Tube microphone preamps, in general.

On the other forum I started a thread about the Tube Cascode Mic Pre schematic just to see if anyone had luck building it. The conversation moved on to some fantastic discussion on tube mic pre design in general. When I came back from lunch eager to read more on this thread, I found the forum had crashed and burned and some weasel saying something about us DIY guys ripping off other's hard work. As if.

Anyway
Though I'm still working on my API 312 clone, I have my eyes out for my next project, a tube mic pre. I've been building tube guitar amps since 1982 so I'm excited to go in this direction. I'm starting to compile schematics of some solid designs and have been interested in the Pultec tube pre, the old RCA stuff, and what really spins my crank is the Telefunken V76.

So here's the question.
How would you guy sum up the most classic designs for use. Like what's good for all applications, what's good for certain coloration, little to no coloration. Including designs that may not be classic, (yet). Sort of a tube mic pre topology primer. I'm just trying to get a handle on what would be a good starting point for my next project, I say starting point as I like to evolve the designs I do, keeps the fun in it.

Thanks,
Kelley

A personal favorite of mine is the Ampex 351. It's a unit that exhibits more size than color. Fast enough for hard rockin' drums, quiet enough for accoustic passages. I did a couple sessions at Steve Albini's Electrical Audio. That is where I first saw and heard these beautiful beasts. I was 21 and fresh out of recording school. I learned so much in a non-stop weekend of tracking. I learned techniques, but more importantly I heard the gear for the first time. Their 351's have been gone through and simplified. Check them out.

http://www.electrical.com/~greg/351.pdf

and some photos

http://www.electrical.com/~greg/351-photos/

I have a 351 in the works. It's a very early one, ser. no. 301. Unfortunately, the boards don't match up with Electrical's. It's so early, the boards are phenolic. They are shot, burned, sagging, and just plain useless. So I'm stuck. But I think the above schematic would be great for a new layout and PCB. I'm so green, I don't know where to start. But I'll learn from you guys and do it one day.

The V72 would be a lot easier to tackle, and its definately a classic, probably more so than the 76.
The gain can be raised but not to the level of the 76 and you need a gain control mod.

You are better off without the original boards unless you want to solder all of those little fork connectors on the edge of the board. They are not reliable. You could use the original boards as a pattern to lay out a "turret" board. Only a table saw (to cut G10 board) and a drill press are necessary to do this and the layout, and presumably the sound, depending on parts, will remain similar.

Thanks for the schematic of that Ampex 351, looks quite tempting! I've never used one and probably should hear one before I build one unless it's such a classic you can't miss, is it? How does it compare to the V76 and V72?

Kelley

I've never used a V72. I have only used a V76 a couple of times. The 351 is a bit more hi-fi, tubey, but transparent. The V76 was also pretty transparent but had more of a saturated quality to it.

Another Ampex pre I like is the MX-10/35 tube mixer. They are very quiet. VERY quiet. My favorite working toy. They remind me more of a V76 than much of anything else. They used EF86's. Next to no headroom though. Great for that distorted motown sound. Bob Ohlsson (sp?) remarked on using one at motown.