Ampex 350 bias noise

[Dmichel123]

Can anybody give me a walkthrough on biasing this thing? I've read the manual and followed those directions to the best of my comprehension. There are too many non-existent or non-adjustable resistors listed in the manual for me to make sense of it. I'm having trouble getting both tracks biased well. I can get one channel biased and sounding great, then the other sounds like a dumptruck pouring gravel.

 

[RuudNL]

The only bias adjustment in an Ampex 350 is C113 (100 pF).
Record a 10 KHz tone on a low level and increase the value of C113 until you get maximum output.
Increase the capacity of C113 further until the level drops again a couple of dB.
(Depending on speed en tape emulsion.)

Another way of biasing is to record a low frequency (40 Hz or so) at a fairly high level and adjust the bias for minimal modulation noise.

 

[abbey road d enfer]

Can anybody give me a walkthrough on biasing this thing? I've read the manual and followed those directions to the best of my comprehension. There are too many non-existent or non-adjustable resistors listed in the manual for me to make sense of it. I'm having trouble getting both tracks biased well. I can get one channel biased and sounding great, then the other sounds like a dumptruck pouring gravel.

There is no reason for this discrepancy between channels. Assuming your channels are of the "with bias buffer" type, there is no interaction between channels. Do you mean that aligning one channel interacts with the other?
The documentation available on line for the 350 is relevant to the mono electronics (without bias buffer) so may not be adequate for stereo set-ups.
A tricky part of the 350 record electronics is the "noise balance" that injects some DC into the record head "cold" point in order to compensate for the leakage of the output capacitor AND to counteract whatever 2nd-order distortion of the bias current.
DC in the head or distortion of the bias current can result in sounding "like a dumptruck pouring gravel".

 

[abbey road d enfer]

The only bias adjustment in an Ampex 350 is C113 (100 pF).
Record a 10 KHz tone on a low level and increase the value of C113 until you get maximum output.
Increase the capacity of C113 further until the level drops again a couple of dB.
(Depending on speed en tape emulsion.)

Another way of biasing is to record a low frequency (40 Hz or so) at a fairly high level and adjust the bias for minimal modulation noise.

I think you are referring to the mono version of the 350. The stereo version is a very different animal. Bias adjustment is via a trimpot that controls the input of the bias buffer. There's also an additional "2nd harmonic "control that optimizes symmetry of the bias waveform. And finally the "noise balance" control.

 

[RuudNL]

Yes, you are right. It seems I only have the schematic of the mono version of the Ampex 350.

 

[Dmichel123]

I'm trying to understand the noise balance circuit. Magnetized heads can cause the noises I'm hearing. DC on the head will magnetize the heads. The noise balance circuit adds DC to the heads.  I'm trying to understand how providing more of the problem fixes the problem.  How can I measure the erase and bias current? What level of current should I be getting? The noise seems to be present and barely changes no matter the position of the noise balance pot. I used to be able to dial out the noise to a somewhat satisfactory level. The noise has gotten louder over the past year, to the point that the machine is not really usable now. I have demagnetized the heads, followed the manual for bias/erase/noise balance adjustments and the noise hasn't really changed much at all.

I wrote yesterday that I could get one channel or the other quiet. Yesterday, I was able to setup the master without the slave and the noise was much lower. As soon as I connected the bias interconnect to the slave, the noise jumped up. The slave channel is much more quiet.

This machine has a full track erase head, with Nortronics record and playback heads. I don't know if this is of any consequence. We've used this machine for almost 2 years with this setup, and it has worked well with just a little noise.

 

[Dmichel123]

Ok, I'm ashamed to admit that I didn't change the reel of tape before attempting to get rid of this noise. A new reel has brought the noise back down close to levels we were seeing before. Any tips for further reducing the noise? Are there any mods for the bias section to reduce noise or increase bias current capability (I'd like to try newer tape formulations, we're using Scotch 203 now).

 

[abbey road d enfer]

I'm trying to understand the noise balance circuit. Magnetized heads can cause the noises I'm hearing. DC on the head will magnetize the heads. The noise balance circuit adds DC to the heads.  I'm trying to understand how providing more of the problem fixes the problem.

The way it's connected, it can add or substract DC.

  Yesterday, I was able to setup the master without the slave and the noise was much lower. As soon as I connected the bias interconnect to the slave, the noise jumped up. The slave channel is much more quiet.

It seems like the slave channel overloads the master oscillator.
When you did this, was the erase head connected?

 

[reelersanddealers]

I'm trying to understand the noise balance circuit. Magnetized heads can cause the noises I'm hearing. DC on the head will magnetize the heads. The noise balance circuit adds DC to the heads.  I'm trying to understand how providing more of the problem fixes the problem.  How can I measure the erase and bias current? What level of current should I be getting? The noise seems to be present and barely changes no matter the position of the noise balance pot. I used to be able to dial out the noise to a somewhat satisfactory level. The noise has gotten louder over the past year, to the point that the machine is not really usable now. I have demagnetized the heads, followed the manual for bias/erase/noise balance adjustments and the noise hasn't really changed much at all.

I wrote yesterday that I could get one channel or the other quiet. Yesterday, I was able to setup the master without the slave and the noise was much lower. As soon as I connected the bias interconnect to the slave, the noise jumped up. The slave channel is much more quiet.

This machine has a full track erase head, with Nortronics record and playback heads. I don't know if this is of any consequence. We've used this machine for almost 2 years with this setup, and it has worked well with just a little noise.

Your unit sounds like it the one with a master and slave electronics that utilizes the single mono erase head. Your unit probably has only the erase head output on the master and then a bias interconnect and erase head out on the slave. These units are tricky to get working right- I prefer a pair of modified masters, or 2 slaves modified to work right- took forever to get our 3 track to operate right, IMHO the 350 electronics setup is a nightmare for multitrack 2/3/etc machines, the 351 is better for it- the 350 was meant for mono from day one with extra things done to make it work in different setups.