Tube biasing

[jcn37203]

In an amp that doesn't do that self-biasing thing, is it required to have the amp rebiased in order to change out any of the tubes? Or only the preamp tubes? Or only the power amp tubes?

 

Exactly when is a rebias required?

[themeanreds]

 

Originally posted by jcn37203 In an amp that doesn't do that self-biasing thing, is it required to have the amp rebiased in order to change out any of the tubes? Or only the preamp tubes? Or only the power amp tubes? Exactly when is a rebias required?

 

 

Re-bias is required for power tubes only.

 

What amp are you thinking of re-tubing?

[jcn37203]

 

Originally posted by themeanreds Re-bias is required for power tubes only. What amp are you thinking of re-tubing?

 

 

None really, I was just curious.

 

Seems like it would be a serious pain in the ass to have to take an amp in to a tech any time you got any tubes for it.

[idiot savant]

preamp tubes are almost always cathode biased(self-biasing) so they don't need rebiasing, just the power tubes, everytime you change 'em. even power tubes that are cathode biased should be checked out from time to time to make sure current draw of the tube is just right.

[palthegiraffe]

There are devices like the BiasRite that allow you to do it at home with a digital multimeter.

 

Now if I could just remember how to work that digital multimeter...

[Mr. Twang]

 

Originally posted by jcn37203 None really, I was just curious. Seems like it would be a serious pain in the ass to have to take an amp in to a tech any time you got any tubes for it.

 

 

Depends on the amp I think. But then again, I don't really change my tube all that often.

 

You don't automatically need to have the amp biased if you get a matched set of tubes and you're replacing the old tubes with another set of the same brand.

[jcn37203]

 

Originally posted by Mr. Twang Depends on the amp I think. But then again, I don't really change my tube all that often. You don't automatically need to have the amp biased if you get a matched set of tubes and you're replacing the old tubes with another set of the same brand.

 

 

Huh, I didn't know that.

 

I've always wondered though, these guys who do tube comparisons. Sure they aren't rebiasing fifteen times just to compare all the different tube types.

[jcn37203]

This tube {censored} confuses the hell out of me.

 

Using my own amp as an example:

 

So given this schematic...

 

link

 

And this picture...

 

link

 

 

Using the photograph as reference, the leftmost tube (5Y3GT) is the rectifier tube, which apparently is a type of power tube. Then the two next to it are the power tubes (6V6GT).

 

Then apparently the next two are the preamp tubes (6EU7). Then the two next to them, both in weird metal housings are the reverb and trem tubes, 7199 and 6EU7 respecitively.

 

Is this correct?

 

Also, if so, is there any way to know which is the reverb and which is the trem tube?

 

 

And what are the effects, if any, of swapping out the reverb and trem tubes? Will different tubes create different sounding reverb and trem?

[ginnboonmiller]

Dude, I am totally with you in terms of bebafflement and enconfusingness about this.

 

My Princeton needs new tubes. But because it only needs a pair of 6L6s, I got it in my head somewhere that it doesn't need biasing ever. But I don't know how that got in my head, so I'm afraid to change the tubes. I also think that both leaving tubes unbiased and trying to bias them yourself are 100% certain to cause cancer, immediate electrocution and sterility. Guess I'll have to call my tech.

[jcn37203]

bump

[idiot savant]

 

Originally posted by jcn37203 This tube {censored} confuses the hell out of me. Using my own amp as an example: So given this schematic... link And this picture... link Using the photograph as reference, the leftmost tube (5Y3GT) is the rectifier tube, which apparently is a type of power tube. Then the two next to it are the power tubes (6V6GT). Then apparently the next two are the preamp tubes (6EU7). Then the two next to them, both in weird metal housings are the reverb and trem tubes, 7199 and 6EU7 respecitively. Is this correct? Also, if so, is there any way to know which is the reverb and which is the trem tube? And what are the effects, if any, of swapping out the reverb and trem tubes? Will different tubes create different sounding reverb and trem?

 

 

the 6eu7 preamp tubes are alot like 12ax7's but use a different pinout so they aren't interchangable. the 7199 is a triode/pentode which drives the verb. there are more pentode/triode combo tubes, but i'm not very familiar with them. prob shouldn't mess with that. the 6v6's are obviously power tubes, changing these will probably affect the sound of the amp the most. the 5y3 is a rectifier tube, which provides power to the rest of the amp, apparently you can supposedly swap different recto tubes for more or less "sag" but they seem tomore or less emphasize the charecteristics of the amp already.

 

looking at the schemo you posted, the power tubes are cathode biased(self-biasing), so you can pop in some different 6v6's without much fuss.

 

you could also have the preamp tube sockets re-wired to take 12ax7's, but that would affect the value of the amp.

 

it also looks like there is an unused triode half of V3, you could use that somehow.

 

unless you want to re-wire everything, simple tube swaps are the best way to get different sounds from the amp.

[idiot savant]

 

Originally posted by ginnboonmiller Dude, I am totally with you in terms of bebafflement and enconfusingness about this. My Princeton needs new tubes. But because it only needs a pair of 6L6s, I got it in my head somewhere that it doesn't need biasing ever. But I don't know how that got in my head, so I'm afraid to change the tubes. I also think that both leaving tubes unbiased and trying to bias them yourself are 100% certain to cause cancer, immediate electrocution and sterility. Guess I'll have to call my tech.

 

 

what kind of princeton, all the ones i've seen have 6V6's, and are fixed bias.

[JimVW]

You must rebias your amp when you replace power tubes in a fixed bias amp. Cathode biased amps do not have a bias adjustment.

