Ampex 350 mic pre new power supply and servicing - recommend a tech!

[Farringdon]

Hello, we're in the UK (London) and need to find someone who can build us a new power supply for our Ampex 350 so that it can work as a standalone mic pre.  We're happy to ship it  anywhere within reason.

Any recommendations?  Alternatively if anyone knows where we can buy a power supply that'd work for the Ampex that'd be super too.

Thanks!

 

[jwhmca]

Do you need the 350 repaired or modified? Or just a power supply?

 

[mjrippe]

A quick glance at the schematic shows that you need 385 VDC, 12 VDC and 6.3 VAC.  I didn't add up all the tubes, but the 385 may be 100ma or less, the 12 and 6 may be a few amps.  No off the shelf solution jumps to mind unless you find a loose 350 supply on ebay.  Any good tech could build you one though, especially if they are experienced with valves. 

 

[radardoug]

If I were you I would spend my money on something better. The 350 is not a great preamp. I know, I've converted 3 of them. The input transformer is not great, the end result is pretty grungy. Unless of course all you want is grunge.

 

[gridcurrent]

Opinions vary.  Some listeners regard many of the recordings of the '50's to be superb.  350 and 300 models were mainstays in the studios of that era.  Some 350's were fitted with UTC HA-100x, some with Peerless, all measure excellent by my standards.

Here is a partial checkout of a 350:
Ampex 350 restored and modified as follows:
1.  V402 gain stage bypassed.  re-routed signal from wiper of record level control to metering switch, bypassing R428 (record calibrate control).
2.  rewired V401 as triode (strapping pines 8, 3, and 6).  microphone preamplifier gain reduced from 97 dB to 85 dB.

Test Conditions
150 ohms source Z
600 ohms load Z

Maximum gain microphone input to line output: 85 dB @ 1000 Hz
Weighted noise, microphone input: -42.1 dBm (400 Hz and 30 kHz filters)
Un-weighted noise, microphone input: -39.5 dBm
Weighted Equivalent Input Noise : -127.1 dBm (400 Hz and 30 kHz filters)
Input loading:  less than 0.5 dB

Frequency response microphone input to line output, 0 dBm output
20 Hz +1.6 dB
100 Hz +0.1 dB
1000 Hz  0 dB
10 kHz +0.9 dB
20 kHz +1.8 dB

Maximum Output
1000 Hz: + 30 dBm @ 0.35 %  total harmonic distortion

A suitable power supply can be fabricated from readily available components.  One needs filtered and possibly regulated B+, low noise 12 VDC, and 6.3 VAC.  The power supply schematic is included in the 350 manual which is online.

 

[sahib]

I am based in Glasgow. PM me.

 

[radardoug]

Mostly in the 50's and 60's those recorders were used on the line input. So in order to make it into a mike pre you have to modify it as you have done. Some of the recorders used pcbs with very poor connectors, and those can be troublesome. If you have lots of time and money you can do almost anything! But your figures are good, better than I got.

 

[Farringdon]

Yes we're looking for a grungey sound, we already have lots of good non-valve mic pres like Api, Chandler TG, Daking, SSL etc.

Our Ampex 350 has been recapped but after the new PSU is fitted I imagine there'll probably be other things to look at before it's ready.

 

[mjrippe]

gridcurrent - are you using an Amber 5500?  Love mine!

 

[PRR]

> No off the shelf solution jumps to mind

Looks like a Fender Champ supply to me. Champ-repair/clone parts are readily available. Not off-the-shelf but an easy build.

Or an Epi Valve Jr head.

Plus a 12V DC supply which these days could be a switcher-brick.

 

[njm]

I've built a couple of power supplies for these, pretty straight forward. I'm in kings cross/battersea, PM if you're still after someone!

 

[Brian Roth]

Awhile back, I uploaded some docs with my added handwritten notes.


http://groupdiy.com/index.php?action=dlattach;topic=48883.0;attach=23239

 

[Farringdon]

Thanks Brian!