Repair tech recommendations for old tube recording gear

[Bob Dunham]

I have an early 50's Ampex 400a preamp that I recently got (a predecessor to the 350). It seems to be working fine at lower gain settings, but at higher settings the audio starts to disappear for some reason.

I saw in earlier posts that some people recommended Alan Garren for tape machine repairs, but I wanted to see if anyone knows some techs they recommend for older tube gear (which can be pretty quirky to track down issues).

Thanks!