Class AB1 Bias Current?

[BackwardsBoB]

I spend lots of time at the Carvin forums - carvinbbs.com and carvinmuseum.com. I give simple advice and whip up simple mods for folks to try. For amps with 4x6L6GC or 4xEL34 (plate voltage is around 450V), Carvin recommends biasing to 100mA at the standby switch on amps that have bias pots (most do now). Screen resistors, depending on vintage, are 270 or 470 Ohms, though a few early 50W models have no screen resistors.

Now, some folks are recommending bias settings (at the standby switch) up to 180mA. An article at http://www.carvinmuseum.com/pdf/Ka-Boom_v1a.pdf recommends 70% to 80% of rated plate dissipation, and it measures and subtracts out the screen current, preamp current, and phase inverter current.

I've used 65% to 75% of Pa measured on each tube with a bias meter (this provides a cushion, since the screen current is included in the measurement), and I generally install 1K screen resistors.

I went through Kevin O'Connor's TUT series, and he recommends 50% of Pa measured with a 1 or 10 Ohm cathode resistor, but he admits 33% to 66% is safe.

I understand that there's no sense turning up bias current if you can't hear a difference, and lower increases tube life.

I did a bit of searching here, because the topic seemed likely to have come up alot, and perhaps it generated heated arguments (pun intended). A 70% of Pa figure comes up a bit, and the question doesn't seem to be beaten to death.

So let me ask you:

For 6L6GCs or EL34s in a class AB1 amp with 450V plate voltage and 470 Ohm screen resistors:

1. What percentage of Pa do you try to stay under, measuring current at the cathodes, a) on your own gear, and b) on other people's gear?

2. If you did a standby switch measurement, and subtracted out a measurement with no power tubes, what percentage would you try to stay under, given that you now have to take into account the normal variation of tubes from various sources that claim to be "matched", and the fact that the set's match may have drifted over time. I always advocate the safer bias meter (nooby users, and Eurotubes will sell you a bias probe attachment for $25 now), but the standby switch is the usual method that people are using.

[Wilder Amplification]

I spend lots of time at the Carvin forums - carvinbbs.com and carvinmuseum.com. I give simple advice and whip up simple mods for folks to try. For amps with 4x6L6GC or 4xEL34 (plate voltage is around 450V), Carvin recommends biasing to 100mA at the standby switch on amps that have bias pots (most do now). Screen resistors, depending on vintage, are 270 or 470 Ohms, though a few early 50W models have no screen resistors.

Now, some folks are recommending bias settings (at the standby switch) up to 180mA. An article at http://www.carvinmuseum.com/pdf/Ka-Boom_v1a.pdf recommends 70% to 80% of rated plate dissipation, and it measures and subtracts out the screen current, preamp current, and phase inverter current.

I've used 65% to 75% of Pa measured on each tube with a bias meter (this provides a cushion, since the screen current is included in the measurement), and I generally install 1K screen resistors.

I went through Kevin O'Connor's TUT series, and he recommends 50% of Pa measured with a 1 or 10 Ohm cathode resistor, but he admits 33% to 66% is safe.

I understand that there's no sense turning up bias current if you can't hear a difference, and lower increases tube life.

I did a bit of searching here, because the topic seemed likely to have come up alot, and perhaps it generated heated arguments (pun intended). A 70% of Pa figure comes up a bit, and the question doesn't seem to be beaten to death.

So let me ask you:

For 6L6GCs or EL34s in a class AB1 amp with 450V plate voltage and 470 Ohm screen resistors:

1. What percentage of Pa do you try to stay under, measuring current at the cathodes, a) on your own gear, and b) on other people's gear?

2. If you did a standby switch measurement, and subtracted out a measurement with no power tubes, what percentage would you try to stay under, given that you now have to take into account the normal variation of tubes from various sources that claim to be "matched", and the fact that the set's match may have drifted over time. I always advocate the safer bias meter (nooby users, and Eurotubes will sell you a bias probe attachment for $25 now), but the standby switch is the usual method that people are using.

For amps in the 450-500 volt range I usually try to stay around 30-35mA between the OT primary and pin 3 on each tube.

[stokes]

I dont necessarilly go by numbers,per se.I start with about 70% and then tweak it by ear.Every tube has its "sweet spot" and if you spend enough time listening you will find it.I was taught this method back in the late '70's by a very knowledgable amp builder and have stuck with it ever since.On quite a few of my 6L6 amps I am biased up in the 80-85% range and have not fried any tubes and dont notice any severe loss in tube life.One amp I built for onew of my sons had a pair of GEC KT66's with 400 plate v's and biased at 60ma's,those tubes lasted 3 years,gigged 3-6 times a week and a couple of rehearsals thrown in,I've replaced them,but those tubes are still useable and in my spares box.A couple of my 6V6 amps are around 90%+.Mind you,I only use NOS tubes and wouldnt recommend doing this with current production tubes.

