For sale is a rare vintage stereo pair of Ampex 350-2U microphone preamps with power supply. These started off as two mint units and then were fully restored by Patrick Grenham with mods as two Mic Pre’s. They are fully functional and sound fantastic. Kept in smoke free professional home studio of a famous singer/songwriter, well taken care of and lightly used. You aren't likely to find a nicer looking and working pair.
Modification notes from Patrick Grenham are as follows:
Top left 1/4" jack is a HiZ D.I. input
Left knob is gain
Center knob is input control: left = 150ohm stock, C 2.2k padded (20db), right D.I. selected
Meter switch - on/off
Right knob is master volume
Basic gain structure is as follows:
- Microphone goes to an input transformer.
- The transformer fees a tube that has a lot of gain.
- The knob labeled "record level" controls how much signal goes from that tube to the next.
- The now amplified signal goes through 2 more tubes each with gain.
- The knob labeled "playback level" then attenuates the signal before feeding an output stage.
- The output stage is a 6F6 tube which drives a lot of current into an output transformer.
- The output transformer drives a 600ohm balanced xlr jack and the VU meter.
- The output has 2 switches, one to disable the VU and a second on back to "terminate" the line with a resistor. Use the switch on the back for best noise and it really depends what you are feeding as to the position.
Phones is now a DI input for bass or guitar or keys etc. In order to switch it on you use the small input transfer switch set to "unbal bridge". This setting is hi impedance and bypasses the input transformer.
Record Level is the "input gain" but since that follows a transformer and gain stage you may find at times you want to use an in line input pad between the microphone and the Ampex. There is more than enough gain with this stage to drive things to distortion. Start out trying to set this knob between 2 and 10. Passing 10 on most sources is going to either distort or be noisy, Ampex provides another 50dB of gain after this knob, so no need to crank it.
The Equalization switch is disabled.
Input transfer selects the input between stock "microphone" which is 150ohm and the most gain. Bal Bridge has been modified, essentially followed the textbook numbers for a U47 microphone to dial it in, padding the input and bridging the 2.2k impedance that mic would like with the 150ohm the transformer would like. This will likely be the setting for most mics designed after around 1955. Finally all the way to the right selects the DI input.
Meter and Output switch basically serves to turn off the VU needle if you want.
Playback level should be considered a "master volume" and yes it helps deal with all the gain after the first knob but also it lets you make sure you are coming out at Zero dB and not too hot. The rabbit hole of tone lies in the interaction between record level and playback level. The distortion you can get driving the record level can be great on snare drums, or going for that Billie Holiday vocal distortion that is on the transients. keeping the record level dialed back and pushing the playback knob a little harder will sound more like Bobby Blue Bland or a lot of Andrew Bird recordings.
Of course there a multitude of places in between, many of them with loads of really clean gain.
Try starting with both knobs at 4 and then turn just one knob to get the needle to Zero and see what it sounds like, then return them to 4 and try it the opposite way.
The DI can deliver insanely clean bass, but you can get really great guitar as well and cranking the first knob.
Final note. The output tube is a 6F6, but you might also enjoy the tone of doing a drop-in replacement with a 6V6. Note regarding shipping: These will be individually packed and safely shipped for transit. Shipping costs are not included. The pictured rack case is not included in the listed price, but can be included for extra $150 + shipping.
