The Canadian Guitar Forum banner

1 - 20 of 24 Posts

·
Registered
Joined
·
2,665 Posts
Bias can be beautiful...

Bias with guitar amps only applies to tube designs. Solid state amps handle biasing in a different way that is fixed for life on the printed circuit board.

All a player needs to know is that the output tubes in his amp have a constant minimum current running through them, even when there's no guitar signal. It's like the idle speed in a car engine. Bias refers to a control voltage that adjusts the minimum current happening through the tube(s).

Most amps have a bias adjustment inside for a tech to set the idling current to the proper figure. What IS the proper figure? There's a usable range where if the idle current is too low the tube is hardly working and will last for years. The tone will be kinda thin and trebly, 'though. Less power too. Adjust for higher idle current and the power comes up and the tone gets progressively "warmer" and "thicker". This is what most players want.

If you run things too high the tube is operating too "hot" and the life expectancy goes WAY down - a few months instead of a couple of years or more. In extreme cases (like where some idiot says "What's that there inside my amp? I'll just take a screwdriver and crank it on 10"!) you can burn the tube out in minutes.

Tubes were never perfectly the same and nowadays even more so. So when you replace a pair of outputs it's a crap shoot as to them being anywhere near the characteristics of the old tubes and the existing bias setting. It doesn't mean the new tubes won't work as well or better than the old ones, just that you absolutely should have the bias set for the new tubes.

Setting bias is a techie thing. You have to get inside the amp, take measurements and do some calculations to know what the proper figure should be. Tube amps have high voltages that if you're extremely unlucky can killyou! Most of the time if you get shocked it just hurts like hell. There are FAQ's on websites telling you all about biasing and how to do it. Only a nimrod would dive in before he got educated first and that takes longer than one quick read! You should already know how to service/build amps before you try it.

So just as most folks just want to drive a car and not have to become a mechanic most players take their amp to a tech to bias new tubes.

It should cost maybe $40-$50, especially if you're buying the tubes from the tech. Some techs are better than others. If you talk to a guy who says "I ALWAYS set Marshalls to 35 ma. of idle current!" turn around and leave. There is never a universal setting, even for the same model of amp. Things can vary inside and you have to take measurements and calculate for each amp, to do it proper.

There are amps that don't need biasing. Cathode biased amps like Vox AC30's, some Fenders, Gibsons and lots of others use a "self-biasing" circuit. It means much less power. A pair of 6L6's normally run 40-50 watts with what is called "fixed" bias (even though it's adjustable!) and with cathode bias you'd be lucky to get 30, probably 25. The difference is that cathode bias has a much different sound that many players love. Think Neil Young's "Rockin' In the Free World".

There is a type of circuit/bias setting called "Class A" which is famous for the warmest and bluesiest vintage tone. Some amp makers advertise their amp as Class A simply because it is cathode biased. This is "suit" crap to get you to buy the amp. HOW you bias a tube doesn't matter! The tube doesn't care about anything but the amount of bias voltage. Most cathode biased amps are actually running in a class called AB1, the same as most adjustable biased amps.

This is most offensive only to techies and don't let it stop you from trying out such amps! The suits in marketing may be lying but the designer may have still cooked up a killer sounding amp!

Boogies usually have no bias adjustment. They pre-test their tubes and expect you to only buy from them. They also may set the idle current on the low side so that the tubes will never fail during warranty and cost them a bit. Of course, if you have a favourite brand of your own or you want a warmer sound a good tech can compensate for you. It wouldn't be smart to just buy your own tubes and jam them in without making sure all factors are covered. Boogie is not just trying to lock you in to buying tubes from them. Their method ensures that nobody will blow up the amp from jamming in aftermarket tubes without making sure the bias is ok! So there's good and bad reasons here, depending on how you look at it.

A lot of amps leave the factory with the idle current deliberately set low for that reason. Usually the player never knows his amp could be made to sound better. If he complains to the store that he can't nail the vintage tone he thought he would they just con him into buying some kind of "POD PROCESSOR! GUARANTEED TO MAKE YOUR AMP SOUND LIKE ANYTHING AND EVERYTHING! HONEST!".

There's more of course but that's all the basics a player really needs.

Have fun!

