Using an Ampex preamp with a DAW

This is intended as a guide for those who have decided to use the vintage tube preamp sections of Ampex tape decks as a "front-end" for their DAW or as microphone preamps. Certain Ampex experts have belittled the use of these units as "phat toob mic pres" and for good reason. These electronics sections contain a lot of circuitry whose only purpose is burning up power when these units are used as microphone/DI preamps. This adds unnecessary noise and heat. One would be much better off building a dedicated tube preamp from scratch if that is what one wanted. All this does not change the fact that some of these units can be used as microphone preamps/DI boxes with excellent results. With the cheapest true tube (not starved plate) mic preamp selling for $400+, with no meter and only fair components, it is no wonder people are doing this. Some units; however, suffer from major drawbacks for use with typical modern recording techniques and microphones.

The Ampex 350, 400, 401

These units are some of the most desirable as preamps because of their large input and output transformers, octal tubes (12SJ7,) limited use of negative feedback, point to point wiring, remote power supplies, nice meters and high build quality. There were several different transformer companies used: UTC, GTC, Triad, Altec/Peerless and perhaps others. Some are more sought after than others. They are also valuable as tape electronics and I don't recommend chopping up the electronics with mods like I do on the 601s. A tuned 350-2 makes a great deck for analog mastering and there are things about the circuit which make it less desirable for converting to a preamp, including the size and weight. If you have one of these units which has already been altered I'd be happy to convert it for use as a preamp. I prefer not to alter units in original working condition, other than recapping and restoring.

The Ampex 351

The Ampex 351 is also highly thought of for use as a mic preamp. (not by me really, because ...) It uses miniature tubes (12AX7) instead of octals, printed circuit boards and lots of negative feedback. The input transformer was made for Ampex by several companies. It is cylindrical and of nice size. Many have gold pins. They plug into an octal tube socket. The 601 uses the same input transformer. The 601 and the 351 are the only units which use these transformers. I believe they were designed to replace the UTC A-11 input transformers which were sometimes installed on the Ampex 600 as a factory upgrade. I have built units with UTC A-11s and others with the Ampex octals. They sound very much alike. The 351 uses negative feedback from a special winding on the output transformer for the line amp section. It has local negative feedback in other stages through the unbypassed cathode resistors. It uses printed circuit boards instead of point to point wiring. These were early printed circuit boards and there are a variety of issues that make modding them or repairing them difficult. The ones with blue boards are better. I prefer the pentode/triode combination (EF86/12AY7) of the 601 over the 351's dual triodes (12AX7.) The negative feedback in the 351 and the printed circuit boards, make repurposing parts of the circuit more complicated.

As tape deck electronics, the 351 may be the favorite tube unit. They are reported to be less muddy and have the clearest top end of the Ampex tube recorders. I haven't found this difference to hold when they are used as mic preamps only.

The Ampex 600

The 600 has a lot of potential, but without an input transformer it has limited function in a modern recording setup. It isn't compatible with low impedance mics like a shure 58 or an LDC. I also prefer the EF86 tube of the later 601's to the 5879 tube in the 600 and early 601s. Some 600s have a UTC A-11 as a factory upgrade.
I have it from a high authority (Dave Royer of Royer Labs) that one very good use for a tube preamp like the 600 (without an input transformer) is with the output of a tube microphone (usually vintage.) It is especially good if one bypasses the microphone's output transformer and feeds the unbalanced signal straight into the tube grid via pin 3 of the XLR input on the 600. This only applies to tube mics with a cathode follower and the rare exception of a low impedance, plate loaded circuit. When a tube mic with a plate to line output transformer is used, the output transformer should NOT be bypassed. The 600 has an input impedance of 2.2 megohms.
I'm looking forward to trying this myself.

The Ampex 601

The modified 601 is my favorite Ampex for use as a mic preamp. There are a couple happy accidents that make it so good for mods. The 600 has only an unbalanced, high impedance output like the MX-35. The broadcast community wanted balanced 600 Ohm output for sending audio over phone lines, so Ampex made the 601. The engineer was told to add the capability to the 600 and change as little as possible. This means that the 601 is single ended and uses no negative feedback from the output transformer (the thing the engineer added.) These are the very traits that have become so popular with tube heads: single ended circuits with no negative feedback. You see, negative feedback lowers distortion and flattens response - great for a tape recorder. What most people want from a tube preamp these days is that "tube sound" which I equate with low order harmonic distortion from single ended tube circuits and no negative feedback. From what I've read and heard, this distortion is similar to the effect of an Aural Exciter. (or vice versa)

Setup and controls:
If you are using a stock Ampex 601 as a preamp, these are the controls of importance:
the front panel controls (Mic Rec level, Line Rec level, Input/tape switch, and the power switch) plus the Hum Balance and Rec Cal pots on the back. The phones jack on the front is a mono, Hi-Z output suitable for feeding a guitar amp or mixer input. This is an often overlooked feature of these units. The Phones output does not go through the output transformer, so it has a little bit different sound compared to the balanced 600 Ohm output.

