Ampex 350's: Conversion info requested.

[goldstache]

I know there are several threads on this topic, but I was unable to glean exactly what needs done for standalone Mic preamps.
I have two, and I gotta cobble some power rails together, but am totally interested in topology and wiring.
Seems that most folks convert V401 pentode to triode mode, taking the record level wiper over V402 tube, then where?
Based on gain, I'm thinking you can just grab that front end tube and tie it into V406 grid from Record level pot, and on to V407 and OT???

I have one with UTC iron, and one with Triad.

Hoping for some guidance, comfy working with the bottles.
Looking forward to your info.

Thanks in advance,
Joe

 

[Brian Roth]

If it's totally stock....merely flip the monitor switch to "input".. Original signal path with all the "charm" and "warts". Easy check.

Bri

 

[Brian Roth]

OH...the PSU...it wasn't magic

 

[Brian Roth]

https://groupdiy.com/threads/ampex.48883/
Bri

 

[iampoor1]

Look up the "electrical audio" mods. Thats what a bunch of people do. If you want to do it properly its quite a bit of work!

 

[Brian Roth]

You need the B+ voltage as well as filtered 12V for the input bottle filaments as well as 6.3 VAC for the rest.

A 350 was the best Ampex could do in 1950 with available tech. Hence the octal tooooobz. 1950 to 2023......cool.......70+ years ago!!! <g>

Bru

 

[beatnik]

Seems that most folks convert V401 pentode to triode mode, taking the record level wiper over V402 tube, then where?
Based on gain, I'm thinking you can just grab that front end tube and tie it into V406 grid from Record level pot, and on to V407 and OT???

3 tubes signal path should have enough gain for mic preamp use.
With V1 in triode mode it would become a similar configuration as the Gates SA-134, which has a paralleled 6SN7 instead of the 6F6 output stage.
V1 in triode would be less hassle to find a low microphonic tube

If you need to make the power supply from scratch it might make sense to replace the 12SJ7 with 6SJ7 and powering all the filaments with just a single 6.3V rail.