Rebias every time or only when switching types?

[B2D]

Ok, I need to set the record straight for myself…

I’ve always been told that when you change the tubes in tube amps, that you must rebias the amp every time you change power tubes.

I’ve also been told that if you replace the old tubes with the EXACT same type of power tubes, (i.e. replacing GTEL34R’s rated at 5 with the same) that’s a safe alternative and you can get away with it.

My amp has a switch that will allow it to accept EL34s or 6L6s. Do I still need to rebias every time I change tubes or can I use one type and not rebias until I change types? Had the amp (it’s a Carvin Legacy) adjusted by a factory tech for the specific tubes I’m using.

Help clear my clouded mind.

 

[The Golden Boy]

Re: Rebias every time or only when switching types?

My understanding is that you should rebias when you change tubes.

Unless the tubes you're replacing have the same values as the ones you're replacing them with...

 

[Boogie Bill]

Re: Rebias every time or only when switching types?

Or, if you're using a fixed bias amp, like a Mesa or Peavey, your can simply use tubes within a range of the set bias of their amps. No muss, no fuss.

Bill

 

[Gearjoneser]

Re: Rebias every time or only when switching types?

Two things every tube amp owner should have is...

Bias tool socket, which is a 2 sided socket that fits between the tube and amp.

Volt/amp/ohm meter, which connects to the lead on the bias tool

If you see my Jubilee biasing thread in the Vault, you'll see how easy it is to monitor the amperage of the tubes. As long as you're in the proper range, the amp will sound good.
From there, you can tweak the bias pot so the amp sounds it's best. Play it as you bias it, and let your ears do most of the work. The meter assures that you're in the right ballpark.

 

[Young Angus]

Re: Rebias every time or only when switching types?

I think the answer you're looking for is that every tube has a certain "rating", and unless you know that the tubes you are replacing the old tubes with have the same rating as the ones that were in there last time it got biased then you probably would want to check the bias.

A way you might know that your tubes are all the same rating is for example if your amp takes two EL34's, and you get a matched QUAD of EL34's but only use two of them initially. You will know that the remaining two are rated the same because they were MATCHED so you should just be able to slip them in with no fuss (providing no one has messed with the bias in the meantime).

Aside from that unless you had all the instrumentation to measure the tubes and everything (in which case you probably wouldn't be asking us this question) then I'd get the bias checked when putting in new/different tubes even if they are the same type...ie. replacing EL34's with EL34's...still get the bias checked!

 

[Fresh_Start]

Re: Rebias every time or only when switching types?

Or, if you're using a fixed bias amp, like a Mesa or Peavey, your can simply use tubes within a range of the set bias of their amps. No muss, no fuss.

Bill

If you're referring to a "fixed bias" amp that doesn't have a bias adjustment, then you have no choice (unless you want to tinker with a resistor inside the amp). Fender's Blues Jr. is that way too, probably for cost reasons However, "fixed bias" does not mean non-adjustable. Many fixed bias amps have bias adjustment and you should use it if you got it.

Cathode biased amps are a different story - these are effectively self-biasing and almost never have a bias adjustment.

BTW before you go out and buy an expensive bias meter, check out the "bias checker" on Doug Hoffman's site. The build information is in his Library and he sells all of the parts. If you can't build one, you shouldn't be fooling with a multimeter or your amp's bias anyway.

Chip

 

[casblah]

Re: Rebias every time or only when switching types?

You probably should re-bias...if applicable, to your amp.

 

[Lake Placid Blues]

Re: Rebias every time or only when switching types?

I'm of the opinion; that yes you should always re-bias if it's an adjustable bias amp. In my experiance there is always a fairly wide "rating" range for tubes of the same type. An matched set of EL34's, for example, may still work okay in the amp, but it won't be ideal. You could unkownly be giving up tone, and/or the service life of the tubes may be shortened. I've seen cases were the bias was off enough to cause a melt down. If the bias is a little cold, your tone will be harsher than it need be, although the amp might work just fine.