Yes, that does make sense what your tech said, and IMHO the tech is spot-on.
Back to tape selection and operating level, LPR35
IS good tape to use. It is the BASF/EMTEC/RMGI/RTM equivalent of the tape type the machine was designed to use; +6, 1mil class tape. So you're good there. I'm just bringing the whole +3 thing up because, as an option, if (with a proper setup) you wanted to experiment with ways to achieve higher tape saturation with minimal impact to the amplifiers and dbx circuitry, the easiest and best thing to do is just use a tape that saturates easier. It doesn't mean you *should* do that or *have* to do that. My recommendation is use the machine as it is setup with the LPR35 you have...experiment with tracking hotter and see what it does. Drive it for awhile, and then if you find there is anything lacking, THEN ask the critical questions and seek options to address those limitations if they even exist at all for you.
But I had been getting artifacts using dbx, especially on bass frequencies. A “pshh” sound surrounded the bass notes. Is it possible that using +3 tape and biasing to factory would get rid of that?
First of all, your statement about using +3 tape and biasing to the factory 150mV is tying two things together that aren't necessarily related. All the tape type and its designated operating level (i.e. +3, +6, +9, etc.) means is at what level over the original Ampex standard (+0 if you will) the signal to tape distorts to 3%. So "hotter" tape can handle more signal before it distorts. Generally speaking, the higher the operating level of the tape, the more oxide is on the tape, and the more bias signal is needed to do its job. By the way, if you didn't already know, all "bias signal" is is a high frequency signal that hits the tape ahead of the record head when recording. Why? During the dawn of analog audio tape recording it was realized that frequency response drastically increased and distortion drastically decreased if the magnetic particles in the oxide on the tape were "excited" or prepared for receiving program signal by a high frequency signal just prior to program signal hitting the tape. I think the frequency of the bias tone on a 388 is 150kHz. Too little bias signal during record and distortion is not abated. Too much bias signal and you diminish the HF response. Every tape has an optimum bias signal requirement to maximize frequency response and minimize distortion. So that's why we set the bias on the machine when we put new tape on. There can even be slight variation in bias signal requirements between different batch runs of the SAME tape brand and type. Professional studios made it common practice to set the bias on the machine with each new reel of tape regardless of the make and tape type.
Now to hopefully address your question, yes, if you switched from LPR35 to some +3 tape (like 407, which is a +3, 1mil class tape) you would certainly want to reset the bias, but neither the standard operating level (i.e. "SOL", the +3, +6, +9 designation) of the tape nor the bias setting may have anything to do with your issue your experiencing with your bass track. My gut-hunch is what you are hearing is possibly dbx mistracking artifacts, or a bias issue, or a combination of both. Might you fix that by plopping different tape on the machine and blindly setting the bias to 150mV? Maybe. Probably not. If you left the operating level of the machine the same as it is currently and otherwise continued tracking at the same level, you would be distorting the tape more (saturating the tape more...right? Because if you left everything else the same and put tape on the saturates easier you will instantly have more tape saturation), but that in itself may increase the potential for dbx tracking errors (the dbx circuitry may have issues dealing with the distortion), AND if you put that +3 tape on the machine, which typically would require LESS bias signal than the factory specified +6 tape (remember what I said above? Generally speaking, a "hotter" tape has more oxide requiring more bias signal level, and therefore the converse is generally also true...a less hot tape generally requires LESS bias signal level) but you INCREASED the bias to 150mV, my hunch is you would have an over-bias situation with compromised HF performance as a result. In summary, it might sound like ass. Or you might like it. Only your ears will tell you the truth relative to YOU. BUT...here's what I'm thinking...you might just try tracking your bass a little less hot and see if that addresses the issue. I don't know how you have your bass setup, how full it is on the bottom end...since your tech set the machine up at 250nWb/m = -2VU, if you are running your average levels on the bass track at around 0VU or something a little more, and your bass tone is full, you might just be causing some dbx tracking artifacts. So start there, see if taming your average levels a bit helps with the "pshh" sound. If that doesn't do it, I would try increasing the bias 10mV at a time on the bass track only to see if that helps. I'm not sure where your tech actually set the bias level. You can ask your tech, and it sounds like he/she possibly set it a little different for each track in conjunction with the record EQ control to maximize frequency response which, IMO is okay...the whole point of the bias and record eq calibration controls is to line up the frequency response of each track and get them all within factory spec...IIRC that's +/- 3dB from 30Hz to 16kHz on the 388. But maybe your bias level is a little shy overall which is helping the HF response but on the bass track the LF content and dbx processing is revealing some distortion. I don't know. But those are the things I would try if it was on my bench, FIRST just easing up on how hot you are tracking to tape. And by the way if you DO decide to mess with your bias level I totally agree with your tech you want to use a multimeter that is acurate to at least 100kHz. Why? He/she says that because, remember, I said bias signal is high frequency. Maybe I'm wrong on the frequency of the 388 bias signal...maybe its 100kHz and not 150kHz...I don't have the manual in front of me. But anyway your tech's point is that if you are going to measure/set your bias level you need to use a meter designed to measure that high of a frequency. That makes sense, right?
And here's a little tip...there's nothing wrong with experimenting with your bias level outside of that sweet spot of minimum distortion and maximum frequency response. That's right. You never know. Maybe your kick drum might actually sound better if on the kick track you lowered the bias a little...the extra bite of a little more distortion and increased HF response might actually sound good. That's part of what makes working with a tape machine pretty great. Between bias settings, tape types, operating levels, record levels, etc., there's a diverse palette of things a tape machine and its tape can do to compliment your source material.
Hey, anybody, its been a little while since I've mind-barfed this stuff, so if any of my information is backwards or otherwise incorrect PLEASE correct me.
Thanks.
