Bugera 6262 help!!

[mattbreaker]

Hello, I've already posted a question on here and no one answered so I'm trying again.

Got a cheap bugera 6262 which is all valve, power section is 4x6L6's

I have bought a matched set of EL34's to replace which according to Bugera is totally fine, the amp has a bias switch (from el's to 6L6's) and it as a Bias adjust pot.

I can't get any help from bugera and behringer are useless. I want to know what the base current should be to set the new bias to with EL34's in circuit.

If anyone knows or can advise where else to look it woul dbe a big help for me.

Thanks folks.

Matt.

[MWJB]

Well if the amp runs 6L6 it probably doesn't run more than 520vdc max on the plates so 30mA looks safe enough.

[Steve Conner]

Switch the bias to EL34s, pop the new tubes in, and see if the tubes seem happy. If you run them too cold, they sound bad. Too hot, and you can feel waves of heat rising from them, smell funny smells, and in extreme cases, see the grey metal plate inside glowing a dull red. If you don't notice any of these problems the bias is probably OK. If not, mess with the bias pot until they go away.

The recommended current for EL34s is around 30mA per tube vs. 42 for 6L6s. But does the 6262 even have jacks for a meter to check the bias current, and do you have a meter and know how to use it? If there were no jacks, do you know how to open the amp and read the bias that way?

[MWJB]

It has a switch to switch between tube types, plus an external bias pot & meter pin jack...it just doesn't give any details as to how to utilise them...or whether the meter point is measuring one, two or 4 tubes...bit of an educated guess need here.

[mattbreaker]

yeah thats the problem I've had,

The amp does have a jack plug for the bias adjust and yes I've got the tools and collegues to help me out, they wouldn't touch it unless they knew what the base current was as neither the manual or the websites have been of any use at all.

When I had my triple xxx I switched to EL34's and the rating for those were 30mA so I guess I'll go with that.

Thanks for your input guys much appreciated.

Cheers,
Matt.


www.myspace.com/breakerband

[stokes]

EL34's should dissipate about 15-16 watts at idle.

[Enzo]

Amps are amps, a set of 6L6s operating at 500v will want the same current regardless of the amp its in if it is a conventional push pull AB amp like most are. There is nothing special about this amp circuit. Quite conventional. Any tech should have zero problem setting the bias.

If there is ONE test point for bias, it will either be for the supply itself or for all tubes. They won't bring out a test point for only one tube in a set of four. A volt meter into teh test point will quickly determine its purpose. If you get -40v it is the supply, the way PV did it. If you get less than a volt, it is a current monitor.

[PaulyWally]

I've attached Bugera's valve bias instructions for those that have asked.

I also have a question: I'm installing new Electro-Harmonix EL34 valves with a measured plate voltage of 24. Not sure what bias voltage I should be adjusting for since the attached instructions relate bias voltage to (Behringer) valve "grade".

[Enzo]

A measured plate voltage of 24? SOmething is very wrong with that statement. Power tubes in these amps run on about 500 volts, not 24. If you measured 24 volts there, either the amp doesn;t work, or your meter doesn;t or you are not using the meter correctly.

If your tube has the number 24 printed on it somewhere, that is not a voltage. Tubes have no voltage of their own, they only see the voltage of a circuit they are installed in.

If you measured voltage at the test point, that is not plate voltage, that is bias voltage.

As to their grade system, they just made it up, similar to that used by Groove Tube. Set it to 48 for the 6L6 and 42 for EL34 and see how it runs. If the tubes start to get red hot, increase the setting a volt or so.

[PaulyWally]

.... Ack

Meant to type "plate current" not plate voltage.

Sorry.

[Enzo]

Well if you are measuring plate current, then just twiddle the bias adjust to get the current level you desire. For it to mean anything you also do need to know the plate voltage. Plate volts time plate current yields plate dissipation, which is the target. 17 or 18 watts being a common goal for EL34.

[PaulyWally]

Unfortunately I'm not measuring plate current. What I was trying to say is that the tubes I bought were matched quads and they came with a marked plate current. As for this amp, I can measure (via an RCA plug) and adjust (via a pot) the bias voltage. What I don't know is what target I should be adjusting for. I'm currently measuring 43.1 vdc

[Enzo]

OK, matching numbers are ONLY good for comparing tubes set to set form one seller. That current rating is how much current the tubes drew UNDER THE TEST CONDITIONS AT THE TUBE SELLER. Whatever those conditions might have been, when the four tubes all operate the same, they are considered matched, and they will likely operate as a set in your amp. But the number on the tube has nothing to do with your amp.

