Do I really NEED to 'bias' my amp?

[Reckless_Desire]

Saw this in the Tube Amp FAQ

"Do I really NEED to 'bias' my amp?
Only if getting the best sound possible is of any importance to you, but in a fewer words—probably not. As long as you keep using the same type and brand of tubes you previously had, then the bias shouldn't be that far off. "

I'm just curious if this is true, lets say I currently have JJ EL34's in my amp and I replace them with another matched set of JJ EL34's, you no longer need to bias ?

[SteveD]

It's not essential but it's recommended.

[gtrman]

The tubes will last longer if biased ....

[Bailz]

The tubes will last longer if biased ....

Surely that would depend on which way the bias had drifted?

[jagermike66]

In my experience, they can drift quite a lot. I put new EL34Ls in my amp last year and after about 2 months I found that they had drifted from 36mA to 31mA (doesnt sound like much but it is!). I was told it depends on the change in conditions that the amp is subject to, for example going from a cold dry storage room, to a hot and humid stage in a short period of time can effect the components and therefore sway the bias. Dunno how much truth there is in that but it seems to ring true for my amp.

Also never assume new tubes are matched even if they company you bought them from say so. I just bought some KT77s, one was running at 35.5mA (good) and the other was running at 20mA (waaay too cold). Either theres something wrong with my amp (which as far as I know there isnt, as it still works and sounds good), or the tubes werent matched.

[Dave W]

Those tubes should be replaced by the tube dealer you bought them from and they should pay the shipping to and from.

I know its not essential for tubes to perfectly match, but I would rather they are as close as can be.

I always bought my tubes from Eurotubes and they always matched in quads within 1.5 ma usually within 1 ma. I started using another Tube place because of their free shipping on orders $99 and more and because they run discounts at times. Four tube sets and all varied between 4-6 ma in a quad. Ordered 8 ecc803's and 2 were bad at installation.

Eurotubes shipping is a little more but will not use anyone else anymore. I at least believe they actually do the testing they say they do and thats worth the few extra bucks for shipping.

[slowpokerhino]

Eurotubes shipping is a little more but will not use anyone else anymore. I at least believe they actually do the testing they say they do and thats worth the few extra bucks for shipping.

+1. I've ordered many tubes from Bob @ Eurotubes and have never gotten a bad one. He's also very helpful if you need his assistance.

[jagermike66]

Those tubes should be replaced by the tube dealer you bought them from and they should pay the shipping to and from.

Yeah good idea! They are fine at the moment, still sounding good. I'll be checking the bias again in a couple of weeks after a few rehersals and a gig. If they have drifted alot then yeah the tubes will be going back and they can refund me.

I've always found eurotubes to be the most helpful. I always get a detailed reply to any emails, and some good suggestions. Only problem is they do seem to be JJ only from what I can tell. Other places usually give me a sentance reply n thats it!

[MartyStrat54]

Eurotubes shipping is a little more but will not use anyone else anymore. I at least believe they actually do the testing they say they do and thats worth the few extra bucks for shipping.

+1. I've ordered many tubes from Bob @ Eurotubes and have never gotten a bad one. He's also very helpful if you need his assistance.

There are a few things I do not like about Eurotubes. For one, JJ's are all they sell, so they make it seem like JJ's are the best. This simply isn't true. Secondly, Bob sells his customers a "balanced phase inverter" tube at an additional cost. A balanced PI isn't necessary since the circuitry isn't balanced to begin with.

The guys I roll tubes with all use a non-balanced high gain PI tube. In fact, some players use a 12DW7 in the PI slot. This is a highly lopsided tube. Also, if your signal coming off of the OPT is lopsided, it is richer in even order harmonics and your guitar will sound a lot better.

[surfnorthwest]

Hey Marty, how many "I love or hate Euro tubes" threads have you read in forums. Funny how this always comes up, reminds me of the Bill Murrey movie "What about Bob". While I will say JJ valves are better then they used to be, I have such a bias and pet pieve against them I freak out anytime I see a JJ snuck into a amp I just bought or had serviced.

[Hot Tubes 70]

For best results , always rebias your amps .....

[Marshall Mann]

Is rebiasing essential? Would you change your oil and not change the filter?

Tube amps are designed with a bias trim pot for a reason. It's an essential part of proper tube amp maintenance.

You might get lucky and purchase a new set of tubes with natural bias close to the set you removed, but that is leaving a lot to chance. If they bias too cold, you'll end up with crossover distortion. Too hot and they will be over compressed and die prematurely.

[joeellistsl]

Is rebiasing essential? Would you change your oil and not change the filter?

Tube amps are designed with a bias trim pot for a reason. It's an essential part of proper tube amp maintenance.

You might get lucky and purchase a new set of tubes with natural bias close to the set you removed, but that is leaving a lot to chance. If they bias too cold, you'll end up with crossover distortion. Too hot and they will be over compressed and die prematurely.

I agree. I have 2 matched pair of vintage Mullard EL34 XF2 tubes and they are fairly far apart in current draw. When the set I'm currently using become unusable and I install the other set, I will have no choice but to rebias these tubes. Bot sets sound very good but there is a substantial difference in current draw.

[Harlequin Tusk]

Eurotubes shipping is a little more but will not use anyone else anymore. I at least believe they actually do the testing they say they do and thats worth the few extra bucks for shipping.

+1. I've ordered many tubes from Bob @ Eurotubes and have never gotten a bad one. He's also very helpful if you need his assistance.

+2 here, have ordered many full sets and never had a problem, I deal with a Michael there..good guy.

[BigD]

I understand that not biasing an amp can cause problems for tubes, but can it cause damage to the amp if left totally unchecked with really miss-matched tubes?