previous: lion Sorry gentlemen – I think you misse... -- 2/3/2000 12:18 PM

Re: Driver tube mandatory w/p.tube change?

2/4/2000 3:11 AM

SteveR

Re: Driver tube mandatory w/p.tube change? Lion,     I think I'm the same boat as you where it comes to acquiring knowledge of this stuff. I'm not as knowledgable or experienced as a lot of these guys, but I do know what gets said here.     A resistor over time will drift up and become more resistive. It is not "defective" in that it hasn't completely failed, but it does not function in a way you would expect it to if you went by the color marking on the side, and it no longer functions properly in the circuit for which it was originally installed. You can determine the real value of the resisor by measuring it with an ohmmeter.     What I think you are asking, if we switch tubes for resistors, is how to determine the "real" value of a tube which has gone "bad" or worn out but hasn't completely failed.     I think by their lack of directly answering your question R.G. and Greywater are essentially saying, "No, there is no way to quantify the 'state' of a tube without using a device specifically designed to measure what tubes do--control the flow of current." The measurement of how well a tube can do this is transconductance.     You can't get a meaningful measurement of a capacitor with an ohmmeter, other than, "It's shorted, or it doesn't charge at all." And just knowing the transconductance doesn't necessarily tell you the whole story about the tube.     Visually, there are ways to tell if a tube has failed completely. You can look for broken, slagged, or shorted grid wires. Scorch marks are a pretty good indicator too, but if a tube has failed you probably know before you've even taken it out of the amp. Hopefully you don't have a bunch of failed tubes lying around mingling with good ones.   There really isn't any way to tell visually if a tube has given up the ghost from wear.     The only other measurable value I know of is interelectrode capacitance. But frankly I don't know how to measure this, and couldn't tell you how to interpret the condition of a tube based on this data. For one, you'd need a meter that measured capacitance which, unless you are a tech, is probably as obscure an item as a tube tester. And the other thing is I think the values published for a tube are under operating conditions. You would need either need to measure the values with the tube in the amp like you were checking voltages, or again, have a tube tester that can measure this.     I think the advice of performing an empirical comparision of the tube with a known good tube will provide the best balance between convenience and accuracy.     It would be really cool though if manufacturers could put little tester strips on their tubes like Energizers and Duracells, or if they had little counters that ticked off their hours of use or maybe even watthours.     Regarding your point about tubes sounding different depending on what amp they are in, well, this is why people who do matching give a rating to their tubes. You should be able to replace your tubes with new ones of the same rating and get more or less the same tone and not have to rebias. Also, it's possible that the guitarists who retubed and lost their tone either didn't re-adjust their bias, or they had techs set their bias properly when, before retubing, their bias was way off. And I'm not even going to get into the debate about the sound of one manufacturer vs. another. Please refer to the taste tests.     I know this wasn't the answer you were looking for, but I hope this helps.     Take care,     SteveR