We don't know how much voltage their tester applied, what the load resistance was, or what any other parameter might have been. So the numbers are ONLY for matching.

If you don;t have the means to check current, then we are back to the previous recommendation. I suggested 48v for 6L6 and 42v for EL34. If the amp sounds OK on 43v then leave it, my 42 was just a mid-range average number.

Bias is nowhere near as critical as many people think.

[PaulyWally]

Enzo - thanks for all your help. As for you comment wrt the importance of biasing. My opinion is that it "can" be important since running hot can severely damage an amp. Running cool will simply sound bad.

[Steve Conner]

The plate current printed on those tubes is simply the current that they passed in the test rig used for matching them. The matching guy tests them all one after the other, writes the numbers on them, and then sorts them into sets with the same numbers. So the label is really for his convenience, not yours, and therefore it doesn't really have any bearing on what current you should run them at in your own amp. I have a pair of 6550s from Hotrox that came with "97" written on them in magic marker, but I don't run them at 97mA, in fact nearer 50-60.

The Bugera amp under discussion doesn't have any means of measuring the bias current, which is presumably what you're wanting to know. The jack on the back measures the voltage applied to the grids. To measure the actual current caused by that grid voltage, which is also influenced by the screen voltage, B+, and so on, you have to buy or make some bias probe adaptors, or bust the amp open.

Running hot severely damages an amp by burning and cooking components. You can smell and feel that without any sort of meter. With EL34s, any bias voltage colder than 40V will probably not cook them. I say colder because the voltage is negative, and if I said "greater", well is -41 greater or less than -40?

[PaulyWally]

Originally Posted by Steve Conner

The plate current printed on those tubes is simply the current that they passed in the test rig used for matching them. The matching guy tests them all one after the other, writes the numbers on them, and then sorts them into sets with the same numbers. So the label is really for his convenience, not yours, and therefore it doesn't really have any bearing on what current you should run them at in your own amp. I have a pair of 6550s from Hotrox that came with "97" written on them in magic marker, but I don't run them at 97mA, in fact nearer 50-60.

The Bugera amp under discussion doesn't have any means of measuring the bias current, which is presumably what you're wanting to know. The jack on the back measures the voltage applied to the grids. To measure the actual current caused by that grid voltage, which is also influenced by the screen voltage, B+, and so on, you have to buy or make some bias probe adaptors, or bust the amp open.

Running hot severely damages an amp by burning and cooking components. You can smell and feel that without any sort of meter. With EL34s, any bias voltage colder than 40V will probably not cook them. I say colder because the voltage is negative, and if I said "greater", well is -41 greater or less than -40?

Steve - I'm actually not trying to measure the plate current. I'm simply wondering what bias voltage I should be "looking" for? As for the label, I think it also somewhat for my convenience since I will request replacements with the same plate current measurement if I order from the same supplier.

[Steve Conner]

The bias voltage/current is a matter of personal preference, there is no exact answer. If you play blues, you'll like a hotter bias than if you played thrash metal with a distortion box in front of the thing.

[Enzo]

Pauly, bias that is way out of whack can burn up tubes, sure, but my point is that running the tubes at exactly 38.42ma, or some such is silly. Think of the air pressure in your car tires. SOme pressure like 35 pounds might be "optimal," but if they are sitting at 34 pounds, the car will not explode. In fact the pressure could be 37 on one tire, 34 in another, and 32 on the left rear. The car will still drive mom down to the store and back just fine.

There are bias levels that are way too hot and way too cold, but there is no "right here" correct spot, there is only a wide range of prefectly safe, reliable and acceptable bias levels.

The bias voltage is not a reliable measure of tube function. Get ten sets of tubes, set the bias to one voltage, and try each set of tubes. Each set will draw a different amount of current. And it is current, not bias voltage, that determines the tubes' dissipation. Dissipation is what determines whether a tube is going to overheat and melt down, not voltage.

Frankly, you can set it by ear pretty much. people holler bias so much, the marketing people at the amp makers throw it in. These test points are not some critical adjustment, they are a marketing tool. Set it in the middle range as mentioned earlier - it will be fine. The only way the realy know if the setting makes sense is to use a current measuring method.