bias setup

[madmax]

bugera 6260 how do i set the bias on this amp? any help is appreciated. I am new to tube amps and to this site so be gentle.

[D2o]

Apr 13, 2009 21:46:04 GMT -5 madmax said:

bugera 6260 how do i set the bias on this amp? any help is appreciated. I am new to tube amps and to this site so be gentle.

Hi madmax, and welcome to Thunderdome GN2!

Not a silly question in the least - an extremely prudent question, in fact.

I will be most interested to hear anything other than advice to take your tube amp to a good and qualified amp technician, which is all I would do with a tube amp ... that's serious business.

Hang tight and welcome to the show.

D2o

[sumgai]

mm,

Hi, and welcome to the NutzHouse!


Nobody's posted a schematic yet to web, but most folks seem to think it's a knock-off of the Peavey 5150. Sadly for me, there are some conflicting opinions about now closely the Bugera cloned the Peavey, so I'm not gonna rely on the 5150's schematic to answer your question.

If you already have this unit, did you perchance receive a schematic diagram with it? If so, could you post a copy, please? And if not, can you get one from your local dealer? As a last resort, you can try the company itself, they're still new enough that they may value customer support.

In general, you are correct to be concerned about your amp's bias setting, but given that you've observed the power tubes, and they don't look like they're about to imitate Chernobyl, then you can safely play as much as you want. The tone might be a bit sterile, that would mean that the tubes are biased too much, but there's no damage going on. Going the other way, a bit too little bias will probably sound OK, or even quite good, but the tube life will be a bit shortened... fortunately not drastically so. The actual bias voltage isn't an exact number, there is a range of acceptable values that are all pretty much "on the money".

The procedure for actually setting the bias voltage differs among the various types of circuitry. We need that diagram before going much further with this, sorry to say.

HTH



sumgai

[cynical1]

For what it's worth, I found the Bugera Valve Bias Instructions on-line.

Happy Trails

Cynical One

[ChrisK]

...they're still new enough that they may value customer support.....

;D ;D ;D

So true.

[ChrisK]

......on-line.

Cheater! You got on the google and used the internets.....

[cynical1]

Apr 14, 2009 16:19:38 GMT -5 ChrisK said:

......You got on the google and used the internets.....

Yeah, once you realize it's not just for porn you find out it can be quite useful...

[ChrisK]

Really? You can get porn over the google?

Wow, can you get guitar porn?



Note: Postings have been somewhat lean of late. Apparently some members have succumbed to the will of others and have been distracted into doing what they are presumed to know that they are supposed to be doing in the Spring. Perhaps this will help return the lost to things of true import.

[newey]

Apparently some members have succumbed to the will of others and have been distracted into doing what they are presumed to know that they are supposed to be doing in the Spring.

Ahh! Spring! When a young man's thoughts turn to love, and the old men think of guitar porn!

I'd post some here, but that would be seriously hijacking a serious thread asking about Bugera biasing.

Or maybe the "will of others" refers to the list of landscaping chores which the distaff side has lately demanded hereabout.

EDIT:



"A tone technician tests every Bugera amp" reads the caption on this photo from their website.

This was the closest I could get to guitar porn and still stay on topic.

[madmax]

i downloaded the manual and the bias instructions they are lacking. I can take pics of the bias controls and post as soon as i figure out how but for now it is a single bias adjustment for four tubes using what i think is an rca jack, this jack is labeled "test jack". there is a switch to swap between 6l6's and el34's, it came with 6l6's. since there is one bias adjustment that means that it is for all four tubes at once which means i absolutely must buy matched sets of four. which also means that individual tube readings would have to be multiplied by for to set it correctly. im just guessing here please correct me. I should be able to read the rca jack with a volt meter. red lead inside the rca jack and black to ground or the outer sleeve of the rca jack. i email bugera and got no responce its been two days.

