One of the preferred platforms for the mod community has been the amplifier from Ampex 351 reel-to-reel player, as shown above. In fact, there's a fair amount of documentation online (especially at the Electric Audio forum, courtesy of Greg Norman), describing how to mod one of these units into a preamp. Free hdfview for mac.
The Ampex 600's and 601's were mono reel to reel decks with tube electronics common to many units of the time. Not many people have a use for a mono tape deck these days, but these units have both a mic and a line input jacks and a line out. So they're being bought as a way to get a real tube microphone preamp for not much dough. Not a bad idea, but you do need to be pay attention to what you're actually getting for the money. A tube mic pre requires an input transformer to work well with modern low impedance microphones. In vintage tube mic pres this transformer frequently plugs into an octal socket like the ones used for large power tubes. Here's the empty socket from an Ampex 601 that came in for a recapping job: Good quality input transformers don't come cheap. If this socket is empty, think of spending just shy of $100 to put a transformer in there. Here's what the socket looked like in this unit when it came to me:
Deville: It appears you have the electronics interchangeable with the later 300 series and the 351 series Ampex recorders. The VU meter looks like the older Burlington meter but the printed boards, etc. Look very familiar. I see the Ampex input transformer in the picture taken oblique from the rear of the amplifier. Winged warrior ii. Apr 22, 2020 Ampex Plug-in Mic Pre 1971 (p/n 4010066B, has pictures), 1.57mb Ampex Plug-in Mic Preamp, description of 4010066B, 230kb Ampex Plug-in Input Transformers - describes line input transformers, 12.54kb Transformer pages from 1968 Ampex Parts List. 340kb MR70 Preamp, 40dB, 01-96440-01 MR70 Preamp, 60dB, 246kb, 01-96440-04. 40 results for ampex 351. Save this search. 2 Vintage Ampex 350 351 AG440 Plug-In Bridging Input Transformer 4580200-01 Pair. Time left 4d 2h left.
That's how the input transformer plugs into the socket. Here's the base of the part that came in this Ampex:
Line Input Transformer
The base looks just like an octal power tube and it plugs in in the same way. Let's look a little closer at this one. Here is the piece standing on it's base: Here I've popped off the metal shield so you can see the transformer windings:
Ampex 351 Input Transformer Parts
Audio Input Transformer
If you can't make out the windings, don't be concerned. There aren't any. This is a simple dummy plug inserted into the transformer socket. Since transformers are so expensive most of these Ampex 600 and 601 tape decks shipped with no input transformer. All they had was this jumper plug. This was cheaper and worked fine with the high impedance microphones designed to work with this sort of input back in the day. It won't work well with microphones you are likely to own now though. So if you're thinking of buying one of these vintage tube pres for recording ask if there is a microphone input transformer before you make an offer, and remember that you're going to be spending close to an extra $100 if there's not one.