biasing a 388 for 0db tape

I have no problem with a proper test tape. Not in everyone's budget, though.

The good advice would be to leave the Tascam alone, as it is, probably, set-up and 457 ready to go (pot sweeping is regular maintenance).

The dude was wanting to BIAS for consumer tape and I suggested straight out he was barking up a flagpole
 
I get it re: affordability, I need a second MRL myself and I've put it off for months for lack of funds. But if you are dedicated to recording with tape it will be essential that you get a MRL for your machine/s. It's not just good hygiene, it's critical to making good sounding recordings.

Good luck!
 
Been quite a while but I am pretty sure that 10kHz will not come anywhere close to '0VU' ref 1kHz?

At 15ips we always setup at neg 10 and neg 20 for cassette. (mind you, flux level standards were a bit different this side of The Pond.)

Dave.
 
Been quite a while but I am pretty sure that 10kHz will not come anywhere close to '0VU' ref 1kHz?

At 15ips we always setup at neg 10 and neg 20 for cassette. (mind you, flux level standards were a bit different this side of The Pond.)

Dave.

Good, good point, Dave.

The 388 level set procedures specify 400Hz at -10dB.

10kHz should reproduce close to that...response spec is +/-3dB 30Hz to 16kHz, and my experience is when setting a 388 up its no problem to get 0VU (referenced to -10dB at 400Hz) at 10kHz. But that's with doing what I recall to be a couple hours of bias level, record level frequency response (rinse and repeat) procedures for all tracks.
 
Thanks for that. I'd put wkrbee's input before mine.

TBH...I missed his post when scanning the thread, and I was mainly commenting based on how well you broke it down for the OP, and specifically covered all his questions...and again his follow-up questions.
I mean, it was all laid out for him and easy to follow....you really took the time. :thumbs up:

I wasn't implying that no one else had anything worthwhile to say. :)
 
Thanks...i getcha.

i try, but somebody like wkrbee has a vast ocean more background and experience with Tascam gear than i do...so I'm respectfully quick to redirect if he chimes in. :guitar:
 
Firstly, just want to thank everyone who has taken the time to help me out here. Really appreciate it and to say that I have learned a lot would be a total understatement.


I've realized I've created a lot of confusion here by simply forgetting that 35-90 is +3 (what I would have called +3db once upon a time :facepalm:) and not standard reference.

If I'd titled the thread biasing 388 for +3 tape then perhaps things would have made much more sense.


The old Ampex reference was 185nw,+3 is 250nw.The 388 was set at 250nw.

I'm going to do tests of 100,1k,10k with both 35-90 and lpr35 to what happens. If everything looks relatively okay I think it's best if I leave things as they are for the moment. There's some other small issues I've been having that I could attend to first before I buy a test tape, audio voltmeter etc.

However, I'm now back to thinking, why is tascam saying the 388 is setup for 357 (355nw?) if the electronics and meters are setup for 250nw?

Maybe I'm just not getting it here, I'm sorry if this is the case. I do understand now how a tape's fluxivity can be different now despite being classed as belonging to the the same reference standard (lpr35 and 457 for example). Wouldn't using 35-90 (200nw) be better however if my 388 was (and presumably still is) setup for 250nw? I don't mind the earlier saturation point and would rather not deal with cross talk if that's the biggest factor.

Again, sorry for confusing everyone. I'm think I'm getting there though?
 
They have to design a machine around a BIAS setting/s. 3M, and the like. could play fast 'n furious seemingly.
I'll attach the last version of 3M specs that was in PDF form
 

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I'm going to do tests of 100,1k,10k with both 35-90 and lpr35 to what happens. If everything looks relatively okay I think it's best if I leave things as they are for the moment. There's some other small issues I've been having that I could attend to first before I buy a test tape, audio voltmeter etc.

However, I'm now back to thinking, why is tascam saying the 388 is setup for 357 (355nw?) if the electronics and meters are setup for 250nw?

Maybe I'm just not getting it here, I'm sorry if this is the case. I do understand now how a tape's fluxivity can be different now despite being classed as belonging to the the same reference standard (lpr35 and 457 for example). Wouldn't using 35-90 (200nw) be better however if my 388 was (and presumably still is) setup for 250nw? I don't mind the earlier saturation point and would rather not deal with cross talk if that's the biggest factor.

Again, sorry for confusing everyone. I'm think I'm getting there though?