[Ronsonic]

Bypassing the s'by switch gives you current for the entire amp, not just the power tubes. This could be misleading unless you've got a specific spec for a particular amp, such as Carvin publishes.

Moving on from there, here are a few things I actually know about biasing amps.

A lot of people mistakenly believe that biasing an amp hot makes the amp sound hot. Generally, it is hifi amps that like to be biased hot. I cannot tell you how many clients ask that their amps be biased hot thinking they've got to cook the tubes to get a good tone. They are generally wrong. Often I'll sit down with them and let them play it as I adjust - it is always far cooler than the tube cooker setting.

In the world of guitar amps, there are characteristic sounds associated with different amps which are influenced by the way they are biased. As an example, Marshall used to recommend that their amps be biased on an oscilliscope, cold enough that there was a hint of crossover notch just before clipping. This sounds like a classic Marshall. Grab a copy of Humble Pie Rockin' the Fillmore if you have any doubt about what that might be.

Another comparison point, the Fender Twin Reverb was traditionally biased warmer than that, without a crossover notch and this is apparent in its sound.

Off on the other extreme, we've got the AC30. Setting aside the whole "is it really Class A" thing. They burn those poor little tubes up and it does indeed sound good.

All of these are very different amps and to compare one to another is apples and pomegranates. Optimum bias points in music reproduction are determined by the specifics of the circuit; transformer loading, driver impedance, etc. In music production they are also affected by taste and tradition and considering the guitar, FX, amp, speaker combination as a whole.

Let's try an experiment with your Carvin, or any other amp you have handy with adjustable bias. Set the amp up open, with a speaker, your current measuring device in place for insurance, we don't want to melt anything, and a guitar plugged in. Be careful here, you're going to be reaching into the amp and the strings are grounded and we don't want any cross-chest incidents.

Set the bias as cold as it gets. If you aren't sure do this by measuring DC at the grid and set for most negative. Power up, plug in the guitar, set the controls on a clean channel with the tones in a middling position, Master volume (if exists) all the way up and the volume set low but just loud enough for the amp to "clear its throat" a good clear level, maybe 2 on most amps. Turn the guitar up and strum a big open chord, a G is good. Listen to the tone and the sustain. You'll hear the amp sound a bit tinny and the sustain will die early and usually ugly, as if gated. Repeat this, strum listen to sustain and note die out, slowly bringing up the bias a little at a time. At some point you'll hear a substantial change - the tone will get fat and the sustain will snap into place and trail gracefully. This will be a pretty damn good minimum bias point.

Check and record your current measurement and if available look at what this does on the scope with tone into a dummy load. (incidentally, 1kHz is traditional for audio measurement and nearly useless for our work since that's sort of well past the notes we mostly play. The balance of guitar music is closer to 300Hz)

Continue to bring up the bias repeating the open chord test, until you reach a reasonable safe maximum for that amp. Note the change / lack of change.

Now it's power chord time. Yeah!!!!! Dial the bias back to the point you determined where the open chords got fat and sustained nicely. Wind the amp up to concert levels, again with the master up, crank the gain/volume up to the point that it's starting to break up. Now pound some low power chords and bringing the bias up, listening to how the amp responds. It wouldn't be a bad idea to try settings you know are too cold, just to get an earful of it and learn about the reaction. Again watch your current measurements we don't want to kill a set of tubes here. And note how much more current you're drawing at volume than with the amp idle. But remember it is the idle measurement that we're using. If you find a sweet spot for your power chords that is different from what you found on the open chord clean test, make a note. In fact make notes all along.

Try the amp on lead lines. Again, start with the baseline you got from the clean chord test and slowly work up.

Now sit back, review your notes and let's see what you've learned. You've just spent what should have been an enjoyable hour experimenting with and learning about your amp and you now know something. In fact you know a hell of a lot more than some rote percentage that was advised by a tube seller who wanted a handy number that would guarantee that anybody could dial it in without sucking.

Somewhere between your "clean chord minimum" and that 70% recommended max you'll find a pretty broad range of good useful tone. Pick the point in there that is the coolest that makes your ears happy and your guitar feel right when playing in anger.

[BackwardsBoB]

Thanks for the very short book. I think I really should try that. When I'm biasing, I'm all set up for it. It's much cheaper than swapping in different speakers four times (and I've been known to do that).

