How hot do you Bias your Amps??

[Dale B]

I used to do 70% back in the day, but it's mostly 55% to 60% now. It also depends on the amp and power tubes.

The Soldano SLOs I had were good at 50%.

 

[Smash]

This thread makes me feel better about my laziness. But more than that, some great information is dumping.

So colder bias has more punch. I would assume colder would also be more tight because you arent saturating to the point of sag even as you crank the MV (which is better for most of us metalheads). Would this be accurate?

 

[7704A]

Generally I try to keep my stuff between about 33%-66%, but I haven't done a lot of biasing. The last one I did was for a friend, he didn't care how it was biased to so long as it worked, so I just stuck it at 50%. He seemed to like it.

 

[Racerxrated]

Andy has been into that for quite a while. In the 90’s, when I was using him as an amp tech, he would state his preference for around 25mA and I can’t disagree. One time with a JMP 2203, I experimented with the limits of bias with how it actually sounds and with the B+ on that particular head (over 450 but I don’t remember exactly), I didn’t hear a major difference between 18mA and 38mA or so. So 25mA seemed very reasonable with enough room for low stage voltage at a gig but also in normal voltages, giving great tube life.

With how modern tubes are now I would definitely err on colder. Shit, my Stiletto set to Tube Rec, spongy and on a dirty setting gave me readings of 13mA. Set it to clean, Diode and bold it would be at nearly 30mA. I was bored one time and got the adjustable bias mod put in and literally have not taken one reading since I got it done and that was probably 7-8 years ago…lol
I’ve gotten so lazy about amps in the last 10 years, I figured he set it after the mod to a good setting and I won’t worry about it as long as it sounds normal.
Got a ‘75 Marshall 5 years ago or so and have never taken a reading once. It sounded good when I bought it and I figured why worry about it. If I smell something bad, I’ll worry about it. Sheer laziness.

18ma in a JMP? Ugh, that was the number a 79 2203 I once had came to me with. Jumped that up to 34 or so, WAAAY better. Mesas can get away with unusually low bias, but in my exp a Marshall will sound terrible. Or, unusually high as well.

 

[FourT6and2]

I'll probably give the more tech-y crowd conniptions by saying this, and I'll admit 5 years ago I would've hated this too, but
I just stick a matched set of tubes in and I don't touch it again until a tube pops or something is obviously wrong.

I used to meticulously measure each bias, and I also tried the "bias to tone" thing where I'd play, tweak, play, tweak, etc and it's all such a chore.

I've yet to have any problems just sticking any set of tubes in a given amp, and I have a lot of amps. The difference between amp circuits is far far more than any little bias tweak would be, but if I only had one or two amps I'd probably be a lot more concerned with squeezing the very best out of it. I barely have time to play at all lately so when I do, I want to fuss less and play more. Same reason why I stopped putting together project guitars.

You've just been lucky. But if it works for yah, go for it lol...

If the new set of tubes are rated within ballpark of the outgoing set, no need to re-bias. But if the new tubes are vastly different in rating, then who knows what will happen. Either bias will be too cold (totally safe, just tonal difference), or too hot (to the point where you get red platting and a tube pops, maybe taking out other components). You mention you don't replace a tube until one pops. That's telling. I have had only two tubes "pop" in the last 30 years. So the fact that you even have tubes "pop" means you're doing something wrong.

I had a Hiwatt (which are notorious for bias problems) that a new set of tubes wouldn't actually bias up within range because they were rated too differently than the outgoing ones. To the point where they were operating at well over 100%. If I hadn't caught it, they would have "popped" and taken out the OT along with 'em.

 

[FourT6and2]

I'd also like to point out that most people don't actually know what their amp is biased to because most people don't actually take the drop across the screen resistor into account. So y'all think you're biased to 70%, but it's really 65% or 60%. Which is fine, because it bakes in a margin of error.

 

[rallyfingers]

I go by plate voltage 450-460vdc on plates generally.

 

[stephen sawall]

Mine are all over the place. 19 - 52 ma depending on the amp.

 

[TotallyRadGuitars]

You mention you don't replace a tube until one pops. That's telling. I have had only two tubes "pop" in the last 30 years. So the fact that you even have tubes "pop" means you're doing something wrong.

I've only had a single power tube pop since I started playing in 2008, and it was on my TSL which I meticulously biased to 65%. So I'm on track for your twice in 30 years if I keep going like I'm going. Plus it's a TSL so it's more likely the amp's conductive PCB causing bias float that caused the problem than the tube/bias itself.

Like I said, if I put a set of tubes in and they start red plating, I know something's up and would take the time to bias. Whenever I swap tubes I keep a close watch on them for a while until I feel confident things have "settled in." But so far my success rate of 99.9% makes me feel like the time I spent in the past trying to nail the perfect bias was relatively wasted. Or maybe my amps just all happen to be biased conservatively/cold, but I have a pretty big sample size so I'd expect at least some of them to have problems.

Anyway, I would've had the same reaction you had just a few years ago, so I get where you're coming from.

 

[FourT6and2]

I've only had a single power tube pop since I started playing in 2008, and it was on my TSL which I meticulously biased to 65%. So I'm on track for your twice in 30 years if I keep going like I'm going. Plus it's a TSL so it's more likely the amp's conductive PCB causing bias float that caused the problem than the tube/bias itself.

