Bugera 6262 help!!

[PaulyWally]

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.