Here's the problem I see. Some composite guy posts "Bias at 180mA at the standby switch! It's like a different amp! Scoop the mids! \m/!" Then I get an email from a guy who bought a bias probe, and he's measuring 20mA, 21mA, 28mA and 39mA. That's about 129mA at the standby switch, and if a different guy goes up to 180mA at the standby switch with a similar tube set, the 39mA tube is going to fry.

There are guys like this poor soul...Carvin Museum :: View topic - legacy bias and standby mA . The document he's using wants plate current (the method actually splits off the screen current) at 70%-80% of Pa. That's something like 88% measured at the cathode with screen and plate power combined.

Then you get stuff like the attached photo.

I wish I could just get these guys to let Enzo take care of it, but they're in there with their multi-meters and bad advice.

My big problem is that I don't know how much power dissipation goes up when you play loud. It's hard to measure. I can simulate it, but I don't know if the simulation is accurate (the tube model probably isn't).

It is a conundrum. There's a mixed bag of users. The bedroom heroes and occaisional jazzer don't need to push things very far since the power stage never clips and they don't have grid conduction problems. The guys with the big master-volume V3s that don't live in a state where it's legal to turn the master to "10" fall in this catagory too. But there are alot of gigging musicians that find the 10spot alot.

Any more advice on how far you can push it, and how far you'd go on somebody else's amp?

[stokes]

"Any more advice on how far you can push it, and how far you'd go on somebody else's amp?"
Anything that sounds good and doesnt cause your tubes to red plate,either at idle or when played full bore.

[BackwardsBoB]

I think we're starting to reach a consensus I didn't expect...

[Steve Conner]

Ronsonic: That was a great article on biasing, thanks!

I agree with the findings, I've also heard that ugly, trashy sustain when the amp is biased too cold, and if I turn up the bias till it goes away, it seems to work fine.

30-40mA per tube in guitar amps always worked fine for me. As for how that current increases when driven: A 50 watt guitar amp should draw about 300mA at maximum undistorted output, and a 100 watt one about 600mA.

Some high-gain metal amps ran the tubes colder. I think the reasoning behind this is that it causes a kind of "expansion": the opposite of compression, with low gain for small signals and higher gain for large ones. This reduces noise on the dirty channel and seems to make the sound somewhat tighter. Of course this expansion is the very thing that makes the clean channel sound like crap in that ugly, trashy way. But if you always play with screaming, balls-out gain then it might be worth a try.

Peavey added series diodes in the dirty channel of some of their high-gain designs to produce this effect, so they could have that sound and still run the power tubes hot for a good clean tone.

One thing to bear in mind is that, when you overdrive the power tubes in almost all guitar amp circuits, they charge up their grid coupling capacitors and bias themselves colder. You'll see a crossover notch start to appear as you overdrive the power stage more and more. The extra bias dissipates with a time constant of some tenths of a second when the overdrive goes away.

I don't think there's a consensus on how this affects the tone, except that it might be cool in classic guitar amps, but bad in tube bass amps and behemoth guitar amps for modern metal. But I'm sure it helps to protect the tubes against overdissipation.

[Ronsonic]

Thanks for the very short book. I think I really should try that. When I'm biasing, I'm all set up for it. It's much cheaper than swapping in different speakers four times (and I've been known to do that).

Like I say, it is educational and for anyone geeked enough to belong inside a guitar amp, it should be fun. At least once or twice. If you look inside the Blackstone Appliances overdrive on the circuit board near the trim pots, "Quite obsessing and play your guitar!" It's good to learn a bit about this stuff, but at some point just getting it tuned up well enough and then learning to work the instrument is more important.

Here's the problem I see. Some composite guy posts "Bias at 180mA at the standby switch! It's like a different amp! Scoop the mids! \m/!" Then I get an email from a guy who bought a bias probe, and he's measuring 20mA, 21mA, 28mA and 39mA. That's about 129mA at the standby switch, and if a different guy goes up to 180mA at the standby switch with a similar tube set, the 39mA tube is going to fry.

That's exactly the problem with that sort of bias measurement. Try installing a 1R resistor between cathode and ground on each power tube. Use a 1/2W and it will behave as an effective fuse for that tube as well.

There are guys like this poor soul...Carvin Museum :: View topic - legacy bias and standby mA . The document he's using wants plate current (the method actually splits off the screen current) at 70%-80% of Pa. That's something like 88% measured at the cathode with screen and plate power combined.

Then you get stuff like the attached photo.

My main point is that there's simply no need for biasing most amps all that hot.

