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Ampex 440b calibration questions
Old 15th June 2019
  #1
Gear Nut
 
SunkenCity's Avatar
Ampex 440b calibration questions

Please forgive the long winded and exhaustive details.

This is probably something only someone with extensive Ampex/440B experience and knowledge can answer as the calibration procedure is a bit different than a standard tape machine cal.

While calibrating my newly acquired 1/4" half track 440b I ran into some confusion.

I calibrate my MCI Jh-24 before every use and between the manual and the analog rules guidelines I found it pretty straight forward the first time other than the previous owners settings for daw interfacing that had to be changed back to 0 Vu = +4dbm at 1.23 Vac.

Both my machines are running at 15ips.

I thought the Ampex would be just as simple if not more so but a few things caused snags.

Repro cal was the easiest part going exactly as it should for a two track machine. The MRL(15 ips NAB eq) is at +3/185 nwBm and I set the meters to read -3 Vu for a +6/185 nwBm calibration

First hiccup was the output strapping was set for +8 and I had to rewire it for +4 to get proper voltage readings based on input signal.

The manual briefly mentions that with the input transformer in place the input level/recording level is a balance between the rec cal trim pot and record level knob.

I must have spent a ridiculous amount of time trying to find the balance between these two controls to get the proper output voltage to match my input reference level, one effecting the other so much just going in circles for hours. Still not sure of their relationship and how to get things to work better to find the proper setting. I did eventually get there but I'm dreading the next record calibration.

I'm I supposed to use a dummy plug for this?

Next was confusion with from the bias procedure.

On the MCI you input a 10k tone at 1.23 Vac adjust for the Vu scale, find your peak and continue rotating clockwise to get your drop for the tape you are using.

The Ampex manual suggests biasing at 1k(I went with 10k based on what analog rules guidelines said and couldn't find a good answer about this) with the output selector switch set to bias you adjust the erase adjust to find your peak fiddling with the bias cal to get it on scale.

Once the peak is found you change the output setting to repro and adjust the bias adjust control to find peak voltage at the output.

Next switch your output selector to bias again and adjust the bias cal to put the meter at zero.

When doing this second or third time I noticed that by adjusting the bias adjust for peak voltage at output and switching back to repro the meter showed around the same db drop from zero that was suggested for the tape I was using.

I figured/assumed that by adjusting the bias adjust was the same as finding the peak then overbiasing for the tape formula.

I'm not 100% sure about this so I figured I would ask here.

Lastly the Record high speed adjust for high frequency is unresponsive at a 10k 1.23 Vac signal, reading off scale. I backed the signal down on the console to read Ovu on the console meters and was able to adjust the high speed pot for 1/2 a db above 0 Vu but with the pots at the very end of rotation ( the screws almost coming out of the pot).

I went ahead and mixed down to it, the results sounded like my mix with no perceivable abnormalities.

I had to increase the record cal/record level output voltage to get the proper output voltage from recording tones at the head of the tape. About a 0.010 Vac increase.

Anybody who can clear these things up thanks in advance
Old 16th June 2019
  #2
Here for the gear
 

I own a 440b that is running very nicely and sounds great. I have it calibrated so it’s very transparent, but adds a nice tone. I am however far from a proficient tape op/tech. I am self taught, but may be able to provide some help.

First off, my machine has the dummy plug, not the transformer on input. This may change the relationship between “Record Level” and “Rec Cal”. I don’t know. On my machine the “Rec Cal” trim pot is for adjusting the level while monitoring “Input” on the “Output Selector”. In other words, once you have your Repro calibrated using your MRL tape, adjust the “Record Level” knob until you are outputting 0vu (I use 1k tone) while monitoring “Repro”. Now flip the “Output Selector” to monitor “Input” and use the “Rec Cal” trim pot to match the level to 0vu. Now when you switch between monitoring “Repro” and “Input” the level should be matched.

