Using an Ampex electronics 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 better off building a dedicated tube preamp from scratch if that is what one wanted.

When these units are used as mic preamps, the signal only goes through the mic preamp section and the output section of the tape deck electronics. The record amp, playback amp, and bias sections are unused. 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. None of these will sound their best without work. If you have the DIY skills, these can make inexpensive, high quality preamps. The paper/wax and electrolytic caps in the audio path are usually seriously deteriorated - no high end and muddy. These units need a full recap/rebuild to sound their best.

In all the Ampex tube preamps, the record calibration control is very important. It serves as the master gain control when using these units as preamps. If it is dirty, the sound can be trashed. It is labeled "rec cal" or "record calib."

The Ampex 350, 400, 401, 402

The Ampex 350 and 351 are classic American tape recorders from the 1950's. A vast amount of music was recorded on these professional tape machines. From the front, the 350 electronics are hard to distinguish from the 351. The 351 electronics section has a small red dot in the center under the "Input Transfer switch." From the top, back, and electronically, they are very different.

Ampex 350 from above:

Some salient points concerning the 350 and 400 series:

separate power supply
octal tubes (12SJ7 preamp tubes)
paper/wax audio capacitors
balanced line input is a pad on the mic input. If used, the pad should be rebuilt with low noise, metal film resistors.
"record calib." pot is accessible
large input and output transformers
single ended output section with ordinary negative feedback loop through a resistor
(vs the special transformer winding NFB loop of the 351)

400 series
very much like the 350 in a different layout. There are quite a few 400 series electronics with very desirable transformers, Peerless and UTC mainly. The 400 series also uses a tube voltage regulator on the B+. Nice for the blue glow as well as voltage regulation. The 400 series transports are really poor and only suitable for a museum piece. I have no qualms about reworking a 400 electronics as a preamp for that reason.

Ampex 400

History and commentary

If one digs around, one will find Ampex experts who say that almost no one used the mic preamps in the 350/351 and they aren't very good. It seems that many recordists used the unbalanced line input or modified the electronics. Those that used the balanced line input as built by Ampex, were actually using the mic preamp with its high ratio transformer.
They often state that Ampex's emphasis was on line in recording and the mic preamps were so-so. I believe that is certainly true after about 1960. Ampex first cheapened the mic preamps, then eliminated them. The cheapening involved lowering the tube count and replacing the large, expensive mic input transformers with the tiny Beyer Peanut transformer. After the 351, they DID include low ratio, 600 Ohm, balanced line input transformers with no clumsy pad arrangement on the PR-10, the 354 and the 602. This is the right way to do transformer balanced line in. One "trick" that is rarely mentioned is that one can replace the mic transformer of the 351 with the aforementioned line input transformer. Then one can use the microphone position on the input selector for balanced line input. This eliminates the pad. The line transformer has little or no gain compared to the mic transformer which gives approx. 23 dB of "free gain." In some situations, there may still be too much gain with this technique.

The way Ampex designed the electronics of the 350 (400) and 351 speaks volumes. The mic input feeds the mic transformer, which feeds the 1st tube - completely normal. For the balanced line input however, the input feeds into a pad which lowers the signal level to mic level and sets the impedance. Then it goes into the mic input transformer. Now the biggest problem with this is that the input transformer has a high ratio. (1:15) Such a high ratio, that few modern transformer makers will even make one. It is too difficult and expensive to get good performance. It seems apparent to me that the mic input is the one the designers intended as the primary use. The balanced line input was secondary and compromised. I would say that much of the character of the recordings made on these machines is due to the effect of running the signal through the mic transformer and mic preamp of the recorder. It is well known that many of the major studios bypassed the mic preamp section. It's impossible to say what percentage of vintage recordings were made with modified electronics and what percentage went through the mic preamp.

These units are often used as mic preamps in hopes of getting that classic sound of the 50's. They work as mic preamps and can sound good when recapped. They are large, heavy, heat producing pieces of gear with a lot of unused circuitry when used as mic preamps. I ask everyone to leave these units for use with the 350 transports. If you have a 350 electronics that needs work, I am happy to do that, but please do not hack up a decent recorder for the electronics as preamp. There are plenty of them already orphaned. The 600 series and the PR-10 are examples of decks whose tranports are not something many people want to use. If these transports are worn, I have no problem stripping the electronics for preamps.

There's only one real advantage of using one of these over a 601 and that is the input transformer. These beasts will take a lot of signal. In this example, I was able to record the horns using an unpadded LDC microphone. That was impressive. I usually use at least 20 dB of pad with this mic and horns. To me, this is not a good enough reason to use one of these on a regular basis. Something like a Dual Mono 601 takes up much less space and sounds better (a little bit) to me than a recapped 350 or 400.

The Ampex 351

The Ampex 351 is also highly thought of for use as a mic preamp. It uses miniature noval tubes like the venerable 12AX7, 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.)