I wish I could just get these guys to let Enzo take care of it, but they're in there with their multi-meters and bad advice.

My big problem is that I don't know how much power dissipation goes up when you play loud. It's hard to measure. I can simulate it, but I don't know if the simulation is accurate (the tube model probably isn't).

That actually gets pretty complex, since a substantial amount of power is dissipated in the load. The bottom line is that for practical purposes it doesn't matter. For any of the well researched output topologies getting a reasonable static bias on the tubes will see you good. If you want to run extreme loadings, then all bets are off and you're into an R&D project beyond tinkering with an amp or even professional repair standards.

Any more advice on how far you can push it, and how far you'd go on somebody else's amp?

Don't push it. It won't sound any better. Really. Try it and see.

[Bruce / Mission Amps]

That's exactly the problem with that sort of bias measurement. Try installing a 1R resistor between cathode and ground on each power tube. Use a 1/2W and it will behave as an effective fuse for that tube as well. \

Gads, I really think using the standby switch and a fixed current sensing resistor or meter shunt is stupid.
A 1 ohm 1% Mtl Ox resistor on each socket is the only really safe and easy way to go.

But, FWIW:
PT power rating as suggested by Steve, 300ma (50 watt amp) to 600ma (100 watt amp) across a 1/2 watt 1 ohm resistor is still less then 1/2 watt of dissipation so I don't think it will act like much of a current limiting fuse unless the tube is shorted.
It couldn't hurt though.
.6A^2 x 1 = .36 watts

[Joey Voltage]

Ronsonic: That was a great article on biasing, thanks!

I agree with the findings, I've also heard that ugly, trashy sustain when the amp is biased too cold, and if I turn up the bias till it goes away, it seems to work fine.

I hear it more as a hash noise riding on top of the note, almost like slightly overdriving an opamp. where as biasing too warm almost gives kind of a "dull thud" around the attack... close to the sound of slight blocking.

Some high-gain metal amps ran the tubes colder. I think the reasoning behind this is that it causes a kind of "expansion": the opposite of compression, with low gain for small signals and higher gain for large ones. This reduces noise on the dirty channel and seems to make the sound somewhat tighter. Of course this expansion is the very thing that makes the clean channel sound like crap in that ugly, trashy way. But if you always play with screaming, balls-out gain then it might be worth a try.

I don't know Steve, I think you are being a little idealistic in the sense of giving a generous amount of undeserved credibility. my theory is a bit more bleak, and I assume they do it in order to reduce valve failures with in the warranty period most amplifier companies offer on valves. I think in my experience this sounds equally as bad for heavily overdriven tones. and makes that heavily rock vibrato sustain thing not as easy.

Peavey added series diodes in the dirty channel of some of their high-gain designs to produce this effect, so they could have that sound and still run the power tubes hot for a good clean tone.

The only diodes in the signal path in Peavey's, at least that I have seen belong to the "coring" circuit, which is mainly there as a cheap noise gate. the crossover distortion produced by this is more of an understood byproduct out of necessity to keep the hiss away (the funny thing is that the circuit is still noisy! LOL, I can only imagine what it would be like). peavey typically shunts highs in this area, I'm assuming to to reduce any switching transients from the diodes switching on when the signal level goes above .5v or so. Some of those amps are biased just as cold in the output section.

One thing to bear in mind is that, when you overdrive the power tubes in almost all guitar amp circuits, they charge up their grid coupling capacitors and bias themselves colder. You'll see a crossover notch start to appear as you overdrive the power stage more and more. The extra bias dissipates with a time constant of some tenths of a second when the overdrive goes away.

one of the reasons very large PI output coupling caps should be avoided, although at some point it doesn't really matter. Paul Ruby wrote a very good paper regarding this, and a few years back Ray Ivers followed suit and did a lot of experimenting on this. I think he had very favorable results IIRC, and even preferred colder biasing under these conditions.

[Ronsonic]

Gads, I really think using the standby switch and a fixed current sensing resistor or meter shunt is stupid.
A 1 ohm 1% Mtl Ox resistor on each socket is the only really safe and easy way to go.

But, FWIW:
PT power rating as suggested by Steve, 300ma (50 watt amp) to 600ma (100 watt amp) across a 1/2 watt 1 ohm resistor is still less then 1/2 watt of dissipation so I don't think it will act like much of a current limiting fuse unless the tube is shorted.
It couldn't hurt though.
.6A^2 x 1 = .36 watts

Then there's the good side, it won't act as a fuse unless the tube shorts.