Part of what is confusing you (I think) is a particular tape's output level class (i.e. +3, +6, etc.) is based on the flux level that elicits that 3% harmonic distortion level. For a +6 tape that is 355nWb/m (that number may be different depending on whether you are using the DIN standard or NAB standard for instance...let's just say 355nWb/m). Well, are you really going to setup your machine so 0VU means you are already pushing your tape into saturation? The typical practice was to reference 0VU to something that was getting good strong signal to tape, and if you wanted to saturate the tape you would push your average levels during tracking to something higher. Remember, you can set the meters to reference whatever you want, and that may or may not reflect the strength of the signal that's actually printing to tape. Calibrating the machine is about two things:

1. lining up all your tracks so they are the same levels

2. setting your input, record and reproduce levels so you are maximizing signal to noise while staying within your line amplifier's headroom, keeping things controlled for noise reduction processing (if applicable), and hitting the tape with whatever signal strength you want in order to get what you want out of the tape (maybe that's clean, maybe that's saturation, maybe something in between) ---AND--- setting your meters so 0VU means something relevant to your setup.

That #2 is a mouthful and that's why when you are starting out its valuable to set the machine up the way it was specified to be setup by the factory, and then branch out from there depending on what you identify you want to achieve after getting the basic setup under your skin. There are a ton of possibilities and ways to use different tape and machine setups making analog tape a valuable tool for your canvas, but there needs to be some fundamental understanding and experience with it first.

SO...back to your question...457...its a +6 tape which means 3% harmonic distortion occurs when a 1kHz sinus tone is printed to tape at 355nWb/m flux level. But the "standard setup" for a +6 tape is to reference 0VU to 250nWb/m flux level applied to tape, so your average peaks at 0VU are still clean. Again, the VU meter is a visual reference for you during tracking. The "standard setup" assumes if you want to achieve some saturation you are pushing your average peaks to +3VU.

Does that help?
 
That actually helped a lot. What can I say… wow… basic stuff… things are now making a lot more sense. Thanks for clearly explaining how this stuff relates specifically to the 388 for me.

I did the basic tests and here is what I came up with.

First thing to note. When doing the tests, I had the pgm masters all the way up and only pushed the channel fader receiving the oscillator up until everything was at 0db on the VUs. I had to do this to get equal levels across all the meters going to tape.

If I put the channel at unity gain and routed the oscillator through that channel to all tracks on the tape. And if I then adjusted the pgm fader to bring everything up to 0db on the VUs. I found that the levels weren't all perfectly at 0db across all the channels like they were if I put the PGM masters all the way up before turning up the gain on the fader of the channel receiving the tones. They also weren't uneven as a resultof the pgm odd and even being at slightly different levels.

I did this with all channels and couldn't get everything to come up at 0db evenly, though some were better than others. Sure they were only a few channels slightly out but everything is definitely more even when the PGM masters supply most of the gain to tape.
I thought this was peculiar?


Anyway, I did the test with the PGM faders all the way up as that seemed to give me all the channel reading 0db perfectly going to tape.

Here is what the results are when reproducing the tones off both 35-90 and LPR35




35-90


Everything plays back at 0db with the exception of

channel six reproduced -1.5db @ 100hz, 1k an 10k

channel 7 reproduced +1.5db @ 100hz
+1.5db @1k
+2 @ 10k

Channel 8 reproduced all fine except +1 db @ 10k




LPR35

Track 1 = -1db @ 1k

Track 6 = -1db @ 100hz, 1k

Track 7 = +2db @ 100hz
+2db @ 1k


All tracks reproduced @ 10k
T1 = -4db
T2 = -1db
T3 = -1db
T4 = -2db
T5 = -1db
T6 = -2.5db
T7 = +1db
T8 = 0


I thought it was quite interesting how LPR35 reproduced 10k so differently from the 35-90. What's up with track 1 @-4db from 10k?!

I gotta say though, the LPR35 sounds good this time around. I'm not sure why I didn't like it the first time I used it. Wrong instrument perhaps. I'll test it out a bit more now that I'm familiar with 35-90.
 
That actually helped a lot. What can I say… wow… basic stuff… things are now making a lot more sense. Thanks for clearly explaining how this stuff relates specifically to the 388 for me.

I did the basic tests and here is what I came up with.

First thing to note. When doing the tests, I had the pgm masters all the way up and only pushed the channel fader receiving the oscillator up until everything was at 0db on the VUs. I had to do this to get equal levels across all the meters going to tape.

If I put the channel at unity gain and routed the oscillator through that channel to all tracks on the tape. And if I then adjusted the pgm fader to bring everything up to 0db on the VUs. I found that the levels weren't all perfectly at 0db across all the channels like they were if I put the PGM masters all the way up before turning up the gain on the fader of the channel receiving the tones. They also weren't uneven as a resultof the pgm odd and even being at slightly different levels.