Some salient points of the 351:

Built in power supply
Noval tubes (12ax7 mic preamp)
Some paper/wax, some ceramic disc audio capacitors
Balanced line input is a pad on the mic input. If used, the pad should be rebuilt with low noise, metal film resistors.
"rec calib." control is hidden inside the chassis
Same input transformer as the 601, comparable to UTC A-11
Push-pull output with negative feedback from a special O/P transformer winding

Pros: Built in power supply (simpler but noisier,) lighter than the 350, It often works and sounds good without repairs, only one paper/wax cap in the audio path - decent high end without recapping, very good transformers, 12AX7 tubes (cheap and easy to get.)
Cons: There are very many fragile mechanical connections on the edges of the circuit boards. These connectors corrode and break easily. The Rec Calibration control is hidden inside the chassis. There are several ceramic disc caps in the audio path. (Ceramic disc caps are a poor choice for audio. They are generally grainy and harsh in the high end.)

The Ampex 354

The 354 has no built in mic preamps. If a recordist wanted to use mikes directly with the 354, Ampex made octal plug-in mic preamp modules. These mic preamps are not very good. They use very simple transistor circuits with the Beyer mic transformers. This is not the sound you are after if you are here reading about tube mic preamps. The 354 is a stereo machine with insufficient air flow to avoid overheating problems. It isn't as popular as the 350 or 351, but it does have a proper balanced line input structure. (no pads or mic transformers used) The 354 electronics is almost identical to the PR-10-2 (the stereo version.)

The Ampex 600 series

The 600 was a wonder when it hit the market in the early 50's. For the price of a Chevy, someone with a 600 and a 620 amplifier/speaker could go into business. The sound and performance were good for the time, but the transports of these machines have high wow & flutter and are not something a modern recordist would want to use. After a few years of complaints from the broadcast folks who needed balanced output for transmission over phone lines, Ampex instructed one of their engineers to design the 601. He was told to do as little as possible to the 600 in order to add balanced output. For a while, the only difference was the addition of the output transformer - a small, mediocre sounding thing, and some I/O jack changes. At the end of 1957, Ampex decided to upgrade the 601 by replacing the 5879 tube with the new EF86 (6267.) They chose not to give it a new name or number so the only way to tell them apart is to look and see what tubes are used. The EF86 made the 5879 essentially obsolete. If a unit has the 5879 tubes, I change the circuit to use the EF86. There are several modern makers of the EF86, not so for the 5879.

Salient points, ...
Ampex 600:
Hi-Z input, No input transformer was the standard configuration. (UTC A-11 available as factory upgrade.)
Unbalanced "mid impedance" output,
5879/12ay7 mic preamp tubes,
Paper/wax, ceramic disc, and electrolytic audio capacitors,
AC tube heater power,
Tube rectifier for B+

Ampex 601:
Dummy plug standard (Ampex 17331/580022 octal transformer optional)
Balanced 600 Ohm output,
5879/12ay7 or EF86/12ay7 mic preamp tubes,
Paper/wax, ceramic disc, and electrolytic audio capacitors,
AC tube heater power,
Tube rectifier for B+

Ampex 602:
Beyer "peanut" input transformer standard,
Balanced 600 Ohm output, 6DJ8 output section,
Octal socket for optional line input transformer,
12ax7/6aw8a mic preamp tubes (only half of the 6aw8a is used in the preamp,)
Mylar and electrolytic audio capacitors,
DC tube heater power (Selenium rectifier,)
Silicon diode rectifier for B+

The 602 was the upgrade (???) to the 601 that came along in the early 60's. It was released along with other machines that share many of its design features, the PR-10 and the 354. It was with these machines that the mic preamps were cheapened. Gone are the large mic transformers with their multiple windings for different impedances. The preamp tubes were changed to 12ax7 to achieve the gain required with fewer tubes. This saves considerable cost compared to the pentode / dual triode design of the 601 preamp.
The Beyer Peanut transformer is pretty darn good for its size, but it simply doesn't compare very well with the large transformers used previously. It has only one primary winding which they claim is satisfactory for ribbon, condenser, and dynamic microphones with impedances up to 600 Ohms. In many ways the 602 is an improvement, but as far as the sound quality of the mic preamp goes, it isn't. The 602 makes a good preamp with a new input transformer, but it isn't as good as the 601, especially after the 601 has been rebuilt and modified with DC heater power and a different output transformer.

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've been using this technique with a pair of Mojave M-100 small tube condensers. It gives recordings that sound very natural to me. Here is an MP3 of nature sounds and thunderstorms recorded outside my studio window: THUNDER

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.

Other Ampexes

Yes, other Ampex electronics sections can be used as preamps but they all have drawbacks compared to the ones I've described. As time went by, Ampex quit including high quality mic preamps in their recorders. Studios were using consoles and many had been bypassing the mic preamps for years. Units like the 354 or AG series aren't set up for use with microphones.

The PR-10 came in two flavors: a mono version with a built in mic preamp and a stereo version (PR-10-2) with no mic preamps. The stereo PR-10 and the 354 have essentially the same circuits, but in different chassis. They can both use the plug-in mic preamps. The mic preamp in the mono PR-10 is the same circuit as the 602. They also share most of the features listed above for the 602.

Not long after the 602, PR-10, and 354; Ampex released the transistor line. The AG600, AG350, and others replaced the tube recorders. The age of high quality mic preamps in Ampex recorders was over. The Mic preamps which had been in recorders, moved to the mixing consoles. They are still typically found in consoles, but many recordists prefer stand alone mic preamps to those found in all but the finest consoles.

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. A dual mono 601 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 in a recording situation, is to 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.