Bias control

[gpuma]

I downloaded the owner manual of my Mesa rectoverb 50, single rectifier and I found this article written by Randall Smith, President and designer of Mesa.

 

I think it is interesting and, from an engineering perspective makes complete sense:

 

 

 

 

 

 

NOTE: An article written by Randall Smith that we thought you might find interesting.

Here’saquestionweoftenhear:

“Why doesn’t Mesa put bias adjustments in their amplifiers?”

Well, there’s a short answer and a long answer to this question.

 

 

 

 

 

The short answer is that during my 12 years of repairing Fenders, one of the most frequent problems I saw was bias controls tha t were either set wrong or that had wandered out of adjustment due to vibration. As any honest tech will tell you, there’s lot’s of e asy money tobemadebysprinkling“holywater”onamplifiers...uh,whatImeanttosayis“Yourampneededbiasing.”SeewhatImean? What customer is going to argue with that?

It only takes a moment and a volt meter: The Fender diagram shows how: “Adjust this trim pot for - 52 volts.” That’s it. Nothing more.

Now don’t be fooled into thinking that tubes “draw” more or less bias, they don’t. The way a bias supply is connected to a tube is akin to a dead end road, it just trails off to nowhere without really completing a circuit. It’s a static voltage and regardless of what tube is in the socket — or even if the tubes aren’t plugged in at all, it doesn’t change the bias voltage a bit.

So the end of the short answer is this: Since a bias supply needs to put out the right voltage and never vary, I wanted to build amplifiers that were individually hard wired to the correct values and NEVER needed adjustment. And for 25 years, that’s how Mesa/ Boogies have been built. Time to change tubes? Just plug our tubes into any one of our amps and you’re DONE. No tech needed. NO bills and no BS about biasing. And most important: The bias is RIGHT because it can’t change!

Now, you want the long answer? Here’s more information on how our hard-wired bias avoids trouble. Please read on.

But first, let’s make an important distinction. Our business is designing and building high performance amplifiers. And for this we need tubes whose variance is within a narrow range. Our warehouse is full of rejects ...oh, they work — they just don’t perform within our tolerance range. We have a very sophisticated computer - based tube testing system (nicknamed “Robotube”) that matches and measures tubes over seven important parameters. It can even predict which tubes are likely to have a shortened lifetime — even though they work perfectly during the test.

Because our business is building quality amps, we can afford to reject a lot of wayward tubes. The guys you hear complaining because Boogies don’t have bias adjusters are primarily in the business of selling tubes - not amps. They don’t want to throw away 30 percent of their inventory, so they promote the idea that tubes outside our parameters can be used to “customize” amplifiers and they criticize us because our amps can’t be adjusted to accommodate their out-of-Mesa tolerance tubes.

Now you might be thinking, “But I thought you just said that tubes don’t “draw” bias, therefore they don’t effect the bias supply and thus it doesn’t need to be adjustable.”

And that’s right. Tubes don’t effect the bias setting, but the bias setting does effect how the tubes work. But HOW it effects the tubes is difficult to measure.

When you set the bias (whether it’s by selecting the right resistors, as we do, or adjusting a trimmer — which is quicker) what you are doing is establishing the correct amount of idle CURRENT that flows through the power tubes. But you can’t adjust the current directly, you can only change it by adjusting the amount of bias VOLTAGE that goes onto the tubes’ control grids.

 

 

 

 

 

 

 

BIAS ADJUSTMENT (Continued)

Voltage and current are NOT the same. Current is the AMOUNT of electricity, the “quantity” — and is measured in amperes. Voltage is the degree of electric charge — like the “pressure” to use the old water analogy. Let me illustrate how different voltage and current are:

When you scrape your feet across a carpetted floor in dry, wintery conditions, your body can become charged with 50,000 to 100,000 volts of static electricity. And when you reach for the door knob, a spark jumps and you feel it! The voltage is super high but the current (measured in micro-amps) is tiny - otherwise you would die from electrocution.

Contrast this with your car battery, which puts out a mere 12 volts. You can lay your hands right across the terminals and not feel a thing. Yet the amount of current available can run to several hundred amperes .. enough to turn over a cold engine and get it started.

So current and voltage are two totally separate electrical parameters — though when you multiply them together, you get POWER, which is measured in watts.

When you set the bias of an amplifier, you are adjusting the static VOLTAGE at the control grid of the tube in order to produce a desired amount of idle CURRENT flowing to the tube’s plate. A small change in grid voltage, produces a large change in the amount of current flowing — and that’s basically how a tube works. Say that again because it’s super important: A small change in voltage at the grid causes a large change in current flowing to the plate. See, that’s the essence of amplification: A small change causing a large change. And here it’s a small voltage change causing a large current change.

