Do I need Tube bias adjustment?

[Radioboy950]

Hey folks. I am brand new to the "tube scene".
If my amp currently has a pair of JJ 6L6GC, and I replace them with a new matched pair of JJ 6L6GC's, would I still need a bias adjustment?
The manufacturer says to re-bias if you don't get the tubes directly from THEM because they grade their tubes, and discard many that don't make the cut.
Supposedly, if you buy their tubes with the same grade, you're fine. But of course this comes at a price...about 2X what you'd pay elsewhere for the matched pair.
What's your take on this?

[tunghaichuan]

Is this a Mesa/Boogie amp? Their amps usually have non-adjustable fixed bias. You have to swap out a resistor to get the bias set right when using tubes not from them.

Even if it isn't a Boogie, you should have the bias set by a competant tech any time you change tubes. Modern production tubes like JJ aren't made to the same tight tolerances as NOS tubes were, RCA, GE, Tung-sol, Mullard. Not that JJs are bad tubes, just not consistant from tube to tube.

You might want to have the bias checked after a few months of playing the amp as well. As tubes age, the bias can change.

My $.02

tung




Hey folks. I am brand new to the "tube scene".
If my amp currently has a pair of JJ 6L6GC, and I replace them with a new matched pair of JJ 6L6GC's, would I still need a bias adjustment?
The manufacturer says to re-bias if you don't get the tubes directly from THEM because they grade their tubes, and discard many that don't make the cut.
Supposedly, if you buy their tubes with the same grade, you're fine. But of course this comes at a price...about 2X what you'd pay elsewhere for the matched pair.
What's your take on this?

[Spudman]

What amph?
As tung said some have a fixed bias and do require a more in depth session to re bias. Some amps are just fine switching in and out tubes. A few manufacturers have this feature. Otherwise, the rule of thumb is new tubes = re-bias always. Check around the web for information about your amph. Some sites have good advice as to exactly what you'll want it biased to voltage wise.

[Radioboy950]

It's a Rivera Quiana 112 combo.
I checked their online documents, which tell me no re-bias as long as Rivera selects tubes for you, you're replacing the originals and the bias has not been previously adjusted from the factory settings.

[tunghaichuan]

It's a Rivera Quiana 112 combo.
I checked their online documents, which tell me no re-bias as long as Rivera selects tubes for you, you're replacing the originals and the bias has not been previously adjusted from the factory settings.

It seems pretty clear to me: if you buy Rivera's tubes, you can just plug and play. If you buy them from somewhere else, you need to have the amp rebiased.

OTOH, what the online documentation doesn't tell you is that no two tubes are alike; they all age differently. As a tube ages, its bias requirements may change as well.

But as Spudman pointed out, some amps have a tolerance. Bias isn't exact, either. My opinion is that a colder biased tube will not sound as good as one that has a hotter bias. Colder biased tubes will last longer, though.

If you're a touring professional, it might be a good idea to have the amp checked and biased before every show.

If you play gigs on weekends with a few of your buddies, you may get away with going every 3-6 months without checking.

If you play an hour or two a day in your bedroom, you may only want to have the bias checked when you change to a fresh pair of tubes.

tung

[Radioboy950]

Thanks for the input.
It helps to talk it over with others who have more experience at this stuff. :AOK:

[Plank_Spanker]

Tung,

You are indeed "da man"!

I truly hope you are around here if I run across amp troubles that stump me. :beer:

[tunghaichuan]

Tung,

You are indeed "da man"!

I truly hope you are around here if I run across amp troubles that stump me. :beer:

Aw shucks, I don't know what to say :)

Seriously, though, I've learned more hanging out here than I've given back :)

tung

[aeolian]

I'm also quite new to tube amps myself so I can't give you useful advice. But I do recommend calling up Bob at Eurotubes and ask him your question telling him about your amp and all the details. He will give you the straight story about tubes and matched tubes. And he only sells JJs; look on his website to see the different class of matched tubes and when you talk to him ask him about them. He will help you even if you don't buy anything from him.

[Radioboy950]

I'm also quite new to tube amps myself so I can't give you useful advice. But I do recommend calling up Bob at Eurotubes and ask him your question telling him about your amp and all the details. He will give you the straight story about tubes and matched tubes. And he only sells JJs; look on his website to see the different class of matched tubes and when you talk to him ask him about them. He will help you even if you don't buy anything from him.

Cool site with some DIY videos, if you want to see how it's done.

[aeolian]

Bob is located in Portland where I live. A few months ago when I had problems with my tube amp I first called him, then I made 2 trips out to his shop and he and his guys give me terrific service and advice. The 2 trips is because I screwed up the first time. I can't say enough about him. Check the internet, he has a great reputation.

My amp is cathode biased, which means there is no need to bias the amp when I change tubes (don't ask me what that means). I did buy a matched pair the last time because that is what my amp manufacturer recommended.

[tunghaichuan]

Bob is located in Portland where I live. A few months ago when I had problems with my tube amp I first called him, then I made 2 trips out to his shop and he and his guys give me terrific service and advice. The 2 trips is because I screwed up the first time. I can't say enough about him. Check the internet, he has a great reputation.


+1 on Bob from Eurotubes. I've ordered a few times from him and have always gotten extraordinary service.



My amp is cathode biased, which means there is no need to bias the amp when I change tubes (don't ask me what that means). I did buy a matched pair the last time because that is what my amp manufacturer recommended.

Cathode bias is another way of biasing an amp. In a guitar amp, the grid of the tube (where the signal is injected) has to be negative with respect to the cathode. In a cathode bias amp, this is accomplished by putting a resitor between the cathode and ground. In a fixed bias amp, the cathodes are tied directly to ground and a special negative bias voltage supply circuit is connected to the grid.

Generally, it is true that changing tubes in a cathode bias amps is a matter of swapping the old set for a new set. However, it is a good idea to check the bias to make sure that the tubes aren't running too hot after changing the old tubes out. As I've said before, modern tubes aren't made to tight tolerances. But, since Bob at Eurotubes does do a good job of matching and screening his tubes, it is less of an issue.

You can set the bias in a cathode bias amp; it involves taking measurements with the amp powered up, so it is inherently dangerous. Setting the bias is accomplished by changing the value of the cathode bias resistor.

tung