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Ampex 351 output transformer.

Posted by dave slagle (M) on November 14, 2013 at 18:47:07

Was in a discussion on another asylum about transformers and loading when this device came up and I was skeptical about the alleged behavior. It was insisted that loading the transformer improved the overall top end behavior considerably and I didn't buy it.

To cut to the chase I cast out a line among my friends to try to score one and the first strike came from a tape friend with a carcass. The second strike was John Atwood and I reeled that one in because he has a huge amount of knowledge / experience on the 351 and he not only had a sample of the transformer in question but the AP to measure it....SWEET!



and here is his commentary on the measurement method.

Hi Dave,

Attached is the result of my 351 line output transformer tests. According to the Ampex 351 schematic, each half of the 12AU7 driving the transformer is drawing 6.5mA and has a plate-to-cathode voltage of about 275V. Looking at the 12AU7 data sheet, this gives an Rp of just about 10K, hence the 10K resistors driving the transformer. The generator source impedance is 40 ohms, thus negligible. The drop in mid-band level is due to the effect of the 600 ohm load with the 2 x 10K source impedance.

The frequency response isn't very good in either case, although the 600 ohm load flattens it out. It should be remembered that in the Ampex circuit, a fair amount of feedback is used, which will further flatten the frequency response in both cases. In the unloaded case, there will be 7dB more feedback, thus flattening the response even more. BTW, the frequency response is virtually the same if the transformer secondary is connected unbalanced (Blk N/C, Grn grounded).

- John Atwood

This is the first time I have seen this behavior and it took me a few days to come to grips with it. Then it hit me, that 7dB loss of gain is the result of Rsource > Rload. (Rsource = Rload is a 6dB loss).

Now for the ampex fans, here is some more info in the unit from an earlier email from john.

I looked at the thread on audioasylum. He is correct in theory, but you mainly see frequency response anomalies when the transformer is driven by a high impedance source, such as an interstage transformer. Most line output transformers, at least in the pro audio world are pretty conservatively designed and hold frequency response, even with no load. The Ampex 351 line out stage has a fair amount of feedback from a tertiary winding (what He calls the "headphone winding"), and it doesn't work very well without feedback. With feedback, it works well, and maintains good response, even into no load. I know this, since I used to repair them when I was in college, and we usually never turned-on the 600 load, since the distortion went up. Alignment was done with no 600 ohm load. Virtually all the loads at the Columbia-Princeton Electronic Music Center, where I worked, were 15K or higher.

BTW, the distortion went up in all the old Ampex machines when the VU meter was turned-on. This was due to the non-linearities of the built-in copper-oxide rectifier in the VU meter. I can't remember exactly how much it went up, but it was in the order of 0.1 to 0.2%. Those in the know would turn the VU meters off when transcribing a tape. The early units had a VU meter on/off switch, while on the 351 and 354, you would switch the meter to BIAS or ERASE.

I tested the 351 output transformer years ago, and was shocked at how mediocre it was, given how good the 351 sounds. This testing was stand-alone, without feedback.

dave

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