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A basic recorder/reproducer in the 351 series consists of a tape transport for operation at tape speed pairs of 3-3/4 inches per second (ips) and 7-1/2 ips or 7-1/2 and 15 ips, a head assembly for use with l/4-lnch magnetic tape, and an electronic assembly which contains the record amplifier, reproduce amplifier, bias and erase oscillator, and power supply — all featuring etched board.
40 results for ampex 351. Save this search. 2 Vintage Ampex 350 351 AG440 Plug-In Bridging Input Transformer 4580200-01 Pair. Time left 4d 2h left. These Ampex plug-in transformers are used for the input electronics of Ampex tape recorders, specifically Models AG-440B, AG-445B, along with the 500 and 600 series. They were used in place of a dummy plug to provide a Balanced Input to the Record/Play Electronics. They use a standard Keyed Octal plug with gold plated pins.
The Ampex 600's and 601's were mono reel to reel decks with tube electronics common to many units of the time. Not many people have a use for a mono tape deck these days, but these units have both a mic and a line input jacks and a line out. So they're being bought as a way to get a real tube microphone preamp for not much dough. Not a bad idea, but you do need to be pay attention to what you're actually getting for the money. A tube mic pre requires an input transformer to work well with modern low impedance microphones. In vintage tube mic pres this transformer frequently plugs into an octal socket like the ones used for large power tubes. Here's the empty socket from an Ampex 601 that came in for a recapping job:Good quality input transformers don't come cheap. If this socket is empty, think of spending just shy of $100 to put a transformer in there. Here's what the socket looked like in this unit when it came to me:
That's how the input transformer plugs into the socket. Here's the base of the part that came in this Ampex:
The base looks just like an octal power tube and it plugs in in the same way. Let's look a little closer at this one. Here is the piece standing on it's base:
Here I've popped off the metal shield so you can see the transformer windings:
If you can't make out the windings, don't be concerned. There aren't any. This is a simple dummy plug inserted into the transformer socket. Since transformers are so expensive most of these Ampex 600 and 601 tape decks shipped with no input transformer. All they had was this jumper plug. This was cheaper and worked fine with the high impedance microphones designed to work with this sort of input back in the day. It won't work well with microphones you are likely to own now though. So if you're thinking of buying one of these vintage tube pres for recording ask if there is a microphone input transformer before you make an offer, and remember that you're going to be spending close to an extra $100 if there's not one.
I realized today that it's been nearly a year since this blog has been updated, but that doesn't mean that Wombat Amps hasn't been busy! On the contrary, we've now launched an improved web site, I've established a formal Wombat Amps workspace in a building that also houses dozens of bands -- as a consequence I've been doing a fair amount of repair work -- more than I had really intended actually. Nevertheless, I have been working on some interesting projects, including a cool custom amp that was recently completed and a solid state equalizer. But details of these just haven't made it to the blog for want of time.
However, I'm working on a new microphone preamplifier project that I'm really excited about, and I hope to document a fairly complete build log here. This mic preamp was commissioned by Andrew Everding, perhaps best known as the keyboard player for the band Thursday. However, with Thursday currently on hiatus, Andrew has moved on to audio engineering, and is currently located in Auckland, New Zealand, where is an audio engineer working directly with Neil Finn (of Crowded House fame) at Neil's well-known Roundhead Studios.
Mic Input Transformer
In any case, Andrew is a big fan of tube microphone preamps based on modification of old Ampex reel-to-reel players. As a tube aficionado, this makes perfect sense to me - in a world now flooded with sterile-sounding ultra-low distortion preamp choices, more and more musicians are realizing that a bit of distortion, of the right kind, can be just the ticket for producing a warm, full sound.One of the preferred platforms for the mod community has been the amplifier from Ampex 351 reel-to-reel player, as shown above. In fact, there's a fair amount of documentation online (especially at the Electric Audio forum, courtesy of Greg Norman), describing how to mod one of these units into a preamp. So why not just mod one of these, and be done? Well, it turns out that there are a number of good reasons. First, the Ampex 351 is now highly sought after, and prices for these units have skyrocketed. You can easily pay $1300 or more for one on fleaBay, and what you get will be in uncertain condition. And this is before the mod process, which turns out to be not at all straightforward. These were constructed on now crufty old first-generation PCBs that are prone to traces lifting from accumulated moisture, and are not particularly amenable to modification. Also, there is a ton of other circuitry in there that gets in the way and is unneeded after the mod, you're for sure going to have to replace all the power supply capacitors anyway, etc., etc.. Even after you go through all the trouble, you're still going to be left with just a single preamp, not a dual. So you'd need to do this all twice to get a dual setup! And this single would not really be optimised in terms of layout, ground scheme, noise, etc. It also wouldn't provide modern amenities such as phantom power and fully regulated DC tube heaters, at least not without having to do a bunch of extra work. But aside from all that, the Ampex is still a pretty damn good reel-to-reel player, and I just hate seeing beautiful old gear being canabilized in that way. So the decision was made early on in the process to scratch-build from new components a dual preamp that stays true to its heritage but incorporates just the components necessary for a preamp. In addition, it should incorporate features such as phantom power and regulated DC tube heater supply, and do it all in an attractive 3U rack-mountable package. Here's the design brief:
- Dual microphone preamp inspired by the Ampex 351 circuit;
- Each section to combine elements of the Record and Repro boards of the original in a manner similar to the Electric Audio preamp mod;
- Tube complement of each section will be 2 x 12AX7 and 1 x 12AU7;
- Tubes provided will be New Old Stock (NOS) selected for low noise, gain, and balance;
- Custom wound transformers audio transformers from Sowter UK;
- Custom wound power transformer from Heyboer USA;
- 3U rack-mountable enclosure with laser-engraved faceplate;
- Black anodized aluminum contrasting with large high-quality brushed aluminum control knobs;
- Regulated 12.6VDC filament heat for all tubes;
- Tube rectifier replaced with silicon rectifier;
- Switchable between 120V/60Hz and 230Hz/50Hz power;
- Regulated 48VDC phantom power;
- Switchable -20 dB input attenuation on the XLR mic input;
- A separate 1/4' stereo jack line level input with fixed -30 dB attenuation;
- A front panel Mic/Line selector switch;
- A mono 1/4' instrument input directly to the input tube grids;
- Phase switch;
- Variable output attenuation using a Bourns 600Ω T-Pad Attenuator;
- Bypass switch for disengaging output attenuation;
- Primary gain control and gain trim adjustment;
- Ruggedized turret board construction for main amplifier boards;
- Custom hand-etched PCB for phantom and filament power supply;
- The unit will be equipped with an internal EMI/RF line power filter;
- Power supply features 105deg-rated low Z electrolytic capacitors for heat resistance and longevity;
- Signal capacitors are high-quality film types in the nF range, and silver mica in the pF range;
- Audio path wiring is aircraft-grade solid core 20AWG with PTFE insulation, shielded where necessary;
- Controls for gain and gain trim will utilize sealed 2W milspec potentiometers;
- 750V rated 2W metal film resistors will be used for low noise and durability;
- Where accurate balance is required for best CMRR, 0.1% precision resistors will be used;
- The ground scheme will be optimised for low noise, and will adhere to the AES48 standard to avoid the 'Pin 1 problem';