---Wild Bill
 
  • Like
Reactions: GuitarsCanada

·
Registered
Joined
·
169 Posts
To Bias Or Not To Bias

Thanks Bill for the enlightening read. I have a much clearer understanding of the subject. So say I find a thirty something amp with what appears to be original tubes. Re-tubing, biasing, and a tech who knows his stuff, should yeild a better example than new.
 

·
Premium Member
Joined
·
2,577 Posts
great post Wild Bill....
thats what these forums are all about....congrats
 

·
Registered
Joined
·
1,348 Posts
wow that's a good read. I did not know any of that.:thanks5qx:
 

·
Registered
Joined
·
1,865 Posts
That's a lot of good info there Wild Bill, do you know any good amp techs that could tune up my Ampeg SJ-12R, it has a horrific noise problem at the moment. :food-smiley-004:
 

·
Registered
Joined
·
2,665 Posts
I don't get out much any more...

PaulS said:
That's a lot of good info there Wild Bill, do you know any good amp techs that could tune up my Ampeg SJ-12R, it has a horrific noise problem at the moment. :food-smiley-004:
Sorry Paul. I'm in Stoney Creek and don't really know the crowd up your way. There was a guy in Peterborough but I think he's retired as I picked up one of his customers.

I camped out at Lake Baptiste a couple of summers when I was much younger. Caught a nice 5 lb bass, some pike and a LOT of walley/pickerel!

What sort of noise are we talking?
 

·
Registered
Joined
·
1,865 Posts
Well when all knobs are at 0 there is no noise, when I turn the bass knob up to about 3 there is a loud hum in the speaker,above 5 and it increases a bit but peaks. When I tturn up the mid it does it also but not noticeable on the treble control. This is all with the volume at 0. I checked inside for loose or burnt stuff components but all looks good. I tried new tubes all the way around but the noise persists. Who was the guy in Peterborough, was his name tony?
 

·
Registered
Joined
·
2,665 Posts
Dave's not here!

PaulS said:
Well when all knobs are at 0 there is no noise, when I turn the bass knob up to about 3 there is a loud hum in the speaker,above 5 and it increases a bit but peaks. When I tturn up the mid it does it also but not noticeable on the treble control. This is all with the volume at 0. I checked inside for loose or burnt stuff components but all looks good. I tried new tubes all the way around but the noise persists. Who was the guy in Peterborough, was his name tony?
Can't remember his name, Paul. I heard from WillieP, if you know him you can ask.

If i recall (I had a better memory before the kids came) the SJ12R is one of the newer printed circuit board amps. The problem could be a lot of things. I would think the amp is not old enough to need new filter caps but maybe. The fact that there's no noise with the volume on 0 makes me think it's NOT the caps! Definitely, you need a tech! :)

Good luck with it!
 

·
Registered
Peavey Wolfgang EVH Wolfgang Charvel Style 2
Joined
·
1,487 Posts
Nice read Wild Bill I cannot disagree with anything you've said... LOL

I would like to add... for those of you who use tube amps and intend to maintain them and take an interest in whats best for their tube amp read on.

Tubes need to be warmed up before they recieve a full signal so place your amp on stanby for 2-3 minutes before turning the amp "on" and using it.

Never turn your amp off and then quickly turn it back on... the power tranny will hate you for it and is probably the single most expensive component in your tube amp.

Allways allow time for your amp to cool down after being turned off... 5 minutes minimum. Tubes when running have molten parts and need cooling time for max life expectancy.

Never use your ear to bias an amp.. ofcoarse you can hear it when it sounds right but read on...

I do not recommend you use a Multimeter to read plate voltage for the purpase of adjusting bias...

Your ear and a multimeter are a GUESS at best.

Amp biasing should be performed with an oscilloscope. This way the waveform can be seen and the bias adjusted until the waveform is as close to if not perfect.

I do all my own Scope biasing and after taking a few voltage measurements it is clear that the voltage can be different to result in the same wattage due to varying current draw even with the same tubes and same amp. Even a few 10'ths of a volt can make a difference to the waveform.

Tubes get gassy even when just laying around. Tubes change electrical characteristics and values throughout their lives and it is for these reasons that you should consider periodic tube biasing even if you are not changing the tubes.

At the very minimum bias you tubes at every change.

Purchase you own tubes. In matched sets from a quality supplier. Purchase from a tech only if you absolutly trust him.

