#1
Hey i just put some gt 6l6-ge tubes in my jcm 900 and i can't seem to find the proper bias anywhere for them. Also, i replaced a set of jj ruby 6l6gc's (which were the old tubes) are the biases similar enough to just pop 'em in and go care-free? the new tubes are glowing a bit red on the snowflake-looking plate on the top and bottom of it. (i don't know exactly what it's called). they seem to sound fine, but i don't want them to fizzle out prematurely or have my amp implode into another dimension if the bias is off by alot.
HELP!
I only feel like me when I'm behind my ax...
#2
What type of bias probe do you have? Or are you using the marshall internal one?

There are heaters in the tubes- they will glow like a light bulb filament.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#3
Marshall internal probe? This amp was made in '95. Are you talking about the 1/4" jack probe socket accessible from the back panel? Because there is none. It just has the internal pot. I just am asking cause I don't want to pay a tech good money and wait for three weeks just to turn a bloody pot. I'm just wandering if it'd be safe to run it 'till my tube socket probe arrives and I can do it myself. Thanks.
I only feel like me when I'm behind my ax...
#4
Generally if they aren't red plating them you are okay.

There is no way of knowing if they are running hot or cold unless you test them.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#5
-so you are asking the bias for them and don't have a way to bias them?
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#7
Quote by slayer1979
Once my bias probe arrives i will.


Do you know what type of bias probe it is? Some have you take a direct current measurement. Others are voltage based IIRC.

http://www.eurotubes.com/eurotubes-Bias-Probe-Pro-One-current-watts-voltage.htm

Read this if you haven't already.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#8
You can bias a 900 without a probe.

Look for R26 and R27, they are right next to the bias pot. Measure the voltage at the junction of those two resistors and adjust the trimpot until it reads -50V. You can be out by half a volt without worry.
Doing it with a bias probe is more accurate but that ^ method will work fine.


PS: -50V is for 5881/6L6 versions. For EL34 versions it's -39V
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#9
I hope I can do this without completely exploding myself. I know how to drain caps and yada yada. As for the probe it's one that plugs right into a regular 'ol multi meter.
So Cathbard, what about my tubes specific bias setting, and i thought it was measured in mA.
I only feel like me when I'm behind my ax...
#10
You don't measure mA directly for a rebias. You get the bias across a resistor which of course by ohm's law gives you the current.

You also don't bias to a set tube current rating, because voltage varies by amp so that rating would be meaningless. You bias to a percentage of the tube's dissipation after measuring the plate voltage for your particular amp. If you look at a tube data sheet, you'll see that it only lists a dissipation (in watts) and then maybe one or two sample bias currents at respective plate voltages. Of course if your amp doesn't happen to have that plate voltage, the number is useless, which is why you measure. There's no one "bias here" mA listed for a kind of tube.

You bias by setting the idle dissipation, which is just the plate voltage times the cathode current. So you have to know the plate voltage to set the bias, and the amp determines the voltage, not the tubes. See why you can't bias a tube now? The reason you can't find bias current for your tubes is because that's not how you bias. It's like asking what temperature you should set your oven to. You don't set your oven to 400 every time and then adjust baking times, you adjust the oven temperature based on what you're cooking.

Anyway, you should measure your plate voltage first, and then use that to determine what mA you need. I like to set a 6l6 for 60% max dissipation at idle. You can play with that number but it's a starting point. A 6L6 is rated for 30 watts, so .65*30=18. Once you know your voltage you just divide 18 by that to get your target current per tube (in amps). So for instance if your plate voltage is 460 then you need 39 mA of current per tube to get 60% dissipation. Some people like 50, some people do 70. 80 is too high with most 6l6s.
#11
:O Mind blow #1 for today... I see... It's tube caters to amp, not amp catering to tubes... I just don't wanna ruin a $180 set of tubes.
I only feel like me when I'm behind my ax...
#12
Yeah, that would suck!

But yes, it might seem like you're biasing the tubes, since you get new ones and then have to fiddle with things, but in reality you're biasing the amp, the tubes just sit there and glow and do what they're told. There's no adjustable anything in a tube.

If you follow Cath's instructions you'll get a proper bias provided your plate voltage isn't way off for some weird reason. That would be rare, I wouldn't worry about it. Certainly good enough until your probes show up.
#13
If you want to do this right get a cheap bias probe from eurotubes or similar and a mulitmeter.

I have a Blog on biasing amps similar to yours that may help too and has all the safety precautions.


v....I see. Check.
Last edited by 311ZOSOVHJH at Jul 10, 2013,
#14
He's got probes in the mail, 311. This should hold him over until they show up.