Sunday, July 12, 2009

Is the Ampex 600 / 601 a good vintage tube mic preamp bargin?

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:

Ampex 600 601 with empty microphone input transformer socket

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:

Stock Ampex 600 601 microphone input transformer with jumper plug installed

That's how the input transformer plugs into the socket. Here's the base of the part that came in this Ampex:

Octal Base on Ampex 600 601 jumper
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:

Ampex 600 / 601 jumper

Here I've popped off the metal shield so you can see the transformer windings:

Ampex 600 / 601 input transformer jumper wires

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.


Gary Beale said...

Great and instructive web page! I had not realized that Ampex often shipped the units with a dummy plug in place of the transformer. And I thought the dummy plug would look more obviously "dummy". Thanks for teaching me about this.

Michael Hutcherson said...

more Apex 601 posts!!!

Corey Schon said...

I have a matched set 600/620 that I bought from a radio engineer I used to work with in the 60s. They are for sale.


Ron Schon