I also use 10k to adjust bias. I go 2db over bias for ATR Magnetics tape. The way I do it is run 10k tone while the “Output Selector” is monitoring “Repro” and adjust the “Bias ADJ” trim pot until it peaks and rolls back 2db. Flip the “Output Selector” to “BIAS” and adjust the “Bias Cal” trim pot until it reads 0vu on the 440 meter. This way you can quickly check to see if the bias has drifted.

One thing to mention, the “Bias Cal” and “Rec Cal” don’t affect Repro.

I use the “High Speed” trim pot to adjust 10k until it sits flat at 0vu on my console at the very end of calibration. Yours may be acting up, or needs to be recapped because I can get plenty of high frequency gain out of the “High Speed” pot.

Hope this helps some. Like I said I am not a proper tech, so some info here may be wrong.

-joel
Old 17th June 2019
  #3
Gear Nut
 
SunkenCity's Avatar
The High frequency isn't attenuating for me it won't go lower than 1/2 a db above 0 Vu.

So you are using the Bias adj. to find the peak and over bias. The manual says to use the erase peak for some reason.

Wish someone who knew without a doubt could weigh in.
Old 17th June 2019
  #4
Gear Head
 

The record cal pot is really your INPUT cal pot. This should be done as the last step, AFTER setting your record level.

So, do your repro alignment, then set your bias while monitoring in repro, do not do it while monitoring bias. Use the big record knobs to keep your meters readable. I like to peak the bias at 10k, use the big record knobs to put the meter at zero, then overbias from zero to your -2.5dB or whatever it is you want. THEN, if you want to be able to recall your bias amount, switch your monitoring to bias, use the BIAS CAL pot and set the meter to zero. Now -2.5dB overbias (or whatever you chose) should read zero while recording and monitoring the bias.

For record calibration, monitor in repro. Send in a 1k tone and set the level using ONLY THE BIG RECORD KNOBS. Then do your 10k with the small record cal pot on the record card. IF YOUR 10K TONE WILL NOT ALIGN PROPERLY, YOU PROBABLY HAVE THE WRONG RECORD EQ DAUGHTER CARDS! There were different ones for different speeds, and for NAB or CCIR eq curves. Send me a pic if you aren't sure what you have and I can tell you.

If you've made it this far with no problems, then your cards are ok. After setting the low freq eq while recording and monitoring in repro, your final step is to switch your monitoring to INPUT, then adjust the REC CAL pot so that your meter reads zero with a 1k tone.

Unless I forgot about something, that should be it. The confusion, I think, is the Ampex nomenclature of record cal where other manufacturers call it input.
Old 18th June 2019
  #5
Here for the gear
 

Quote:
Originally Posted by SunkenCity View Post
The High frequency isn't attenuating for me it won't go lower than 1/2 a db above 0 Vu.

So you are using the Bias adj. to find the peak and over bias. The manual says to use the erase peak for some reason.

Wish someone who knew without a doubt could weigh in.
Oh gotcha. I can easily attenuate at least -5db. No problems here getting it in the ball park. As Analog Al said, you may have the wrong Eq curve.

The “Erase Peak” pot controls current to the erase head. I think you are confusing the erase head calibration and bias calibration. To calibrate the erase head you use the Bias vu meter.

There is good info on this at analogrules.com
Old 18th June 2019
  #6
Gear Nut
 
SunkenCity's Avatar
So I lowered the voltage of the 10k tone that I used for bias from 1.23 Vac to .700 Vac so it showed up as zero on the console meters and -1 Vu on the 440b meters. I was then able to adjust the high eq for 0 vu increasing up to zero instead down from +2.

Why would I have to lower the voltage like that tho?

The cards are the correct ones 4020270-01 that haven't been modded.

That's what I would have guessed from the name "Erase Peak", the procedure in the manual talks about it like you are finding the peak bias with it. It's all fu*ky.

Yeah I tried using the analog rules guidelines too fill in the blanks from the manual but not really machine specific.

I think it's pretty good now I'm getting the voltage I'm putting into it back during playback.

But when the output selector is in input mode the voltage is higher than what I'm putting in. I don't think I should mess with it(record cal pot)because it turns into a endless loop of adjustment between it and record level in repro mode.
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