Some Basic Tube Circuit Mods, Starting places

I've been getting into a lot of older american tube gear from the 50's and early 60's like ampex 351's, magnecorder, gates, and collins stuff. My plan for all of this gear is to just restore them and get them ready for use for modern recording (i.e tube mic pre's with direct outputs). I plan on first getting everything working and up to spec, but then I want to try and improve certain aspects of the circuits if need be. I guess what I'm after is some basic tips/tricks for tube circuits to increase headroom, frequency response, overall timbre etc. I know a lot of it will depend on the circuit, but if anybody knows of any simple tube circuit mods, useful sub-circuits, or could help give me a basic place to start it would be greatly appreciated!

Just study general valve electronics. There are PDFs posted here regarding small signal valve design, and this should help.

There are no quick and easy universal mods.

This is a slippery slope, because a lot of the gear you mention is actually very good and clean when run "as intended". Sometimes you can get improvements as far as specs go by using quieter, tighter tolerance, newer components.

A lot of these designs are really excellent. In an attempt to get better noise or lower distortion figures, for example, you might change a tube circuit topology (lookup cathode bias versus fixed bias), or go from the old carbon comp resistors to new metal films. And then your noise and distortion specs might be better, and you've found you've lost some of the magical "3D-ness".

There's a lot to learn and there's tons of trade-offs... I still consider myself a newbie and I've been around here for a couple years.

Start with CJ's Electronics 101 thread. Ohm's law. Then learn all about rectification and power supplies. Ohm's law. Then tube amplification. Ohm's law, lol!
Some of this is finally sinking in for me! Good luck!

Read as much as you can on the basics of tube circuits and study as many schematics as you can find. The questions will pile up and then the answers will start to make sense.

Here's a couple of must read items that will serve you very well:

http://www.audioxpress.com/resource/audioclass/index.htm  (any or all of these papers)

http://headfonz.rutgers.edu/RDH4/

To my ears a good number of those older circuits leave surprisingly little to be desired in terms of the overall quality of sound they produce.  They often can be much quieter than comparable transistor designs as well.

Valve / vacuum tube info here:

http://www.freewebs.com/valvewizard/index.html

http://www.aikenamps.com/ (see Tech section)

For deep background info visit Pete Milletts site and download some textbooks…
http://www.tubebooks.org/technical_books_online.htm
Many free classic text books there, and the most comprehensive is called Radiotron Designers Handbook 4.

Consider purchasing Morgan Jones’ book simply called  Valve Amplifiers (from Amazon, etc.)

Good luck,
G