Preamps with DC filament power (602, 350, 351, etc.) won't have a hum balance control. The input/tape switch will stay in the "input" position. The noise balance, bias, playback level, playback EQ, and record EQ controls aren't used.

Before using a 601, one needs to be aware that one of the terminals in one of the cables hanging off has over 300 volts in it. Do not allow the 8 pin cinch plug to get wet or short out. The cable with only 2 terminals is safe.

Do not pull tubes from any tube preamp. Doing so will change the voltages to the remaining tubes. If you are having trouble with the unused tape input circuit oscillating, put a capacitor across the terminals of the 2 prong cable. (almost any value will work - if you want a value: 10uF 25V)

Hum balance: Hook up your unit and turn things up (nothing feeding the inputs) until you can hear the noise and hum. Adjust the hum balance for lowest noise.

Rec Cal Pot: Start with the "rec cal pot" turned almost all the way CCW (ACW for Brits ;) ) Hook up your mic and bring up the Mic Rec level to about 2 or 3. Now adjust the Rec Cal pot until you have a good signal level on the meter (peaks into the red a bit.) The Rec Cal pot may be turned down and the Mic Rec level turned up for more coloration. For less color and more headroom, turn up the Rec Cal pot and turn down the Mic Rec level.

Using condenser mics: At this point you should have your preamp sounding pretty good, but now you want to use that nice LDC microphone. Many modern mics used with modern recording techniques (close miking) are too hot for the input section of these preamps and then there's the phantom power issue. Turning the Mic Rec level down won't help with the input clipping. The Mic level pot comes after the first amplification stage. The best solution is an inline mic pad with phase reverse, and an inline phantom power supply. Hook it up this way: Mic --> phantom power --> pad --> preamp input. You want phase reverse because these tube preamps are "pin 3 hot." The standard is now "pin 2 hot" but many old devices have pin 3 hot. The phase reverse fixes this. The alternative is to swap the leads on pins 2 and 3 of the XLR input jack. The devices below represent inexpensive examples. This is not an endorsement:

If you are using a very low Z ribbon mic (30 - 50 Ohm) the input transformer socket must be rewired as shown on the side of the transformer for best performance. When recording either Mic or Line, be sure to turn the other control all the way down. The Mic and Line inputs interfere with each other if they are both turned up.

The Peanut Gallery

I don't generally like the units with the tiny Beyer "peanut" mic transformers. Sure, they can sound fine, but I always want to replace the transformers and that is just too expensive for most people. The 602, and MX-35/MX-10 are some examples of units with the Beyers. The transformers overload easily which makes modern close miking techniques with hot microphones problematic. Of course you can pad them, but they still have limited headroom. Ampex apparently used some tricks to get full range response from these transformers too. They talk about the resistor they add to the transformer input as allowing the transformer to provide full range response. (paraphrased) This seems cheesy to me.

The MX-35 / MX-10 is one of the most popular Ampexes as a mic preamp. Before I started building my own preamps, I would have thought these sounded absolutely amazing, but I am spoiled now and have much higher standards than I did back then. These are mixers so they don't have all that unnecessary stuff in them that the tape electronics have. I have looked every which way at the MX and to really make one rock, I would want to convert it to a 4 in/4 out mic preamp instead of a mixer, replace at least 2 of the mic trafos, and add at least 2 output transformers for balanced low Z outputs. This requires the addition of tubes and a new second power supply for the new tubes OR installing Op-amps and a power supply for them. This is so expensive that I don't believe it is worth it. The MX uses an EF86 first stage and a 12AU7 cathode follower. This is very similar to the 601. With upgraded mic transformers on the MX, you get a very similar circuit to the 601, but with unbalanced outs only, little or no overdrive ability, lower gain, and no meter.

If someone gave me an Ampex MX-35, I would sell it and buy 601s for conversion. For the typical cost of an MX-35, I can build you a dual mono 601 that will blow away the MX in sound and functionality. (The unmodded MX is really only useful with 2 channels when used as a mic preamp with a DAW.)

IMHO, the best thing to do with an MX is recap it and use it as a dual mono mic preamp. This assumes you don't have a good use for it as a 4 x 2 mixer. Most people don't want signals mixed before recording in this world of unlimited tracks. In my youth I would have loved to have had one (or 2 or 3) of these when I was working in 8 track project studios. (1980's) I never understood that it was the preamps in those Fostex and Tascam mixers that made the recordings sound like demos instead of classy recordings.
Visit my MX-35 recap page for more.