 

Rebiasing is trivial if you have a bias tool like the Weber Bias-Rite and if your amp has a chassis-mounted pot to change the bias. It took me all of 5 minutes to rebias my Carr Slant 6V when I changed tubes in the fixed bias power section a few months ago.

 

If your amp has bias test points, you don't need the Bias-Rite, just a multimeter.

[JimVW]

 

Originally posted by jcn37203 Also, if so, is there any way to know which is the reverb and which is the trem tube?

 

According to your schematic, the 7199 is in the reverb circuit (V2), and half a 6EU7 is in the trem circuit (V4).

 

 

And what are the effects, if any, of swapping out the reverb and trem tubes? Will different tubes create different sounding reverb and trem?

 

 

Careful here, not every preamp tube is compatible pin-for-pin or voltage-wise. It's my understanding that the type of tube in these circuits doesn't play much of a role in the sound of the effect.

[jcn37203]

 

Originally posted by JimVW Careful here, not every preamp tube is compatible pin-for-pin or voltage-wise. It's my understanding that the type of tube in these circuits doesn't play much of a role in the sound of the effect.

 

 

Thanks.

 

I didn't mean swap them one for the other, I meant replace them both.

 

I'm not really planning on doing any of this, just trying to understand as much as I can.

[Jebus0000]

I thought that biasing the amp only makes the tubes run at the right...um..something...and that if you don't rebias an amp after changing the tubes that it just means that your tubes may be running too hot and won't last as long.

 

Can some explain what biasing actually is?

[JimVW]

 

Originally posted by Jebus0000 I thought that biasing the amp only makes the tubes run at the right...um..something...and that if you don't rebias an amp after changing the tubes that it just means that your tubes may be running too hot and won't last as long. Can some explain what biasing actually is?

 

 

From this excellent resource (and great place to buy tubes)

 

Q: Why must I rebias my amp when installing new power tubes?

 

A: Setting the bias is like adjusting the idle of your car. If not set properly, the performance (tone) of your amp will suffer. If the bias is set too high for your new power tubes, you can destroy them in a matter of minutes. You can also damage your amp. If the bias is set too low, the amp will sound weak, thin and anemic.

[idiot savant]

 

Originally posted by Jebus0000 Can some explain what biasing actually is?

 

 

http://www.aikenamps.com/WhatIsBiasing.htm

[ginnboonmiller]

 

Originally posted by idiot savant what kind of princeton, all the ones i've seen have 6V6's, and are fixed bias.

 

 

you're right.

[Jebus0000]

thanks guys

[jcn37203]

You guys rock.

 

One final question...

 

In the photo I posted above, the two small exposed tubes are supposedly 6EU7 preamp tubes (according to the schematic at least), but it looks like one is noticably smaller than the other.

 

Thought?

[idiot savant]

 

Originally posted by ginnboonmiller you're right.

 

 

I figured. I have a silverface princeton. Hoffman amps has a good way to add a bias pot to the princeton circuit. I plan on adding it to mine for sure.

 

http://www.hoffmanamps.com/charts/Biascircuits.htm

 

you can also mod the phase inverter of the princeton for more headroom/volume. i plan on doing this one as well.

 

http://www.people.cornell.edu/pages/mt24/Amp/mm/PRtweak.html

[hookstrapped]

 

Originally posted by ginnboonmiller Dude, I am totally with you in terms of bebafflement and enconfusingness about this. My Princeton needs new tubes. But because it only needs a pair of 6L6s, I got it in my head somewhere that it doesn't need biasing ever. But I don't know how that got in my head, so I'm afraid to change the tubes. I also think that both leaving tubes unbiased and trying to bias them yourself are 100% certain to cause cancer, immediate electrocution and sterility. Guess I'll have to call my tech.

 

 

You can borrow my Weber Bias-Rite (w/ built-in meter). PM me.

 

 

 

 

 

* Re-bias at own risk. Not responsible for electrocution, cancer, impotence, or sterility.

[JimVW]

 

Originally posted by jcn37203 You guys rock. One final question... In the photo I posted above, the two small exposed tubes are supposedly 6EU7 preamp tubes (according to the schematic at least), but it looks like one is noticably smaller than the other. Thought?

 

 

Probably just a different manufacturer. In my Slant 6V I have two pairs of 6V6s--one running fixed bias and the other cathode. They are from different manufacturers and are quite a bit different in size.

[guitarcapo]

I'm starting to believe biasing isn't as important as we all make it out to be. For one thing, a lot of amps are being built today that not only have no bias adjustment, but the amp is set at a bias that runs the tubes WAY hotter than they were built to take. But the amp sounds fine and is being sold by the thousands with no amps bursting into flames.

 

The amp in question?: The Fender Blues Junior. The amp comes from the factory with the tubes burning at around 100% of capacity AT IDLE and there's little you can do about it unless you want to toss the warrantee and have it modded.

 

READ THIS!

 

I hear stories how amp techs at the Fender factories don't even CHECK bias when they install tubes in amps leaving the factories. Amp gurus debate and flame each other arguing what is the best way to set the bias. 60%? 75%? Use an oscilliscope? Just bias until it sounds right? The whole thing starts to sound like snake oil,

 

My opinion lately is that you bias to make sure the tubes aren't running to hot, but the rest of the stuff is debateable...I'm also suspicious of all this "matched tube" business. Basically it's a carryover from the audiophile geeks that might not be as necessary in guitar amps as we've been lead to believe. Again, maybe just keep an eye on it to make sure they are close.