The bias conditions are what determines how much current flows through the big power tubes when you’re not playing. And what drives your speakers is flucuations in that current flow when are ARE playing. If the amount of current increases and decreases 440 times per second, then you’ll hear an A note. If the fluctions in current flow are large and still at 440 per second, you’ll hear an A that is LOUD!

But for purposes of biasing, it’s the amount of “plate current” flowing with no signal applied that’s important. Unfortunately current is hard to measure because the circuit must be interuppted — as in “cut the wire” — and the meter spliced “in series” with the broken circuit. But measuring VOLTAGE is easy. It is not necessary to interrupt the circuit because a voltage reading can be taken in PARALLEL with the circuit intact.

Thus, as a matter of convenience, most bias settings are given in volts at the grid ... even though current through the plate is the important factor. In fact plate current is so inconvenient (and dangerous) to measure that Fender doesn’t even state what the correct value should be. They only give the grid voltage that will produce that current. (That’s the minus 52.) But that only happens if the tubes being used are “in spec.”

As long as the tubes ARE “in spec”, the right bias voltage will always give the correct plate “CURRENT” — but then there’s no need for the bias voltage to be adjustable!

If the tubes are NOT in spec, then the only proper way to re-set the bias is to cut the circuit and measure the current while adjusting the bias ... but no manufacturer I know even STATES the desired current value! Be that as it may, when the original bias voltage is altered far enough, it will compensate for the tube’s abnormal performance and the correct amount of idle current flow may then be restored. Clearly this is something most repair techs should not attempt.

Some newer amps have LED indicators connected to the circuit which will turn on when the right threshold of current flow has been reached. This is an improvement, and almost worthy if you’re willing to except resistors and lights added into your amplifier’s audio path — which we aren’t. The other “advantage” of this system is that it allows some amp manufacturers to avoid matching their power tubes. The thinking is that adjusting the bias to each tube separately eradicates the inherent differences between the tubes by insuring that the same current flows through each one.

Again, this has some merit .. but it’s still not as good as using tubes that are matched in the first place because compensating for the mis-match causes the push-pull circuit itself to become unbalanced. Two wrongs don’t really make a right.

 

 

 

 

 

 

BIAS ADJUSTMENT (Continued)

Some of the other recommended biasing, “methods” — such as -”.. tubes running red hot, increase the bias .. sounds harsh and runs too cool, turn it down ...” are guesswork at best. Luckily, one of the great things about tube amps is that they can usually stand some abuse without causing any real harm ... at least not immediately.

But don’t these alterations imply that you are second-guessing the amp designer and that there’s a better set of operating conditions that the designer missed but the tube sellers have discovered?

Now some players may like the sound of their amp altered by tubes with extreme characteristics and with the bias set to help compensate. But often it is the mere novelty of change that they’re really responding to and when the amp goes back to the proper original way, we’ve seen them be far happier still!

Because every part in every one of our designs has been meticulously evaluated, compared and stressed over — no matter how seemingly insignificant it might be. And with every design we look for a “sweet spot” where all the parameters — including the bias — come together to give the best sonic performance, consistently and reliably. Every part and voltage is important — yet no one complains that these other parameters aren’t available for tinkering.

Consider our patented Simul-Class circuitry where there are two different bias voltages used for separate pairs of power tubes ... and changing one voltage also changes the other. Great care goes into getting this just right and we think we’d be asking for trouble to have it adjustable for the world to play with ... unless you like paying to have your amp messed up. Sorry, I meant to say, “Uh, ... your amp needed biasing.”

If that doesn’t appeal to you, then merely plug a matched set of Mesa tubes into one of our amps and you’re ready for tone. Guaran- teed. You’d be amazed at the number of service calls we field every day that lead to a diagnosis of out-of-tolerance, non-spec tube problems. To think these would be prevented by including a bias adjustment is something of an insult to you and us. If you put the wrong size tires on your car, do you think changing the pressure will make them right?

Please, don’t think this is a blanket indictment of the other guys selling tubes — it isn’t. And their tubes aren’t all bad either. It just doesn’t make sense to pay more of your hard earned cash for tubes that were probably made in the same Russian or Chinese factory and which have the possibility of being outside the performance window we select for your amp. And it pains us to hear the hype and mystique built up around biasing when twenty-five years of evidence affirms our decision to make bias circuits that “never need adjustment”. How much money and trouble that has saved Mesa/Boogie players you couldn’t estimate.

Our rigorously tested and hand selected tubes are available at your nearest Mesa/Boogie Pro Center or from us directly. Nobody offers better price, quality or warranty than we do ... so why swerve?

