As one who has built both auto-bias and fixed (pot-adjustable)bias amps (EA230 etc.)over the years, I can tell you that the only advantage to fixed bias is low cost. This assumes that the auto bias is implemented properly. For auto bias, I have used both digital control (my EA2-150 circa 1976) and analog control with high-speed sample and hold techniques (ZH270). Either works well and holds the proper tube operating points with far better precision than the fixed bias approach. In the case of the ZH270, I even use the bias information developed from the auto bias circuit to tell the power supply shut down if tube parameters are out of spec.
But here is the sonic reason to control bias in typical push-pull amps. Non-gap output transformers used in P-P amps experience core saturation with as little as 1/4 mA current imbalance between the two halves of the push-pull. Toroid transformers, because of their tight coupling, can saturate at half this current. There is no way that hot output tubes will hold their bias to this tolerance without a good automatic controller. The sonic effects of mismatch over 1/4 mA are loss of transparency in the highs and increased distortion in the bass. |
Hi Eldartford, actually semiconductors are nice for self bias as they allow for a higher operating voltage. Since the bias voltage for the power tubes is usually developed across a cathode resistor, that means that the B+ available was the power supply voltage minus the cathode voltage. A tube rectifier would knock an additional 50-75 volts off of that- meaning you had to have 500+ volt power supplies to make enough power with certian tube types. Nowdays the job is easier because you can do the same work with a 450+ volts supply. 450V filter caps are a lot easier to get then 500V! The transformers are a little easier to make too.
The real problem with self-bias is it is really a lot harder to make a high powered amp. And high power is a big deal these days. By high power, I mean over 80 watts. You just don't see that many 80 watt amps that are self biased! |
Atmasphere...Your comments are worth at least 4 cents.
One final comment...the demise of self biased tube operation relates to the development of inexpensive solid state diode rectifiers. In the bad old days, generation of an additional dc voltage for tube bias required another rectifier tube, and all the stuff that goes with a tube. Most peoople didn't think that the improvement was worth the cost. Today it's no big deal. |
My 2 cents: There's a bit of confusion here about autobias and self bias. Self Bias has been around for decades and has been used by many manufacturers over the years.
Autobias is a modern varient of Fixed bias. Fixed bias is the correct way of saying 'adjustable bias' since what is really going on is when one says 'Fixed' is one is referring to the grid voltage being 'fixed' at a particular voltage- often by means of a potentiometer, and now more recently by a modern servo circuit, which might be computor or analog monitored.
Ideally, the bias in an autobias system should be monitored constantly as tubes can drift substantially upon warmup. Computor (or more aptly, controller) based systems are in some ways more adequate to this task as it is easier to prevent the resulting servo circuit from 'hunting' back and forth across the ideal bias point. In an analog-based system, the timing constants have to be chosen to lag slightly behind the tubes and the voltages measured so that stability in the system can be maintained.
Note that as a result in an autobias system the system will rarely be at the ideal bias point but will always be close by. In practice, many users are not attentive enough to monitor the bias level of their equipment (proper use of test equipment being a major limiation on convinience) so often the fixed bias system may be further off at any given time.
So the result is while an autobias system is probably not going to be exact, it probably will be closer in day to day life then a fixed bias system.
Of course, a self-biased setup will not always be correct either, as tube variations usually mean that the self biased circuit will be off by the variation in that tube. But it will always be off by that variation and none other. So if component values are carefully chosen (for example- matched tubes) a good compromise can be reached.
Ultimately: the autobias system is more likely to be the closest to correct from day to day of the three systems currently in use for push-pull amplifiers. The exception is in SE amplifiers, where the tube is operated self bias and class A. In this special case where no other power tubes are involved, the bias level is very accurate.
Sorry for the long diatribe... |
Albertporter...Setting bias just at turnon would seem to be a problem for amps that are left on all the time. Also, if the amp is turned on and off, one would expect the settings to change somewhat as the amp warms up, so that a reset procedure, such as I described, would be useful. |
This is an excerpt from Hank Wolcott in response to questions about how his amplifier works: The circuit has to be made virtually distortionless through optimal loading before positive feedback can be allowed to increase the gain.
The output of the pentode cathode follower is direct coupled to the output stage and constant current sourced by a pair of transistors forming a current mirror.
The grid circuit of the output tubes is connected to a digitally controlled automatic bias circuit which checks and resets individual tube biases at turn-on and then is disengaged from the signal path I don't know if that helps, I know Hank very well and could call him and ask for more details. |
Tubes108...In military equipment that I worked on we had a similar situation.... we were adjusting amplitude and phase of an AC power supply to account for variables of the load. The way to do it is to run the adjustment process until the parameter you are adjusting is within some tolerance, (and then a little longer so you don't always get hung up ON that tolerance). Then, quit the adjustment process unless the parameter drifts outside of a larger tolerance. In other words, introduce "hysteresis".
This is how automatic tube bias adjustment onbviously ought to be done, but...? Does anyone know the actual details? |
A question on auto bias. When would an amp perform said biasing? Unless it was a real smart implementation, seemingly auto biasing would be active all the time the amp is on, that is even when music is being played. Wouldn't that have a negative impact on the music? |
I'd rather have auto bias. I don't have the time or the patience to do it any other way. I have an amp that I turn on and play music, end of story. |
Bin, please see my comments above, which is my opinion of Eldartford being "one of the most knowledgeable here on tube amps". |
Eldartford,
In my opinion you are one of the most knowledgeable here on tube amps and often find myself silently nodding my head to your conclusions. You have provided excellent insight to both the novice and expert. I agree to your last sentence in this threadÂ…So which way would you rather have your tube's biased? :-) |
Bin...Actually it was Ed_sawyer who suggested that a voltage measurement was all that is required, and I merely pointed out that you can utilize a scope as a voltmeter. It doesn't make sense to do that, (like adding on the log scale of your sliderule) but it can be done. Most bias setting is done using a voltmeter, often a meter that is built into the amp.