If you are paying someone to bias your amp make sure it is done with a oscilloscope. Believe me you will pay the same for some guy to use a Fluke Multitmeter as you would for another guy to use a Fluke Oscilloscope so get the most for your money. Any good tech will have the right gear so call around and ask what is used.

I use a Tektronix 422 benchtop anolog oscilloscope to do all my amp biasing.
I only use a multimeter to record readings never to base my bias adjustment on.

Craig
 

·
Registered
Joined
·
2,665 Posts
Oh oh...techie differences of opinion! :)

KHINGPYNN said:
Nice read Wild Bill I cannot disagree with anything you've said... LOL

Tubes need to be warmed up before they recieve a full signal so place your amp on stanby for 2-3 minutes before turning the amp "on" and using it.

Never turn your amp off and then quickly turn it back on... the power tranny will hate you for it and is probably the single most expensive component in your tube amp.

Allways allow time for your amp to cool down after being turned off... 5 minutes minimum. Tubes when running have molten parts and need cooling time for max life expectancy.

Never use your ear to bias an amp.. ofcoarse you can hear it when it sounds right but read on...

I do not recommend you use a Multimeter to read plate voltage for the purpase of adjusting bias...

Your ear and a multimeter are a GUESS at best.

Amp biasing should be performed with an oscilloscope. This way the waveform can be seen and the bias adjusted until the waveform is as close to if not perfect.

I do all my own Scope biasing and after taking a few voltage measurements it is clear that the voltage can be different to result in the same wattage due to varying current draw even with the same tubes and same amp. Even a few 10'ths of a volt can make a difference to the waveform.

Tubes get gassy even when just laying around. Tubes change electrical characteristics and values throughout their lives and it is for these reasons that you should consider periodic tube biasing even if you are not changing the tubes.

At the very minimum bias you tubes at every change.

Purchase you own tubes. In matched sets from a quality supplier. Purchase from a tech only if you absolutly trust him.

If you are paying someone to bias your amp make sure it is done with a oscilloscope. Believe me you will pay the same for some guy to use a Fluke Multitmeter as you would for another guy to use a Fluke Oscilloscope so get the most for your money. Any good tech will have the right gear so call around and ask what is used.

I use a Tektronix 422 benchtop anolog oscilloscope to do all my amp biasing.
I only use a multimeter to record readings never to base my bias adjustment on.

Craig
Hi Craig!

For the lion's part of what you said we're on the same wavelength but I think it only fair and proper to point out that among techies there is some disagreement about meter vs. 'scope biasing. In fact, i would suggest that 'scope biasing is actually championed by a very small minority of tube amp gurus.

In practice I've found that it works out close either way. The problem is that you need a reasonably good scope to have sufficient trace definition AND you have to fully understand what the 'scope is showing you!

Randy the Tone Lizard out west is a 'scope method champion. He's actually the only one I've come across. Also the only one that seems to know what he's talking about! For those sufficiently interested there are excellent sites such as Mr. Aiken's http://www.aikenamps.com that have FAQs on biasing.

I can 'scope bias if the customer insists but frankly I would immediately check it with my meter! :) I use my 'scope pretty well only for audiophile amps, which are supposed to amplify an existing generated sound without changing it and therefore by definition you're shooting for the ultimate lowest distortion. Guitar amps are an extension of the guitar and further shape the sound, which is adding distortion, no matter if it's pleasing. How on earth could someone expect to sound like Van Halen playing through a Harmon Kardon Citation II tube hifi power amp? :)

I only point this out because I personally would never tell players to ONLY have their amp biased by the 'scope method and that metering is wrong. Metering IMHO is always more than close enough for properly designed tube amp circuits whereas a 'scope in the hands of a newbie will likely make him sorry.

Then again, if the tech is truly inept he will find a way to blow things up no matter what methods and tools he uses. :(

Warm up time is always a good idea but I differ about whether or not a lack of it damages the amp or tubes. The usual reason quoted is the phenomenon of "cathode stripping". While I saw lots of info on this when I used to sell Eimac power tubes to radio and tv stations I've never seen boo to indicate it applies to the tiny little cousins used as guitar amp output tubes. I would even quarrel about the need for a standby switch in any amp under 100 watts.

I absolutely agree with letting the amp cool down! Tubes are like light bulbs. When the filament is hot it's very fragile. Everyone has learned that if you knock over a lamp that's not switched on the bulb usually survives just fine but if the lamp is on at the time it ALWAYS kills the bulb!

