Tape Guide (Q and A) (Sept. 1974)

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Adjusting Bias

Q. When adjusting bias on the left channel of my Ampex AG-500, the VU meter reading increases with adjustment, even though peak output on tape has been reached. The right channel will peak and the indicator shows an output drop on the VU meter as the bias peak is passed. Both channels are recording OK, but monitoring from the left channel is guesswork due to the VU needle standing halfway to 0 (zero) VU. Can you please advise me on troubleshooting?

-Howard Levi, Chattanooga, Tennessee

A. Possibly the record calibration of your VU meter, left channel, is incorrect. This is suggested by your statement that the VU pointer fails to reach 0 when recording on the left channel.

From your description, I am doubtful that you are adjusting bias correctly.

Using an input signal of about 1,000 Hz (Ampex sometimes recommends other frequencies), bias is to be adjusted until output signal reaches maximum. It is sometimes recommended that bias be further increased until signal output drops about 1/2 dB. Thus a correct adjustment of bias would call for the VU pointer going up when the meter is set to read bias but down when the meter is set to read output signal.

Possibly on your left channel you are mistaking a bias decrease for a bias increase. A bias decrease would cause signal output to increase slightly, using optimum bias as a reference.

Playback Equalization

Q. I have a question about playback equalization at 15 ips. Is the NAB curve for 15 ips the same as for 7-1/2 ips? I have found that when recording at 15 ips on my tape machine, a modified TEAC A-40105, I can get a flat-sounding record playback response by reaching behind the machine and disconnecting the equalization cable between the transport and the recording electronics.

(This does not affect playback equalization, merely record equalization.) I have never had occasion to run a curve at 15 ips with the above-mentioned cable unplugged; the only criterion I can judge it by is that it sounds good. And good sound is what the whole game is all about.

-Name withheld

A. I don't know why you get sound more apparently flat to the ear with the record equalization cable unplugged.

At 15 ips, the record equalization (treble boost) isn't nearly as much as at lower speeds, so that unplugging the cable may not make much audible difference. Also, unplugging the cable may compensate for some other fault, such as a deficiency in treble response due to a worn playback head.

The NAB playback characteristic is the same at 15 and 7-1/2 ips; the only, and slight, difference in the equalization provided by the playback preamp might be to allow for differences in the playback head's response at very low and very high frequencies at the two speeds. What basically differs between the two speeds is record equalization. At 15 ips, less treble boost is required in order to compensate for losses that occur in recording. The means of adjusting record equalization at 15 or 7-1/2 ips is as follows. First, the machine's playback equalization is adjusted to provide flattest possible response when playing a standard test tape for the speed in question (it is presumed that the machine has first been adjusted for correct azimuth, and that heads, guides, etc. have been cleaned and demagnetized). Then the record equalization is adjusted so as to provide flat response on a record-playback basis (after first setting bias in accordance with the manufacturer's recommendations).

(Source: Audio magazine, Sept. 1974; Herman Burstein)

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Updated: Wednesday, 2018-02-14 21:47 PST