Next time we’ll talk about our part in developing the great Sylvania STR 415 type 6 6 and how we’re on the verge of seeing something fairly close reappear on the market. Remember, we still have some of these super rugged mondo-bottles available for older amps — Boogies only please! Until then, Relax, Breathe and Nourish your soul!

Cheers!

Mesa/Boogie Ltd.

[Thundersteel]

Some people think this is Mesa's way to force customers to buy only Mesa-branded tubes. Some install bias pots anyway.

[JeffB]

I installed a bias kit on my Lone Star and at the same time did one on a Road King.

The end result on the LS for me was dissappointing if I was looking for only tonal changes and improvements. If there were any, I didnt think it was an improvement or worth the bother. Its already a very good sounding amp stock standard.

The Roadking was a different experience and a mixed bag.

I installed the bias kit on the 6l6s and tweaked while someone else played. I kept an eye on the multi meter and went to the extremes of what was safe or recommended. Neither myself or anyone else in the room including the guy playing were wowed or even aware of any real change.

Then I adjusted the already installed bias point for the el34's, This is where stuff happened. This really woke up that amp when used with el34's or a combination of el34 and 6l6. This was worth the time. Even a doubting Thomas like myself could hear the difference in a small turn of the screw.

We dialed it in to what we thought was the best sound and then checked with the multi meter to see if it was alright. It was well within range but mesa had had them set so cold it was a headscratch type of moment.

We then installed bias kits on a Mesa Roadster and a Stiletto and again the el34 based Stiletto displayed more of a change and improvement than the 6l6 based roadster.

I think I have removed the bias adjustment from my LS.

I took a reading on a Electra Dyne right out of the box and it was biased quite warm compared to every othe Mesa at my disposal, which was every current model.

That struck me as interesting as Randall Smith was busy working on the MKV when the ED was released and I think, I could be wrong though, that Randall wasnt the cheif designer of the ED. Im sure he signed off on a lot of stuff with it though.

My Rivera designed Fender Concert is also fixed bias. Rivera went from Mesa to Fender, I think, and took that idea with him perhaps. Either way, it doesnt chew up output tubes and sounds good.

[DetroitBlues]

Some people think this is Mesa's way to force customers to buy only Mesa-branded tubes. Some install bias pots anyway.

 

Despite the biasing issue, Mesa's tubes are sized differently. You cannot use their tubes on many other brands because they size them differently, so proper biasing isn't even an issue....

[212Mavguy]

It made more sense when there was a better selection of tubes being produced. My Mesa improved in tone when my tech installed an original design cathode bias mod. Never saw a Mesa amo that couldn't be improved in tone with a bias pot installed. Mesa's thang works for 85-95% of guitar players, good business model.

[JeffB]

 

Despite the biasing issue, Mesa's tubes are sized differently. You cannot use their tubes on many other brands because they size them differently, so proper biasing isn't even an issue....

Not sure what you mean DB. Do you mean physical size? Ive never had a problem sticking mesa tubes in any amp. They source their tubes from the same places everyone else does, they are not specially made, they are just bog standard tubes except they hit them with a hammer before they sell them

[Thundersteel]

Not sure what you mean DB. Do you mean physical size?

 

They just take tubes they buy elsewhere, test them, an then put their logo on them. How are they sized differently?

[JeffB]

 

They just take tubes they buy elsewhere, test them, an then put their logo on them. How are they sized differently?

Yes

[mad dog]

Just my opinion. The value of user (or tech) adjustable bias pots on a fixed bias amp is primarily for safety, safe operation, only secondarily for tonal experimentation. Tubes - even from the same lot, same manufacturer - can vary in how they bias up. If you like to swap tubes (as I do) that little bias pot is the only safe way to go. True enough, some tubes like to run hotter or cooler. But the bottom line to me is an easily accessed bias adjustment lets me try tubes without risk of damaging tubes or amp.

MD

[DetroitBlues]

Wonder where KBP is? I'm sure he have some tasty tidbits to share on this subject. I understand why Mesa did what they did, considering they want their amps as maintenance free as possible....

[JeffB]

Just my opinion. The value of user (or tech) adjustable bias pots on a fixed bias amp is primarily for safety, safe operation, only secondarily for tonal experimentation. Tubes - even from the same lot, same manufacturer - can vary in how they bias up. If you like to swap tubes (as I do) that little bias pot is the only safe way to go. True enough, some tubes like to run hotter or cooler. But the bottom line to me is an easily accessed bias adjustment lets me try tubes without risk of damaging tubes or amp.