I doubt if snapping the amp on and off will kill the power tranny. While it can cause a big current surge, if done fast enough it would actually be less harmful! Consider that the surge is caused by the filter caps being drained so they appear as a dead short at the moment of turn on and that the filaments also have a near-short resistance when they are cold. If the amp is operating when you snap the switch the caps and the tubes don't have time to completely cool and drain so they have a much higher effective load resistance.

To blow a trannie you need to have it passing excessive current long enough for the wire in the internal coils to overheat to the point where they melt. We're talking about millisecond time intervals of current spikes here when quickly and repeatedly snapping the switch. I doubt if a lab could measure any scary heat buildup in that kinda time frame.

That being said, I would not be surprised if it took out the power supply silicon diodes. Surge currents are much harder on them. A tube rectifier would likely not notice at all or just laugh at it! :)

I would agree that biasing by ear is just dumb showmanship.

As for the need for rebiasing, unless a tube is aging prematurely I've rarely found the need before a year or more. By that time with some brands and frequent playing you may need new tubes anyway! We both agree to always have new tubes biased.

I've learned not to be too anal about close matched output tubes. This seems to be a "hifi" audiophile thing. It does result in the lowest distortion figure but unless you're Mantovani who in their right mind would want that for electric guitar? An electric guitar is SUPPOSED to be distorted! I find that a few ma of difference will result in a bit more harmonics, that give a pleasing "swirl". More than 5 ma. or so can noticeably unbalance the output transformer and give extra hum or stress on the windings but even then I seriously doubt if this amount would be more than an annoyance rather than a catastrophe.

I suspect our differences here illustrate our personal tastes. You're probably more of a hifi guy or if a player you lean to jazz. Me, I'm straight up rockin' blues and classic geezer rock! Let me plug some humbuckers straight into a vintage non-master volume Marshall "Plexi" with a stack of either vintage Celestions or modern Eminence Red Coats and I couldn't care less if the other guy's EL 34's are biased 1/4 of a milliamp closer than mine! :)

This is fun! Might we end up with a Canadian site to rival Ampage?

Oh well, this was just my .02. Now that we've totally confused all our audience... :)
 

·
Registered
Joined
·
563 Posts
Hi all, any recommendations for a good first oscilloscope.....for a DIY guitar amp and studio processor newbie? Is ebay a good place for this? Dot.gone liquidators? Garage sales?

Thanks,

Andy
 

·
Registered
Joined
·
2,665 Posts
'Scopes...

---"Hi all, any recommendations for a good first oscilloscope.....for a DIY guitar amp and studio processor newbie? Is ebay a good place for this? Dot.gone liquidators? Garage sales?"

Well, you can look around for new ones but they tend to have features that make them overkill for audio amps. I've bought 'scopes at hamfests and found the one I'm using today at a pawn shop! Ebay can be a good source as well, as long as you understand what you need. Why pay thousands for a backhoe when all you needed to do your small flower garden was a $10 shovel?

For audio you can get by with only one channel easily. Modern 'scopes have at least 2. It can be tube or solid state. Tube units may be heavier but usually much cheaper.

'Scopes specify a bandwidth, or highest frequency where they will still be accurate. They will go much higher but will not give accurate voltage measurement. As low as 10 mhz or even 5 mhz is plenty for audio. Higher bandwidths are only useful for higher speed digital signals and radio frequencies. Modern 'scopes nearly all offer at least a 100 mhz rating. The one I got from the pawn shop is 2 channel, 100 mhz and I paid $130 including two probes. That admittedly was a steal but you should have no trouble finding something for a few hundred dollars tops.

The absolute most important thing is that the 'scope be calibrated. The input channel will have a switch to set the voltage range per division on the screen. It might have 10 mv as the most sensitive setting (great for looking at the signal from the guitar!) and scale up to 50 volts/div or higher.

This means you can read the voltage level of the signal right off the screen, accurately! Cheaper 'scopes like many of the old tube-based Eico's will only have a level control at the input. You can adjust it so that you can see the whole signal without it peaking outside the vertical height of the screen but you have no idea what the actual voltage strength is...you can see obvious distortion but how can you tell if that preamp stage has enough gain?

