Your 185nWb/m assumption is in agreement with mine. I realize I didn't mention that directly although it was implied. I also agree with you that 180nWb/m at 1khz and 185nWb/m at 700hz is six of one, half a dozen of the other. As stated in my previous post, all but 2 of my machines were biased with a 185nW/m Teac test tape. However, in all modesty, I was unaware at that time about the different fluxivities. You're right also about the voltages in the manual. A few years ago, I became aware that Revox lowered the playback level to 1.55v but I made no changes to the previously set up machines based on the premise that if it works, don't fix it.
I did, at one time, accidentally not correct for the +3dB difference between the 185nWb/m and the 250nWb/m fluxivities. The result was an inability to bias the machine to get a relatively flat response to 20khz; it fell like a rock somewhere above 13khz. Of course, the type of tape used can also affect that. Finding the cause was somewhat frustrating and when found, my first thought was "dummy". I never did intentionally experiment with over biasing. Ten years ago when I started messing with restoring A77 tape recorders, I knew little to nothing about their modi operandae. Even after a decade, I'm not about to challenge the expertise of people like Willi Studer and John McKnight.
Personally, I don't see an advantage to over-biasing a tape recorder despite that it may work with one type of tape but may fail drastically with another. It's kind of similar to over clocking a computer microprocessor. Sure, it'll run faster but it's more prone to instability. Similarly, nitromethane added to the gas in a car will add power but at the cost of engine life.
So, yes, the A77 will perform OK with higher bias but at the cost of some high frequency response. However, most will not notice that as there isn't much musical content above 15khz, not to mention that most people above 40 can't hear that high anyway and if they were exposed to very high SPLs, like rock concerts or industrial noise, many can't hear above 13khz by the time they get into their 30's. As for distortion, such a practice can drive that to several percent. Typically, the machines I've set up have less than 2% (spec is 2% at 7.5 ips) but that's measured at 500hz. At 10khz, the first harmonic is 20khz, at the threshold of human hearing. However, at say, 1khz, we can have 2khz, 3khz, 4khz, etc., the sum of which can audibly change the sound of the fundamental. However, this isn't as serious as it may appear as without knowing how the fundamental sounds (especially in music), no comparison can be determined, audibly. Loudspeakers are notorious for their harmonic distortions which is one reason why people prefer some speakers over others - subjectivity. That is one very good reason why all but the highest priced speakers will publish THD figures. By speakers, I refer to the speaker by itself, NOT a speaker system.
I do have a Tektronix harmonic distortion analyzer (AA501) but measuring THD at high frequencies from a tape recorder is impractical since the tape noise will interfere. The analyzer measures THD+N, total harmonic distortion plus noise. In other words, anything but the fundamental is displayed and in all probability, the noise floor will exceed THD.
The process used to set bias makes assumptions, most likely the tape used. (See Table 6.5-69 in the manual) As engineers say, when the goal can't be attained, check the assumptions. I use a sweep process, recording an appropriate sweep from 20khz to 20hz fed to the input and monitoring the output, with the recorder in NAB. The result of bias and EQ adjustments can be seen on a computer screen instantly. The hardware and software is LMS by Linear-X.
One thing I did notice is that the high frequency response can change from the beginning to the middle of the tape. One major cause was found to be a badly worn pinch roll. I would suspect a new pinch roll could give the same error if the pressure between it and the capstan was not correct. An incorrectly set or worn pinch roll can even cause frequency response variations between two consecutive sweeps. Taking about a minute to run a sweep from 20khz to 20hz, at 7.5ips, that's 37 feet of tape. I've seen it happen with much shorter consecutive sweeps between 20khz and 10khz. Such an affected pinch roll and/or adjustment can wreak havoc on trying to set the azimuth of the heads but in many cases, despite that, the machine can sound OK. Keep in mind that none of these machines of mine, despite some having old pinch rolls, produce audible WOW, even on sustained violin passages. However, a 10 khz or higher signal seen on a scope will drift, wiggle and bounce. This is caused by variations in the intimate contact between the tape and the heads. Under ideal conditions, the scope trace should be as stable as if it were coming directly from the generator.
In the heyday of tape recorders, reel to reel or cassette, many people preferred a certain brands of tape. Neglecting subjective preferences, the difference in sound from various tape brands was mainly the result of the relationship between the recorder and the tape. namely bias. Based on that, if one sticks to the same brand of tape, the machine can be set to spec or even overbiased if the tape can handle it and performance will be consistent.
I'm 76 and can't hear above 12khz yet I've spent hours setting up these seven machines to perform their best; a couple actually exceed factory specs. A couple are biased for older tapes. Why? Nostalgia. When I record 50's and 60's rock n roll, those old tapes and their decorated boxes add a certain flavour to the music. Then there's the mesmerizing effect of those rotating reels. Like fancy wheels on a car which add little to its performance, they look cool.
Not knowing what type of meters you're using to measure these high frequency voltages, be aware that some DMMs don't give accurate results above 3khz to 5khz. While some will, it's best to check them with a constant voltage output signal generator.
I have a website dedicated mostly to loudspeakers but there are some pages on my experiences with the venerable A77. Unfortunately, I have yet to meet the required number of posts to allow adding hyperlinks. I could split this post into several and meet that requirement but that would be cheating and is against my principles.
Good Grief, I'd best stop here. My apologies for the diatribe. In defense, consider MRL's website and all they're doing is explaining the magnetic recording process. Consider the loudspeaker. A visually very simple device. Yet, an engineer can come up with analyses and data that mean nothing to anyone but another engineer. A guitarist doesn't care about the physics of a vibrating string, only how it sounds. Which, in conclusion, brings me to my pet expression on audio, which is, "It's unfortunate that so many pay so much attention to their audio systems that they miss the music."
Robert