately and get it to a service tech and you will save your tubes.

Hint: if this happens put the new tubes back in their boxes, bring them with you to the tech and say you bought these tubes to go in it. This way, the tech won't insist that it needs 'new tubes' and overcharge you for new ones.
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Skybone
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Joined: 04 Dec 2001
Posts: 504
Location: South Yorkshire, UK

PostPosted: Sun Jan 29, 2006 6:31 pm    Post subject: Reply with quote

Marshall and many others use bog standard Sovtek or Svetlana valves, they just have them screen printed with their logo's & then charge more for them (the cynical view!).

Svetlana's are really well respected, as are Sovtek's, JJ's and others.

By your use of the term "valve" I assume you're a UK forumite, in that case, check out:

www.hotroxuk.com - Hot Rox in Nottingham, pretty quick service at good prices. IMO.
www.watfordvalves.com - Watford Valves, have a pretty good reputation.

There's many others if you're not UK based...
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Skybone
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Joined: 04 Dec 2001
Posts: 504
Location: South Yorkshire, UK

PostPosted: Mon Jan 30, 2006 5:14 am    Post subject: Reply with quote

BTW, here's a copy of some biasing info I found about biasing the DSL/TSL amps when I had mine (TSL122). Hope it's useful (knew I had it somewhere!).

Don't forget... High Voltage electricity can kill, so BE BLOODY CAREFUL!



Driver = V1 the first preamp tube, farthest from the power tubes.

Phase Inverter = V4 the last preamp tube, closest to the power tubes.

Some people believe in testing preamp tubes for the driver & PI positions, and some don't. If you do, I opt for the matched phase inverter, where they try to match both sides of the triode. I really do not think personally that matched preamp tubs have a noticeable affect on your tone.

AS for biasing the DSL50, Marshall does recommend a bias setting of 45 mA. Here's the Marshall Service Bulletin on biasing the JCM2000 series amps:

Procedure:
1. Make sure the amp is connected to a load with the proper impedance selected.
2. Power up amp on Standby, and let the circuit stabilize for a couple of minutes.
3. Locate the male three pin molex connector (CON2) with the two mini pots (PR1 and PR2) (located on each side of the molex connector) on both ends found at the bottom of tube bay.
4. Connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector CON2 and positive lead to pin1 on CON2.
5. Take amp off Standby with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from your DMM is connected to and set it to mV voltage that is listed below.
6. Repeat steps 4 & 5 for pin3 mV and until pin1 & pin3 mV readings are the same.

Bias Settings:
DSL50 45 mV TSL 601&602 80 mV
DSL100 90 mV DSL 401 1.375V
TSL100/122 90mV DSL201 .675V

Here's a quick bias lesson:
What you want to do is measure the plate voltage (B+) from pin3 of one of the power tubes. It should measure between 450-500V. Now the max plate dissapation (Pa Max) for EL34s is about 25W. To calculate the correct bias, you want to:
(Pa Max/B+)(.7) = 38-45mA depending on the exact plate voltage of your amp. Check and adjust via the molex connector and pots from above.
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