[sumgai]

mm,

No, don't freak out, you don't need to use matched output tubes, although it'll sound better if you do. In fact, I've never seen a run-of-the-mill tube amp with separate bias adjustments for each tube, whether they be two, four or more in number. One pot usually suffices for all, and for a good reason. Also, you might encounter a balance pot once in awhile. That's closer to what you envisioned, but in most cases, it's not really helpful at all, and can be damaging or destructive.

As for those "instructions", yeah, they're not much help when you don't know what the hell grade of tube you're setting up. I suppose it could'be been worse, but that's really giving them room to stretch for it.......

Long story short, I'm now gonna say, this is a classic fixed bias setup, not a cathode bias scheme. I say that because you can adjust the bias, something that you can't (ordinarily) do with a cathode bias rig.

So, to do this correctly, you need an oscilloscope, a frequency generator, a dummy load that can handle at least 100 watts, and a digital multimeter. Lacking that, you can take a WAG, like I would. For 6L6's try -48 volts DC. For EL34's, try -38 to -40 volts DC.

Bugera's instructions call for "listening to the tone to see if it's alright"..... yeah, and I should read the National Enquirer to see if Obama's doing alright, eh? Jeebus deliver me from idiot manual writers!! Do this instead:

1) Watch the kerbleepin tubes! Crank it, play like there's no tomorrow, and have a friend closely watch the tubes. Or have him/her play, and you watch 'em. Make sure there's nothing glowing red in there (except the filaments, and they're small).

2) Listen closely to the tone, at normal volumes. If you hear distortion when you expect ultra-clean, then the tubes need more bias (more negative voltage). If it's sterile, and possibly underpowered, then the tubes need less bias (less negative voltage). Move in steps of two volts or less, and re-test.

But after every change, re-check the tubes first!! Failure to do so will ensure a visit from your bestest bud, Mr. Murphy!

Now you're in the ball park. If you wanna get it spot on, procure the above list of instruments, and report back here for more training.

HTH



sumgai

[D2o]

Huh! very interesting, sumgai.

Lemme aks ya this, summy ol' pal:

There are lots of electronic surplus stores here, including some good ones that also carry tubes ... only they just sort of "have what they have" and there is no means of determining if any of the tubes are matched (at least no means of doing so in the store).

So, does it matter if they are matched?

Can a guy just buy as many tubes as he needs (assuming the tubes are all of the same type, and made by the same manufacturer) and just pop them in without any concerns?

... or is that where this comes in:

But after every change, re-check the tubes first!! Failure to do so will ensure a visit from your bestest bud, Mr. Murphy!

... what does "re-check" mean, exactly?

D2o

P.S. You get the feeling I'm going to take this off on a tangent? ... I'll try not to!

[madmax]

so is there any way to harm the amp by setting the the bias knob to high or low. and running the amp with fewer than four tubes is harmful. i bought a matched quad set of groove tubes gt-6l6-c they sound eh not so good. is there some kind of break in period for tubes. I read the stock tubes are relabled sovtek tubes they sound much better. i put a volt meter to the test jack and played with the bias knob voltage runs from 50 to about 60 no more no less. that really throws there intructions off for setting bias off since it cant even reach most of the the voltages it asks for. sorry im still a little confused because bugera contacted me and pointed me to behringer service and warranty sight, why not just tell me it was a fixed bias setup. i am waiting for a responce from behringer hope fully they will confirm the setup. I will try and see if i can get it to sound ok by using your directions an i greatly apppreciate the help. ill let you guys know how it goes.

[sumgai]

max,

It's late, and while that doesn't usually stop me, tomorrow's a busy one, so it's off to The Land Of Nod I go......

You've asked a lot of questions, and the short answer is the most important one: no, you're not in danger of damaging either your tubes or the amp. Too much negative voltage will cause anemic volume and tone, which you've described. If negative 50 volts is the closest you can go towards zero volts, then the amp needs work. If you feel qualified, then we'll discuss it. If electronics technicianing isn't what you do for a living, then you'd better like adventure really well, or prepare to seek out a tech with a good reputation. ;D

Manyana.



sumgai