bias-ing.........is it necessary?

[PoisonGodmachine]

ok im buying these new tubes for my 5150 ..but..i have no actual amp repair places withen a 140 miles from were i live.......so...am i screwed or can i just slap these things in and get on with playing....?

[Cleveland Metal]

Re: bias-ing.........is it necessary?

Biased wrong can make the amp sound like ass, wear out tubes, and put lotsa stress on the power supply. I really recommend having it biased by a tech when swapping power tubes.

Preamp tubes, you don't have to bias it when you change them.

[charvel750]

Re: bias-ing.........is it necessary?

So, even if you are replacing old tubes with the same identical brand and style new tubes, you still need to bias? I have a tube head with EL-34's and I replaced them with the same brand and model, I can't hear a difference, but I know I didn't bias them, should I have taken it for that work?

[Cleveland Metal]

Re: bias-ing.........is it necessary?

You'd have to use the tube set they recommend in their system. I think MESA are color coded. You could also call the tubestore, and they could get you a proper tube for that MESA code. I'd choose the latter.

[toejam]

Re: bias-ing.........is it necessary?

Cool. Thanks!

[JerryP]

Re: bias-ing.........is it necessary?

Originally posted by PoisonGodmachine:
ok im buying these new tubes for my 5150 ..but..i have no actual amp repair places withen a 140 miles from were i live.......so...am i screwed or can i just slap these things in and get on with playing....?

<font size="2" face="Verdana, Helvetica, sans-serif">If you buy a matched set you can just throw them in your 5150. That amp is biased so cold from the factory you aren't going to hurt anything. However, if you want the best tone you can get from that amp and your tubes, have the bias circuit modded so it's adjustable and have it set by a tech.
From the factory it's not adjustable and the tubes are biased at about 15ma per tube. Generally most folks shoot for 34ma per tube. It will sound more powerful and warmer when it's biased right. Not as buzzy. I have a bunch of clips on my sound clip page.
Jerry

[monk]

Re: bias-ing.........is it necessary?

When you talk about "plate current" you mean that the amp should have a certain current through the plates, and because each tube has a slight difference in resistance you have to adjust the voltage to get the wanted current?

[Cleveland Metal]

Re: bias-ing.........is it necessary?

Yeah, I'd always heard the 5150 were fixed biased at around 12-14 mA. Theoretically for warranty reasons I guess eh? I noticed my Marshall 900s were good for stock cold bias also.

The two 5150s I had (and bought used at different times at different places) were both adjustable bias amps that looked pretty darn stock, hard to believe they were both modded. Interesting. Every one of the 5150 IIs I've seen were all adjustable also. I usually liked them hot at 38mA+ if I remember right.

The tracks you have with the morley are very cool. But, I thought my hot biased 5150s sounded more alive and crunchy than the others though. Maybe the low gain preamp mod? The morley mod, I would like to hear more about I'd say, seems to really liven them up.

[JerryP]

Re: bias-ing.........is it necessary?

The 5150II's have an adjustable bias but the range is narrow. It doesn't always allow the tubes to be biased as warm as you might want without a mod to the circuit. The Bad Horsie can be modded using many of the same mods out there for the Vox/Crybaby wahs. The BH circuit is the same but it's surrounded by buffers and the switching circuit. That BH has a vocal mod done and the sweep range was warmed up a bit.
The lower gain takes away some of the buzzy saturation of the 5150. It allows more clarity to what your playing too. It could be my crappy recordings. They sound pretty good in person.
Jerry

[Cleveland Metal]

Re: bias-ing.........is it necessary?

Unless you replaced it with a tube having exactly the same plate current, you should rebias the amp.

Same brand and model can have widely varying plate currents. Hence the concept matched sets etc.

Mesa uses a number system that matches an old set with a new set that has the same plate current. You don't have to bias them if you use their tubes and system.

Tubestore also uses a system that if you bias to a certain current, you just keep ordering the same ones each time.

[Cleveland Metal]

Re: bias-ing.........is it necessary?

Sorry I mistated that somewhat, they don't have to have "exactly" the same plate voltage, but darn close.

Timed Edit function, bah

[toejam]

Re: bias-ing.........is it necessary?

So when I need to retube the EL84s in my Mesa F-30, I'll be fine if I use Mesa tubes, but I'll need it rebiased for any other brand? It's got two EL84s and four 12AX7s.

[ November 17, 2003, 12:55 PM: Message edited by: toejam ]