ampex 350: how much work to make it a mic pre?

[u b k]

i have a friend selling a fully functional 350 but the transport needs work. it has stereo innovonics electronics. i have basic competence with diy, and have an amazing tech here in nyc.

i know people convert these all the time but i can't find info on how involved this project is. anyone able to shed some light?

thanks!


gregoire
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ubk
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[David Kulka]

The original Ampex 350 tube electronics included a mic preamp, but I don't think the solid state Inovonics unit had one. If there is no mic preamp, it wouldn't make much sense to try converting it. If you can locate the Ampex original tube electronics, then you might have something.

Were you thinking of using this machine for recording? "Fully functional but the transport needs work" could be the beginning of a very unhappy story. It can take a lot of time and money to get an old 350 running properly. Heads, guides, motors, capacitors, relays...

[Granny Gremlin]

I dunno about the 350, but I have a PR10 preamp in with my tech now. To get it working as a simple mic pre would involve fitting input transformers (if the optional, octal plug in ones are not presant or where not an option on the 350, which I doubt) and some simple rewiring of output connectors. If you're lucky, like I was with the PR10, XLR I/O already exists (check the pinouts tho - Ampex liked to use pin 1 = hot instead of pin 2) - and the signal going to the transport (via round DIN-type connector) is already line level - move it to the playbback XLR out (may have to add an output tranny for balanced signal if not there already - but there should be one in the playback circuit you can use). That would be pretty easy, and I would have done it myself if not for looking at the PR10 diagram, noticing the potential for something really cool that was definately above my skill level - so I took it to my tech. There are actually 2 seperate preamps in the PR10 (1 for input and one for playback) I want them cascaded; that'll make the resulting pre 3 x triode + 1 pentode gain stages, so sick amounts of gain, which is exactly what I want as I plan to use this for running ribbons and larger dynamics (no phantom on it anyway). I thought it might be possible to just bridge the to and from transport connectiors to do this, but wasn't sure... also, due to the extra gain stages, I wanted to have a master attenuator (change over the stock gain knobs for dual concentrics with the second pot could be used as the output atten). Maybe the 350 is similarly blessed.

I have a lot of Ampex lit and spec sheets around here - definately have some stuff about the 350 - I can dig it up if you like.

[Jay Pemberton]

What? Pin 1 hot? I'd like to see that. Unheard of as far as I've ever seen. Pin 3 hot, is much more likely, as that was something of a US 'standard' long ago (reversing the conventions for pins 2 and 3). My 354 (all-tube 2 track from the early 1960s; a contemporary of the PR-10) has pin 3 wired as hot.

[Granny Gremlin]

Sorry, it may have been pin 3 - the point was that it WASN'T pin 2, so check it.

Checked and I don't have anything on the 350 - just the 300, 354 and the 351, the last of which may be close enough. Also, I have a full binder of the operation and Maintenence instructions for the AG500 / AL500 if someone needs that; I certainly don't.

[David Kulka]

With a stock 350 you don't need any mods to use it as a mic preamp. Just switch it to record monitor and plug in a mic.

As for all those PR10 mods, I'd say step back, take a deep breath and reconsider for a moment. The "inputs and outputs" to the transport are the head connections, which are far from line level. They cannot just be connected to each other to cascade the preamps. It might be possible to use the repro preamp as an extra gain stage, but it will be difficult and the project could be a big time waster unless your tech really knows his way around tube circuits. Possibly someone at on of the DIY groups has already explored that idea.

[Granny Gremlin]

Like I said, "above my skill level" - it's with the tech, who is good and works on old tube circuits all the time.

[u b k]

thanks for the replies. i was considering getting it myself for the pre, but didin't realize the electronics were total replacements for the tube originals.

if anyone knows of anybody in the ny/nj/pa area who'd like to grab one of these things for $100, stereo electronics but mono headstack, with russ lang rolling case, ping me.


gregoire
del
ubk
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[paully]

Don't know if this helps http://recordist.com/ampex/schematic...n/350fig01.gif , as I can't read sideways .

Best, Paul