Your method sounds good, although it's not obvious that the end result would be any different from using a voltmeter. |
Ed your wrong on this one.
Only way you can bias a tube amp correctly is to use a scope and a dummy load. A440 cycles when it starts to clip you then adjust so the cross over notch disappears. You have to look at the wave form! If a manufacturer says its autobias its either full class A or if its not class A they are lying to you. |
Bob P. as the saying goes, "If you can't dazzle them with brains, baffle them with bull$#!^" |
"I wonder if Macs were the first ones to implement autobias successful in their vintage gears like the MC240s."
Doubtful. bias as a concept is as old as tubes, e.g. early part of the 1900s, 1910-1920 is when most basic concepts and methodologies were developed. Not much has changed since then. Things rediscovered, e.g. Parafeed, etc.
-Ed |
Eldartford, one cannot add on a slide rule, only multiple and divide and other operations, but no adding (nor subtracting, obviously). The scales are logarithmic to permit multiplication by adding powers. I still have my sliderule in my desk, for nostalgia!
Bob P. |
Ed_sawyer...Actually, you can use a scope to measure voltage if you are too lazy to go to the cabinet and dig out a voltmeter. Or, if you want to impress an audiophile.
I also know how to add on a slide rule (remember those).
And count to 35 on my fingers. (Hint...use two-place base 6). |
I wonder if Macs were the first ones to implement autobias successful in their vintage gears like the MC240s. |
While autobias is certainly more convenient, and I do not know if one is better than the other sonically, I have heard manufacturers claim that adjustable bias normally yields longer tube life.
In fact, Rogue's new amplifiers have made the switch from autobias to user adjusted bias. In addition to the tube life, I have read they feel it also has produced better sonics.
If sonics were equal, obviously autobias is preferable. If there is some audible improvement by not implementing the circuit, it would be good to go in that direction, and make bias adjustment an easy task, with easy accessibility to the adjustment (like AtmaSphere, Cary, CJ, Jolida, Quicksilver, but NOT like Jadis). |
"adjustable bias" - this is actually called Fixed Bias (yeah, great naming convention, I know).
within the realm of 'auto bias' there are many possible ways to do that concept. Same with fixed bias - eg. separate bias power supply, winding, or tranny, various rectification schemes for bias supply, battery-biasing, etc.
bin:
biased with a scope? What the hell are you talking about? Bias is a voltage, nothing more or less. A good voltmeter is all that is needed. it's either accurate or its' not. There's nothing to graph on a meter. Tech's must have been having a laugh at your expense, methinks.
-Ed |
BIN,
ofcourse a tech is going to concur. How would he get paid otherwise? |
Bin, yes there is a substitute. I'm using one! |
I am a bit confused by the terminology. When I was involved with tube power amps there were three schemes which went like this.
"self biased" ....Bias voltage derived by a fixed resistor from the plate voltage. No provision for any adjustment. Output power reduced by about 10 percent compared with adjustable.
"adjustable bias"....Bias provided by a separate voltage generated by the power supply, and having adjustment capability by a pot.
"AutoBias"....Same as adjustable except with the pot replaced by a circuit that makes measurements and adjusts voltage accordingly. |
Does manual biasing always drift over time? So the amp is always slightly off without auto-biasing. So this could be very tube depended. Some output tubes can last quite a while without drifting while other change on a weekly or monthly basis. |
I am sorry to disagree with everyone so far. I have owned numerous tube amps from low cost to very expensive. Some auto biasing and others not. There is NO substitute for having your amp biased by a professional with a scope. None.
Ask a tech and see if they don't concur. |
There are many ways a tube amp can keep self-bias on the tubes. It's neither better nor worse than fixed (Manually adjustable) bias. Some of the tradeoffs are that usually one method (fixed bias) allows a little more power output. For fixed bias an amp usually needs a separate bias supply, which adds to cost/size/complexity. The methods of self-bias vary, the most common is cathode-bias where a resistor biases the tube. Other units (BAT Vk60 come to mind) use a more sophisticated method of bias where its more of an auto-regulated active bias voltage than a simple cathode-bias self-regulating system.
both methods sound good, and adjusting bias manually is not a big hassle. Sometimes it's nice to have fixed bias in that it allows changing the operation point of the tube, which is not generally possible with a self- or auto-biasing amp.
-Ed |
A real easy tube tester for a beginner is the B&K 606. It comes in a small case like those portable 45rpm record players of old. Less options than the ones Albert is recommending, but it's smaller and can test a wide variety of tubes. I got mine in great shape on Ebay for I think around 50-60 bucks. |
I agree that auto bias is a good thing, I had very positive experience with the amps I owned.
As for tube testers, the B&K 707 or 747 are decent and can usually be had cheap at EBAY. Best of all they are transistor and need little or no calibration and won't drive you crazy with too many knobs to turn to get a reading.
Later, if you want a high end tester, the Hickok 752A is one of the best ever made but much more complicated. |
Most auto control processes are as good or better than you can do manually. This would obviously be a requirement on the designer.
In airplanes, even little ones, the autopilot flies the plane more accurately than a pilot. This is hard for macho pilots (like me) to accept, but fuel consumption on a long trip proves the point. |
I have had two tube amps with auto bias, I have no reason to believe it didn't work- the sound was always fantastic. |
I have an autobiasing tube amp (Primaluna) and it sounds fantastic. I'm all for this "gimmick". Sorry, all I can give you here is my opinion. |