MD

The exact reason I put the bias point/adjustment in the Mesa. I had a lot of tubes to try out and didnt want to damage them or the amp.

I arrived at the conclusion that I dont have an ear for subtle nuances in sound in a tube swap but understand the value of a bias adjustment so I can use what ever tube (with in reason) is available at the time.

I was going to sell the Lone Star so I returned every thing back to stock. Decided to keep it but dont recall utting the bias kit back in. I can buy tubes from any supplier that will fall within spec for this amp and dont feel like revisting the time wasting tube search again.

I know people report huge tonal changes and have blanket lifting experiences doing tube swaps or "rolling tubes" but Im not one of them.

[tbonesullivan]

adjustable bias allows you to use tubes that haven't been "tested" by mesa in any amp. There is also bias drift over time, and being able to bias your amp really does help in some situations. I don't run my tubes hot. I run them at the recommended 40-50% static dissipation. Marshall's recommended bias is actually pretty hot, and their amps don't work well with certain tubes. I think my TSL122 would have come stock with SED EL34s, and someone had put JJ E34Ls in there, which wreak havoc with some other amps as well.

 

It all depends on the design of the power amp section. Some are more conservatively designed with higher tolerance components that will work with just about any tubes, while others are designed around specific tubes.

 

Mind you, this ONLY applies for POWER tubes. Preamp tubes are auto-biasing and you can swap them at will as long as they are the same type.

[mad dog]

Jeff:

 

I do hear tonal changes in biasing. The most dramatic is when you get to re-bias a newer amp that came from the factory biased way too cold. I guess they do that intentionally ... the net effect with some amps is that they sound really stiff and nowhere. Even with not great tubes, getting the bias up into a more normal range can improve sound greatly.

 

Smaller tonal shifts within what is considered "normal" bias range for certain tubes. I'm no expert, but it seems some tubes sound better a little hotter. Others not so much.

 

The other time biasing can be very helpful (and safe) is when you swap rectifier tubes. I go back and forth on one amp (Sewell Wampus Cat) between the stock "potato masher" 5R4WGB tube and a GZ34. The former drops B+ voltage significantly, so I'd never swap those rectifiers without rebiasing. Even if you know the circuit and PT are OK for the swap, the power tubes could be at risk.

MD

[tbonesullivan]

See... I guess I'll go back to the Mesa "testing for a minute. Knowing the Mesa business model, I somehow think that what is posted above is only a half-truth, and that the other half is to keep customers dependent on Mesa Tubes, either to increase profit, or increase reliability, or both. Their power tubes are a bit more expensive than the standard ones you can find out there, but they do have a warranty, and should work well. Their preamp tubes are overpriced. I see no reason to go to them for preamp tubes. I believe that almost all of their tubes come from the New Sensor aka REFLEKTOR plant in Russia. Could be wrong though.

 

So, if you have a Mesa Boogie amp, I would recommend using their power tubes. The amps power sections are designed around having tubes are certain specs, and they will most likely sound best with those tubes. And regardless of what Bob at Eurotubes says, running your tubes hot enough to almost melt does not always get the best tone.

[gpuma]

Their power tubes are a bit more expensive than the standard ones you can find out there, but they do have a warranty, and should work well.

 

Based on the article, this should be just a reflection of the higher discard rate due to tighter tolerances. So it is understandable.

I guess for a guy like me who would bring the amp to a tech just for a re-bias, it makes a lot of sense

[Hfan]

Interesting read. Regarding Fender circuits he says “Adjust this trim pot for - 52 volts.” That’s it. Nothing more. Any of you amp tinkerers have a link or can tell me where the test point is for this? Would love to know how to check my bias with just a multi meter. I know how to do my Blues Jr but not my Deluxe or Twin. Spent some time searching w no luck. Sorry if a little off topic.

[tbonesullivan]

I use a bias probe to test my amps bias settings. Static dissipation really is the best way to set the bias, IMHO. There's also the "shunt" method, and the standby switch method. The bias probes are great though because they don't require you to mess with the dangerous stuff.

 

https://taweber.powweb.com/biasrite/br_page.htm

 

They have ones that plug right into a multimeter. I went a bit further and got the "bias rite" with two octal and two 9 pin, as I have a few EL84 amps.

[212Mavguy]

+1 for the bias rite. I have one for my el84 amps. Three of my amps not only have a bias pot, there is a bias pot for bias balance between the two power tubes. Great to have that ability, aids sustain of long notes if you know what you're doing...

[tbonesullivan]

the Marshall TSL122 has two bias pots: one for the PUSH tubes and one for the PULL tubes. all of my other amps just have one trim pot, located on the mainboard. The MEsa DC-5, of course, has no bias adjustment.