Don't forget that you need the probes! New 'scopes normally include a set but with something used they may or may not be in the deal. The cheapest aftermarket probes good to maybe 50 mhz will likely cost you $40 or so!
 

·
Registered
Joined
·
563 Posts
Thanks!!!! I definitely don't want to do the "soccer mom with a Lincoln Navigator driving to the mall to get 2 $350++ (each) 'everest during winter' rated sleeping bags for the kids to have a sleep-over in the back yard" but at the same time, like all my tools, I'd rather buy once.

Andy
 

·
Registered
Joined
·
253 Posts
Nice reading guys.Wild bill,you are obviously a knowledgeable guy.
I NEVER bias a guitar amp with an oscilloscope.I bias an amp where it sounds the best and is within the specs for the tube(a loose spec!).For example: a certain amp sounds best when biased cold at 30ma and another sounds best when biased at 40ma.Which is right?Both are right.We bias tubes so the amp sounds good right?And of course to save those expensive tubes from burning up.
Another point:a Fender Deluxe reverb sounds cold and lifeless when biased at the maximum plate dissipation for 6V6's.They sound best when biased at about 35ma and up/440v.
Don't bias an amp by ear? Why not? If it isn't red plating,humming or doing anything crazy,why not?
And perfectly matched tubes are not the end all or be all for all amps either.Some amps sound marvelous with a purposely mis-matched set of tubes.Of course within a reasonable amount.
Now take all I've said and throw it all out when biasing an AUDIO tube amp.They do not want distortion or cutoff happening so all of what Khngpynn said is true also.Now us guitar players love all that distortion and cutoff and stuff happenning because it sounds great for rock music,so we are classified as rebels.Ever played a class A amp that doesn't distort? Boring!And most push-pull amps operate in class A mode till they clip and then they cross over into class AB and beyond.

www.claramps.com
 

·
Premium Member
Joined
·
1,766 Posts
Wild Bill said:
It's like the idle speed in a car engine. Bias refers to a control voltage that adjusts the minimum current happening through the tube(s).

---Wild Bill
Hate to be a stickler, but this isn't accurate. The idle current is not the minimum current that the tube will see. In fact, in the case of a class A design, the idle current will be significantly higher than the minimum current that a particularlar tube will conduct when the input signal (for that tube) is at it's maximum.

The comparison to a car engine idle is not appropriate to descibe the idle state of a tube amp.
 

·
Registered
Joined
·
253 Posts
Ok.We are not here to educate the world on tube amps,I think Wild Bill was using the analogy to help a newbie understand the idea.The idle current of a class A amp is near 90-95% of it's MAXIMUM current and idle current in a push-pull amp is set so that at MAXIMUM draw it doesn't exceed the limits of the tube.
We know that.Whether or not it is the MINIMUM is not what we care about here.We just don't want the tube to draw too much current and burn itself up.I thought the autombile terminology was a good one,although a little inacurate.
Here's a better one:a class A amp is a 1600cc engine and a class AB push-pull amp is a big block chevy.The little engine sounds sweet for about an eighth of a mile and the big engine gets sweet and very loud near the end of the quarter.
The push-pull amp is capable of 3 times the power output of a similar class A amp.
some people like the sound of a 4cyl at full song and some dig the throaty roar of a big-block.
 

·
Premium Member
Joined
·
1,766 Posts
Adicted to Tubes said:
Ok.We are not here to educate the world on tube amps
But if you are going to post something technical, it should be correct. There's already enough mis-information out there.

Good luck with your amp line...It's good to see some more Canadian builders out there.

Scott
 

·
Registered
Joined
·
253 Posts
I don't think an analogy would classify as 'technical' information.
If he said bias your blackface reverb at 95ma and it burnt down,then he would be guilty of misinformation.
So a direct question like the actual acceptable bias would need an accurate answer.
 

·
Premium Member
Joined
·
1,766 Posts
Adicted to Tubes said:
I don't think an analogy would classify as 'technical' information.
If he said bias your blackface reverb at 95ma and it burnt down,then he would be guilty of misinformation.
So a direct question like the actual acceptable bias would need an accurate answer.
This is the statement that I pointed out as misleading...

"Bias refers to a control voltage that adjusts the minimum current happening through the tube(s)."

I guess it wouldn't cause anyone to fry their amp though, so forget I mentioned it :D
 
1 - 20 of 24 Posts
Top