Bugera 333XL Biasing Instructions

[PepeSilvia]

Hey guys, does anyone know how to bias a Bugera 333XL? It seems like it should be pretty straight forward, it's got the RCA-type jack and bias control pot on it, I'm just not sure what it should be set to.

[Chuck H]

I don't know that amp so I can't say as to how the RCA jack is related to the bias circuit. I can say that if you measure the plate voltage (which I also don't know) and read a tube chart for whatever type of tubes it uses (which I also don't know) an approximation can be made on how much current the amp should draw at idle. There's nothing more to it than that. The best bias setting isn't amp specific, it's tube type specific (and even tube specific to a lesser degree). If you can find your way around the circuit you should be able to set the bias well enough if armed with the plate voltage and the tube type.

Chuck

[Enzo]

Their test point is like the Peavey - it shows the bias VOLTAGE. Hey, I didn't design it...

Bugera has a chart of voltage settings for the various sortings of their brand tubes. The #1 tubes get X volts, the #2 tubes get Y volts, etc. Not very exacting. And not very useful if you don;t buy tubes from them. You can download the chart from the Bugera web site.

One could also buy a bias probe and do it the more conventional way. I have no idea how much range the bias control actually has.

[Bruce / Mission Amps]

It sucks... I just did one and gave up with Bugera's nonsense.
Knowing the bias voltage is totally worthless since I'm not buying prelabeled tubes from Bugera and following a stupid ass chart.
Use the OT current shunt method.
Pull the chassis and find where the red lead center tap of the OT is soldered to the underside of the PC board and put the + lead of your amp metering device there. Follow the blue and brown OT plate lead wires over to the underside of PC board and use the black lead there.
Or if you know how to use your DVM in high voltage, poke your test lead into the solder on the PCB octal socket pattern for the power tubes and read the highest voltage, that is the plate, lug 3.
I actually soldered a larger blob to the PCB so I could use clip leads for this and it was very fast to change the idle current to around 32ma-36ma with a matched pair of tubes.
The bias pot can swing the idle current of an average pair of power tubes pretty well.
If I had another Bugera I wouldn't waste anytime... just whip out the chassis and set it up to do it OT current shunt method... takes about 20 mins total.

[Muskratt]

Hey there, I was just following through on the old post, and just bought a Digital Meter so i can read the actual numbers.. I had tried my regular meter with a needle and it show apprx 39/40 v, so I didn't even touch the biasing on it. I have contacted my dealer and they refuse to do anything with it but plug it up and say ohh.. it sounds real good to me.. But I asked do you have anything to measure the BIAS here to check it? His reply was "we have to send it to our XXXXXXX Store..
So I have a 4 year warranty on it other then tubes, so i was just under the belt when I contacted Bugera about it. They currently said they are waiting for a new shipment of tubes to arrive near the first of the month. Well That was a few weeks ago.
I not saying the support is bad cause they wanted me to send it in, but I think it was really dumb to send the amp in just for tubes..
Now if there BIASing table in there PDF is right then I could adjust that voltage easy. I am sure the true way is to take the reading from the voltage plate. I hope to just buy a BIAS Probe and be done with the hmmm.. I think its right... and just get the plate vvolts and dial it in.. I would like to recheck them to be sure it is performing all the way.

Cheers!

[Chuck H]

Bias is not plate voltage. And bias is not bias voltage. Bias, as it applies here, is how many watts a tube is dissapating at idle. To know this you do need to know the plate voltage, but also the current going through the tube. The voltage at the grid adjusts the current through the tube. The current multiplied by the voltage is how many watts the tube is dissapating. Different amps require different current levels (and therefor different bias voltage settings) because they use different tubes at different plate voltages. Even knowing the plate voltage and the grid voltage will not tell you how many watts a tube is dissapating. The only way to know that is by measuring current. If you use Bugera pre selected tubes and set the grid voltage as per instruction your amp will be biased correctly by Bugera's standards. If you do not use Bugera pre selected tubes any Bugera bias voltage suggestions are useless because you won't know how a given bias voltage makes that particular tube behave with respect to current. You can use tubes other than Bugera pre selected tubes, but in order to bias them correctly you need to be able to measure the current going through the tubes.

[Enzo]

And since it shares so much with the Peavey amps, you can also just set the thing for about -45v for EL34s and about -55v for 6L6s and have reasonable results. PV suggests about -42.5v for EL34s, but either way, all those settings are cool enough that most tubes will likely be well within their comfort zones.

I just did that to a JSX to see, and it put the EL34 JJs at about 15 watts. So that is, what, about 60% instead of the vaunted 70%. Your tone is largely from the preamp on those amps.

[Muskratt]

Hwy, Thanks guys, I did forget to mention that I used a digital meter picked up today and I was at cold -46.1v, at went up to -46.4 then dropped after 10+ mins -46.2v I think they set it to hot from the factory and the BIAS know wasn't sensitive as some folks reported. I tapped it to see if it would change but it never did. The adjuster seems to have pretty good resistence so it don't move. I dropped it to -43.1v to be safe, though I believe its a bit late for that.

So fustrating. And yes I realize I need to get the real bias probe to do the job right.. I plan on getting one if I can ever afford the new set of tubes.

Cheers!

[Enzo]

Too hot from factory??? Who knows what the mains voltage was when they adjusted it. You need not worry if the voltages drift around a few tenths of a volt, your mains voltage is doing that same thing. Every two volts your mains voltage varies means about 1 volt of bias voltage variation. And your B+ is roughly 4/1, meaning for every volt your mains changes, your B+ changes 4 volts. SO voltages and currents in amplifiers changes all the time. In fact I would be surprised if some parameter in my amps measured the exact same thing all the time.

I dropped it to -43.1v to be safe

Not sure what you mean here. Dropping the bias voltage, in other words adjusting the bias from -46 to -43, is making the amp run hotter. SO not sure how hotter is safer in any way.

[Muskratt]

Now this is confusing me, I mean I have it set to the hot side of the -V as per Bias instructions.
It is EL34'S

[Chuck H]

Less bias voltage makes more current through the tube. So less negative bias voltage is hotter, and more negative bias voltage is cooler.

Now this is confusing me, I mean I have it set to the hot side of the -V as per Bias instructions.
[ATTACH=CONFIG]16017[/ATTACH] It is EL34'S

I'm not sure what you mean. Are you saying you are using the el34 side? And what voltage did you choose? and why?

There is no bias voltage that is correct by itself. A tubes specific behavior determines how much bias voltage you need. Since different tubes react differently, Bugera pre grades tubes so that Bugera owners can set the bias by setting the bias voltage.

The really important point here is that diferent tubes, even of the same brand and type, behave differently. So unless your using pre selected Bugera tubes the voltage suggestions on that chart mean nothing.

The first question I ever asked on this forum was "What is the correct bias voltage for a Marshall JMP MKII if I want to use EL34's". That was a very long time ago. I remember having to wrap my head around the idea and being unsure of my measurements. In the end I did go with what was said to be a "safe" bias voltage. This is what Enzo is suggesting. It worked fine for me then and it'll work fine for you now. As long as your bias condition isn't grossley bad your amp will be fine.