Are bias resistors good enough?

[justonwo]

I just built an amp kit that came with 1 ohm resistors to bridge pins 1 and 8 on all four output tubes. The precision band, which is gold, indicates that it's within 5% of the nominal spec.

This means if you read 36 mV on your DMM, the actual current could be anywhere between 34 and 38 mA.

Good enough? Maybe. I'd opt for lower tolerance resistors if I was really concerned about nailing the bias every time.

[bob-i]

I always use 1% resistor, but in reality at 5% you'd only be off by 1-2ma. That's fine.

[Chrome Dinette]

Check the resistors with an accurate meter to see if they are actually 1 ohm.

[justonwo]

Yeah, I don't think my Fluke is all that accurate at low resistance. For that, I think you need a wheatstone bridge.

[donnyjaguar]

This is when a bench-top current-limited supply comes in handy. You dial in 0.1A and then measure the voltage drop. Measuring less than 1 Ohm with a multimeter isn't going to be very accurate. If you do the above you can hand-pick a set of matched resistors. :)

[Blue Strat]

Tubes couldn't care less about being off by a few percent. If you calculate and set for 60% dissipation you'll always be fine.

[Old Tele man]

+1 on Blue Strat's comments...especially when you consider that the CATHODE current (Ik = Ip+Is) also includes the SCREEN current (Is), which is typically about 5% of plate current (Ip) at idle for beam tetrodes (~10% for pentodes).

...hence, the 70% idle equation is already an approximation of an approximation!

[Ronsonic]

Those resistors are fine. Far too much is made of needless precision.

"Electronics is an exact science of 20% tolerances."

Ron

[donnyjaguar]

I tend to agree that "precision tube technology" is an oxymoronism. However, probably given my power electronics background or perhaps due to my initial experiences in tube amplification in the hi-fi realm, I like to have perfectly balanced current through any transformer not intended for class A/single-ended operation. I guess the benefits are maximum power, even hysterisis curve and no danger of magnetising. MI amplification, however, does have different objectives - but old habits die hard! :)

[Old Tele man]

...even back in the GOOD-OLD-DAYS (NOS), vacuum tubes were only 20% tolerance (unless specially "bogie" selected) devices!

...today, they're not much better (aggregate) than 50% tolerance...which is why "matching" is needed (to weed-out the "bad" ones).

[Chris Scott]

Yeah, I don't think my Fluke is all that accurate at low resistance. For that, I think you need a wheatstone bridge.

....and I'd like to indicate that your needle is, er, FLOATING!

[Chris Scott]

Those resistors are fine. Far too much is made of needless precision.

"Electronics is an exact science of 20% tolerances."

Ron

Now THAT is a quote I'm gonna have to remember!!!!

I'm CONSTANTLY amazed at how much has been written in the last 10-15 years on the hallowed subject of BIASING when it refers to a guitar amplifier- what IS it about that stuff?

Maybe it's the word "BIAS"

".....neenerneener neenerneener..... don't touch the vertical, don't touch the horizontal, DON'T PLAY THAT AMP UNLESS IT'S BIASED "CORRECTLY"..........

It's like it has some kind of special, unseen force that just makes people get all silly and stupid.

Or maybe it's just me.

Yup- it's gotta be me......all those other people couldn't be THAT crazy:horse

[hasserl]

Yup, it has to be you. JK! ;)

I think it's because so many people are getting into using tube amps that never had them before, and they're just learning about the subject. It's all new to them. Some people repeat information they've heard that isn't correct, and then that has to be corrected, which means more writing on the subject.

Maybe all the companies that under bias their amps are partially at fault. It would be nice if people didn't have to worry about it at all. But once someone gets their bias probe and uses it for the first time and warms up the bias on their mass produced pcb amp that was running at about 20% of max, and they bump it up to around 60 to 70% and they hear the difference, they justifiably get excited, and then they get on an internet bbs and tell everyone what they found, which starts all the more threads on the topic.

It's a topic that has been done to death, that's for sure, but a lot of people have learned how to do this the last five years, and a lot of people I'm sure have screwed something up trying, but a whole lot more of them have learned to do it right, with no problems, and are the better for it.

[justonwo]

Now THAT is a quote I'm gonna have to remember!!!!

I'm CONSTANTLY amazed at how much has been written in the last 10-15 years on the hallowed subject of BIASING when it refers to a guitar amplifier- what IS it about that stuff?

Maybe it's the word "BIAS"

".....neenerneener neenerneener..... don't touch the vertical, don't touch the horizontal, DON'T PLAY THAT AMP UNLESS IT'S BIASED "CORRECTLY"..........

It's like it has some kind of special, unseen force that just makes people get all silly and stupid.

Or maybe it's just me.

Yup- it's gotta be me......all those other people couldn't be THAT crazy:horse

When I first got into tube amps 5 or 6 years ago, one of the first things I was told before swapping power tubes was to make sure the bias is properly adjusted. I've done quite a bit of reading on the subject since then, and maybe much of that reading is simply regurgitation (in fact, I'm sure it must be), but getting the correct answers even from published texts can often be a challenge. I've even built two of my own amps since then.

Electronics may involve 20% tolerances in some cases, but in some cases it requires very tight tolerances. If that weren't true, you wouldn't see electronic components with such tight tolerances (much lower than 1% in many cases).

While I appreciate your addition to the thread, which, as I dig beneath the layers of sarcasm seems, to imply that biasing isn't as important as it's made out to be, perhaps you could deliver the message without looking down your nose at the people asking the question.

I'm sure all of us have beaten the proverbial dead horse on at least one topic. :-)

I have to say, however, I don't understand what you mean by a floating needle. DMMs don't have the precision to measure very low resistances. Not sure what "floating" anything has to do with it.