Like I said, if I put a set of tubes in and they start red plating, I know something's up and would take the time to bias. Whenever I swap tubes I keep a close watch on them for a while until I feel confident things have "settled in." But so far my success rate of 99.9% makes me feel like the time I spent in the past trying to nail the perfect bias was relatively wasted. Or maybe my amps just all happen to be biased conservatively/cold, but I have a pretty big sample size so I'd expect at least some of them to have problems.

Anyway, I would've had the same reaction you had just a few years ago, so I get where you're coming from.

An amp's safe bias is a fairly large range, so I'm not surprised. But the real issue isn't with the amp, it's with the tubes and their rating. If you don't specify what ratings you want when you purchase from a vendor, there's no telling what will show up. It's just luck of the draw at that point. And it only takes 5 minutes to bias an amp, so I don't see that as a big waste of time.

 

[311splawndude]

^ For sure. That and amp brands and tube brands can vary as well. Apply what you know to what you are working on.

And if you don't know your plate voltage, biasing to a number doesn't really work.

I highly recommend to anyone who wants to learn how or DIY to get an above average probe that has a built in meter, can check plate voltage as well, and has 2-4 octal connectors. If I was smart, I would do that but I still do it the 'hard' way

When I got started I was scared to death to bias my own amp. I did some research and was not to fond of all the tutorials out there (especially Eurotubes) so I made my own I then got it vetted by a few amp techies.

https://splawnfans.proboards.com/thread/7786/amp-biasing-tutorial

tl/dr

1.) Buy matched power tubes
2.) Unscrew chassis from amp and remove it
3.) DO NOT touch the insides of the amp (potential shock)
3.) Rest chassis upside down on something safe, or against the wall. KEEP SPEAKER CAB plugged in.
4.) Install bias probe, then multimeter, then turn on. Read milliAmps. Never detach probe or MM while amp is on.
5.) Turn tiny brass screw (not white plastic one) to the left counter-clock wise is colder
right clock wise is hotter

Use one hand, preferably with a rubber glove, stick your other hand in your pocket, where tennis shoes, use rubber/plastic handled screwdriver.

Nothing embarrassing about that. I ain't an electronics tech so that shit is for someone with the know how. I just play gutiar and sing into the microphone. Entertaining is what my bias is. If someone here doesn't like that well stick your ohmeter where the sun don't shine!!!

Respect that.

But we all have different reasons for messing with amplifiers. You like to gig and sing. I just like electric guitar amplification. Probably more so then actually playing I certainly can't gig nor have any desire. So, if someone here doesn't like that well stick your mic where the sun don't shine!!!

 

[TotallyRadGuitars]

An amp's safe bias is a fairly large range, so I'm not surprised. But the real issue isn't with the amp, it's with the tubes and their rating. If you don't specify what ratings you want when you purchase from a vendor, there's no telling what will show up. It's just luck of the draw at that point. And it only takes 5 minutes to bias an amp, so I don't see that as a big waste of time.

I would guess that most vendors, when you don't specify, just send you a matched set roughly "in the middle" and that's why I haven't had an issue. I've never specifically requested hotter/cooler matching or anything like that.

As for 5 minutes to bias an amp, man I guess so, you must be a lot faster than me. I'm sure it comes with practice, or your amps have external bias pots to make things easier (or both).

 

[shar-vell Dan]

 

[GJgo]

The hottest I go is 60-65%. That said I've learned not to sweat it down to +/- 50%.

Mesas can come anywhere from 15mA to 40mA depending on which amp and what color tubes they packed in them. I have cataloged tons of Marks & Rectos with all the different colors.

 

[shar-vell Dan]

Its important to calculate the realtime wattage output of each pair instead of bias to some random number.

 

[Dan Gleesak]

My 18 watter is about 100%

 

[SpiderWars]

I bias non-MV Marshalls different than MV amps. MV amps get biased relatively cold so <65% and usually closer to 60%. Non MV Marshalls get biased close to 70% if not right at it.

My cathode-bias EL84 amps are generally set (via the resistor) to just under 100%. Whenever I set those hot (over 100%) they get a tone that’s too compressed and runs out of gas. Fenders I set ‘not too hot’ since those amps just get hotter and hotter as they run. An old Fender tilted back on those tilt back legs gets hot af.

And I always measure the voltage drop across the screen resistors and subtract that current from the Ik reading. This was mentioned above but it’s a point worth repeating. Otherwise, wtf are we even doing this for?

 

[7704A]

And I always measure the voltage drop across the screen resistors and subtract that current from the Ik reading. This was mentioned above but it’s a point worth repeating. Otherwise, wtf are we even doing this for?

Unless I'm missing something, there's nothing wrong with leaving that bit 'o current in the Ik reading besides the fact that the bias will be a hair more conservative, right?

 

[FourT6and2]

Unless I'm missing something, there's nothing wrong with leaving that bit 'o current in the Ik reading besides the fact that the bias will be a hair more conservative, right?

Correct. If you don't take it into account, and you bias to 70%, for example, you're actually closer to 65% (depending on the specific voltage/current of the amp in question). So you could just ignore the screen and use that as a bit of a buffer/safety margine.

 

[SpiderWars]

Unless I'm missing something, there's nothing wrong with leaving that bit 'o current in the Ik reading besides the fact that the bias will be a hair more conservative, right?

Right, the screen current is way less than the plate. But if we are talking about actually measuring something and then making adjustments I like to actually measure it and adjust it using the real numbers.

I get your point that it’s typically not a big deal but in the context of this thread I think it’s worth mentioning (or repeating). If I measure something I want to measure it accurately.

I also think it’s a good idea to measure screen current because that can be an indicator a tube is getting worn or is suspect. The screen current tends to creep upward as a tube wears.