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[john abney]

I hope this forum is OK to post this question in.

I have three mono channels of Ampex record/playback pre-amps and I'm trying to determine if any would make especially good mike pre-amps. One channel is part of a 350, which I suppose is more retro (octal-based tubes) than a 351. Good thing? Bad thing?

I also have a couple of Ampex Cont. No. 31544 record/play pre-amps that I have had no luck in finding any information on (e.g. which Ampex machine they would have been used with). They are each 3 rack-spaces high with rounded aluminum edges, small gray knobs, rounded bias-hatch flip-down cover, and a drop-down slanted back. The style is very similar to the Cont. No. 35211 which is the stereo record/play pre-amp that came with my PR 10, if that helps.

I would appreciate any information or opinions on these Ampex pieces.

best,

john

[electrical]

the 350 is the hard-wired octal predecessor to the 351. I have not used one as a mic preamp, but they were great machines in their day, and I don't see why it wouldn't be a fine mic amp.

It will probably be easier to do the modifications on a point-to-point device, if you want to do them. They consist of:

Removing: eq, removing bias oscillator, removing playback switch and electronics, leaving input transformer, gain blocks and output transformer. This requires re-building the power supply voltage divider.

Adding: Phantom power, polarity switch, 600 Ohm output termination switch, -20dB input pad, output attenuator.

Considered, but not done: B+ regulation, current-regulated DC filament supply.

I think I've got all this right. Greg, where are you?

Greg?

The other units, I don't know. You should talk to the folks at the Recordist Ampex group.

[Brian Roth]

The Ampex 350 requires an external power supply for B+ and filament supplies..I hope you have that unit since an original is made from Unobtanium.

As for that 31544 unit, I recall Ampex sold what was a variant of the PR-10 electronics for custom-ordered multitrack machines.

As Steve suggested, sign up for the Ampex list and post the question there.

http://recordist.com/ampex/apxlist.html

Bri

[john abney]

Thanks steve and Brian,

I subscribed to the list, and I'll try searching the archives before I ask too many questions.

Steve, my gut instincts tell me you are right about the 350's potential as a microphone pre-amp. I know octal tubes are supposed to be more susceptible to microphonics, but there are those audiophiles who swear by them.

Brian, what you say makes sense about the PR 10. I've seen pictures of it with the stereo record/playback pre-amp cased with the MX 10 (or 35?) for microphone inputs, but never of the PR 10 with the twin mono pres. Of course with those, there wouldn't be as much incentive to buy an extra mixer, so if I had been in Ampex marketing, I wouldn't have touted the mono pre's either. As for the power supply for the 350, it is present and appears to be complete.

The dream was to get eight channels of Ampex, managed to get five. Then I decided to go for eight channels of quality tube tape pre's, and got a pair of early Crown pre's and a pair of Revox G36 pre's. But I never got the transport nor the eight-channel heads, nor the budget to complete the project. I tried selling the eight channels (minus the 350) together, but no response yet. So it looks like I need to start researching the value as microphone pre-amps. Sure could have made a killer eight-channel recording device, though.

best,

john

[Brian Roth]

Ampex sold BOTH mono and stereo PR-10 recorders.  Keep in mind the era of those machines....AM radio dominated, and was strictly mono.

Hence, Ampex offered one and two channel versions of the electronics units.  The mono chassis included a built-in mic transformer and preamp, but not the stereo chassis.  It had an octal socket for each channel that could be equipped with a line input transformer, or the much-more-rare mic preamp "can".

Bri