I did this with all channels and couldn't get everything to come up at 0db evenly, though some were better than others. Sure they were only a few channels slightly out but everything is definitely more even when the PGM masters supply most of the gain to tape.
I thought this was peculiar?


Anyway, I did the test with the PGM faders all the way up as that seemed to give me all the channel reading 0db perfectly going to tape.

Here is what the results are when reproducing the tones off both 35-90 and LPR35




35-90


Everything plays back at 0db with the exception of

channel six reproduced -1.5db @ 100hz, 1k an 10k

channel 7 reproduced +1.5db @ 100hz
+1.5db @1k
+2 @ 10k

Channel 8 reproduced all fine except +1 db @ 10k




LPR35

Track 1 = -1db @ 1k

Track 6 = -1db @ 100hz, 1k

Track 7 = +2db @ 100hz
+2db @ 1k


All tracks reproduced @ 10k
T1 = -4db
T2 = -1db
T3 = -1db
T4 = -2db
T5 = -1db
T6 = -2.5db
T7 = +1db
T8 = 0


I thought it was quite interesting how LPR35 reproduced 10k so differently from the 35-90. What's up with track 1 @-4db from 10k?!

I gotta say though, the LPR35 sounds good this time around. I'm not sure why I didn't like it the first time I used it. Wrong instrument perhaps. I'll test it out a bit more now that I'm familiar with 35-90.

You are working your ass off here Mr B and good on ya but. It really is bad practice to use the TRs meters to line it up on. At least not until you have checked THEIR calibration.

The manual will give instructions such as "Inject 400Hz (say) at XdBu with record controls at max clockwise rotation" (usually). You then set the meter amp's sensitivity pot to the required reading.* Once so calibrated, you could then use them for line up but it is far better to use an external and vastly more accurate mV meter. E.g. How do you know the VUs have a flat response?

*A few years ago, my son wanted to get some tape distortion and as I feared for the needles on the A3440 I RE-calibrated track One meter to under read by 20dB. Must put that right one day!

Sorry but things keep coming back to me! I recall some m'track recorders used to have a switch in each meter circuit to increase sensitivity by 10dB iirc. This puts the line up tone AT 0VU and thus at the THE most accurate part of the meter scale. You should know that even quite expensive MC movements are only about +or-2% accurate at FULL scale and get worse lower down.
Such machine's meters were designed for line up work, not all were, I don't think.
Dave.
 
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..."as I feared for the needles on the A3440 I RE-calibrated track One meter to under read by 20dB".


Makes sense to me ! hahahah

Ya, a lot of this last experiment deserves a sharp ear, but I doubt the oxides are giving the OP a fair shake either.

Peen in some alignment marks and get those meter pots wiped
 
Yes...meters MUST be calibrated first.

And don't expect signals to come through those PGM MASTER faders evenly. Those are short-throw standard quality faders with 4-channels ganged in each fader. Set the PGM MASTER fader to the shaded area and then adjust your levels with the channel faders *after* you calibrate your meters. :)
 
haha too funny... well... at least I'm learning I suppose!

As my DMM AC voltage specs starts from 220mV, am I okay to calibrate the meters with this?

The manual says I need I need to apply a 1kHz signal at -60db into the MIC IN XLR and adjust the channels trim for a .3v reading from the the ACCESS SEND jack.

Is this too close for comfort?
 
haha too funny... well... at least I'm learning I suppose!

As my DMM AC voltage specs starts from 220mV, am I okay to calibrate the meters with this?

The manual says I need I need to apply a 1kHz signal at -60db into the MIC IN XLR and adjust the channels trim for a .3v reading from the the ACCESS SEND jack.

Is this too close for comfort?
Could be! What is the accuracy specc' at 220mV?

Thing is, if the test kit is not up to snuff you have to get smarter. For instance, that DMM WILL be very accurate at 1V rms sine at 400Hz, so generate a signal from a PC at close to that then make up a 10dB attenuator with a total resistance of say 20k (Google will find you a calculator)

Now you can bob the meter on the full V end then the attenuated point and check the diff'. For an exact 1V rms in you should get 316mV.

Or, you can build a 'precision' 10dB amp (or any reasonable gain). 10k in line and 30k in the feedback loop give a gain of 3. Any op amp will do but there is no reason I see to deviate from the NE5532? BTW 1% or better resistors but you can always buy a gaggle and measure for closest value.

If the above is out of your comfort zone then IMHO you need to do some learning. Maintaining tape machines is a technical job. Even if you could afford/find the old test gear you would still really need to understand the principles involved.

Levell A.C. Microvoltmeter Type TM3B (147) | eBay

Just e.g. You would still really need a good audio sig genny.

Dave.
 
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