[hasserl]

I spend lots of time at the Carvin forums - carvinbbs.com and carvinmuseum.com. I give simple advice and whip up simple mods for folks to try. For amps with 4x6L6GC or 4xEL34 (plate voltage is around 450V), Carvin recommends biasing to 100mA at the standby switch on amps that have bias pots (most do now). Screen resistors, depending on vintage, are 270 or 470 Ohms, though a few early 50W models have no screen resistors.

Now, some folks are recommending bias settings (at the standby switch) up to 180mA. An article at http://www.carvinmuseum.com/pdf/Ka-Boom_v1a.pdf recommends 70% to 80% of rated plate dissipation, and it measures and subtracts out the screen current, preamp current, and phase inverter current.

I've used 65% to 75% of Pa measured on each tube with a bias meter (this provides a cushion, since the screen current is included in the measurement), and I generally install 1K screen resistors.

I went through Kevin O'Connor's TUT series, and he recommends 50% of Pa measured with a 1 or 10 Ohm cathode resistor, but he admits 33% to 66% is safe.

I understand that there's no sense turning up bias current if you can't hear a difference, and lower increases tube life.

I did a bit of searching here, because the topic seemed likely to have come up alot, and perhaps it generated heated arguments (pun intended). A 70% of Pa figure comes up a bit, and the question doesn't seem to be beaten to death.

So let me ask you:

For 6L6GCs or EL34s in a class AB1 amp with 450V plate voltage and 470 Ohm screen resistors:

1. What percentage of Pa do you try to stay under, measuring current at the cathodes, a) on your own gear, and b) on other people's gear?

2. If you did a standby switch measurement, and subtracted out a measurement with no power tubes, what percentage would you try to stay under, given that you now have to take into account the normal variation of tubes from various sources that claim to be "matched", and the fact that the set's match may have drifted over time. I always advocate the safer bias meter (nooby users, and Eurotubes will sell you a bias probe attachment for $25 now), but the standby switch is the usual method that people are using.

I used to hang out around the Carvin bbs years ago (never did hang out at the new Museum forums), it was one of the better forums on the net for a number of years until, like so many other things, the idiots invaded and screwed the place up and Carvin made major changes to the forum and it's never been the same. But I still pop in occasionally.

I was around when Brian was putting together that biasing guide and I did quite a bit of conversing with him about it. I wrote the chapter on the Vintage amps (available elsewhere, I'm not sure where it's been stuck but I can get you a copy), and I am the one who taught Brian to measure the voltage drop across the dropping resistor and to subtract that from the standby current to determine power tube current draw.

As crude as the Standby switch method may be, and as much as it may make some of you shudder to consider it, that is Carvin's preferred method of checking the bias on their amps and is how they instruct their customers to do it. What we did way back then was to try to make it a little more accurate and to help users warm up the bias a little, as it is traditionally set very cold at the factory. IIRC Carvin recommends something like 100ma across the standby switch for their 100 watt 4 - octal tube amps, that is ridiculously cold. We were just looking for a way to help people warm them up using Carvin's preferred method, and it worked. I have set the bias on dozens and dozens of the Vintage Series amps using the standby method and have had absolutely zero problems with it. None, Zilch, Nada. It is fairly easy to check your adjustment using the OT method (measuring resistance in OT wiring between CT and ea leg, then measuring current drop and dividing by the resistance), but this only gets you down to 2 tubes. In order to get down to individual tubes in a 4 tube PA you need to measure the draw independently, and on a pcb amp the easiest way is to use a probe inserted between the tube and socket. But I find this is way over kill and hardly necessary, unless you've got severely mismatched tubes. And even then a little juggling side to side will easily show that if it's there. So the old tried and true standby method works just fine. Trust me Bob, it's fine.

[BackwardsBoB]

The method measures total Pa. It subtracts out the screen power. Measured total current is about 9% lower than what you'd get with bias probes, assuming screen current is only 10% of plate current. The recommended values, 70%-80% of Pa, when measured with a bias probe, which includes screen current in its cathode current measurement, would be around 43mA to 49mA for EL34s on average. That's up to double the stock value. Assuming +/- 10% variation on the tubes, we can get a tube running at 54mA cathode current. A slightly spunky 30W 6L6GC can be drawing 64ma, and that's recommended. 56mA is borderline too low.

I'm here to get opinions from guys like you. Do you think that this is good advice? They're the highest recommended values I've found.

[ThermionicScott]

If you really want the cathode-current-measuring resistors to act as fuses you could bump up their value to 10 ohms. Makes it a little more accurate if you're using a 2V scale, and all you have to do is remember to divide by 10. Still has a negligible effect on bias.

- Scott