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Discussion Starter · #1 ·
How much does it usually cost? Where can I take an amp to get it biased? I'm asking because I'm looking into tubed amps and I'll probably change the tubes sooner or later and I don't know to much about tubed amps and biasing them since I never owned a tubed amp.

Thanks!
 

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The importance of biasing is often overstated. Many amp designs don't allow biasing (Fender, Mesa) and are designed to operate with tubes spanning large tolerances. (+/- 25%)
Sometimes tube classes are defined (within tolerances of ~5%) relieving you of the task to bias as long as you replace tubes like for like.

If an amp is designed to be biased via trim pots, a qualified tech could bias your amp in five minutes. If they charge you more for it, they are ripping you off.
Also consider that a full complement of powertubes (four tubes for approx 100W) will cost you 100$ anyway.

Lastly tubes are built to take a lot of beating, but they also will change reponse over time.
Improper biasing is mostly felt in tone and response, sometimes causing a transformer to heat up a bit more than expected,
but the act of biasing itself (via trim pots or resistor replacement) also changes the tone and response, as it changes the impedance of those circuits.
Some people get so anal about that they absolutely want their amp biased in a "sweet spot" (disregarding tube drift) , but there is no guarantee youwill get the amp back into that same sweet spot with a change of tubes and a change of bias.

So if you really think you need biasing to get that perfect tone then go ahead, but I wouldn't worry about it.
UNLESS you want to change tube types (from EL34 -> 6L6 or EL34 -> KT88 for example)
but there are other issues with such a switch.

BTW I've owned a Marshall for 20years and have serviced it with my dad, who holds a MoEE and who was lucky enough to attend classes by the professor who invented the penthode.
 

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Discussion Starter · #3 ·
So if I replace 6L6 tubes for new 6L6 tubes and not bias it, it wont really matter? Thanks for the info!
 

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It could affect tone, distortion and longevity, but taking care of your amp (cooling down, don't be too rough whe hauling gear) is much more beneficial to long tube life. Tone or distortion are very personal.

What 6L6 amps are you looking for in particular?
 

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Discussion Starter · #5 ·
I was looking at a used 90's Marshall JCM 900 4100. If I do get it I was going to change the tubes to some Sovteks if the old ones were bad. I believe the JCM 900's use 5881/6L6 power tubes.
 

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I have a 4100 too but it uses EL34 tubes.
It was originally designed and sold with those, then EL34 supply dried up in the early 90s and they switched to 5881 for a while. So I wouldn't call the JCM900 a 6l6 amp ;)
But if it has those it was either from that particular time period or the previous owner had it rebiased for those.
You could have it rebiased if you want it to accept EL34 tubes again.
The EL34 is considerd to be the typical Marshall tube, but they have used others.
For example in the early Clapton/Hendrix era Marshall didn't use the EL34 penthode at all, but the KT66 beam tetrode.
In mine, the OEM tubes (Svetlana?) lasted a long time in mine, replacement tubes have never sounded as great (Ruby) and have never lasted as long (Sovtek).
Most of the stuff is now Chinese...

When shopping for a used one I would really only get one if it sounded right straight away.
No messing about with "I'll get new tubes" because on a 700$ used amp, another 100$ for a quarted is a big deal and the poor sound may come from busted caps or a shorted tranny.
Backup power tubes are almost compulsary when gigging anyway as are some extra fuses.
The good thing about the 4100 is that it can run on a duet of powertubes (50W), when one in the other pair fails.

So make sure it sounds right, also at high gain and high volume, test all the knobs (for crackling).
Also be weary of bent pots, they are mounted directly to the PCB.
Haggle down if it doesn't come with the footswitch.
External damage (tears etc.) means the amp was handled improperly.
Most of this applies to other amps as well.

Lastly, check out this thread for some more info on the 4100:
http://www.jemsite.com/forums/f30/m...jcm-900-hi-gain-dual-reverb-212-a-103048.html
 

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Discussion Starter · #7 ·
The tubes I am interested in were Sovtek 5881/6L6's. So I'm guessing they work in 5881 and 6L6's? So if the 900 uses 5881's these tubes would work, right? The 6L6 part is confusing me!

And thanks for all the info! It's really helpful!
 

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6L6 is a very broad designation.
5881 is a specific military version of the 6L6.
There are more specific versions of the 6L6 such as the KT66 or EL37.

http://www.vacuumtubes.com/6l6.html

If a Marshall is set up for 5881 tubes you cannot just dump in any 6L6 and expect it to work correctly.
These amps were changed from their original EL34 design (high plate voltages, max 800V) to allow 5881 (max 350V plate) but this design can easily be reversed.

It's not complicated, but if you're going to that trouble there are more options to consider (KT66, some other 6L6, an exotic like the 6BG6GA etc.)
Depending on real estate perhaps even a KT88 or 6550 might fit (looking at mine it will probably be a tight fit)
The octal base allows many tubes, replacing some resistors will get your amp to work with any of those, but if it sounds good is entirely subjective.

As I said, I would focus on obtaining an amp that sounds good first, without any mods or maintenance and only then think about new tubes.
The preamp on the JCM900, (or later SL-X, JCM2000) isn't to everyone's taste anyway.
There is also a huge amount of misinformation on these things floating around the interweb.

Note that it's perfectly feasibly to buy a second hand amp, try it at the seller, sound great.
Get it home, play a gig and blow a tube. That's just the nature of the beast.
At which point you will need to think about replacements.
The good thing about a 100W JCM is that it has quad power tubes.
It can blow one tube (protected by a fuse) allowing you to continue playing at 50W.

You may also want to look out for a 6100.
 

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my laney vc50 has a switch on the back for 6l6 or el34 it came with 6l6 and currently has el34 in it and all i have to do is slide a switch. i wonder how effective the switch is really? i just read the manual and it says it comes with 5881 , but the switch is 6l6 and el34 weird i guess.
 

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Not weird, as I said: the 5881 is a 6L6. A particular type of 6L6, but a 6L6 nonetheless.

The VC50 has an internal bias adjust pot, as well as a switch to basically bias from 35V to 44V.
Marshall specifies going from 40V to 50V in the same location when switching from EL34 to 5881 but they also change a lot of other stuff, that is for example because the JCM900 has a half power (triode) switch and the VC50 doesn't, but there are other fundamental differences in operation between those two amps, so a direct comparison becomes impossible.
 
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