GrannyGremlin
Active member
If you're not in for a bit of a story, you're in the wrong thread. I am feeling loquacious which is probably not the best thing for my first thread since introducing myself, but here we are.
So I have had this late 60s Ampex PR-10-2 preamp for ages. Made some good records with it. Jumped on a second one when I had the chance. But now neither of them are reliable. Put off doing anything because they are such a pain to work on (you'll see in the pics later; very cramped - it's the same circuit as an Ampex 354 but in a chassis 1/2 as big, ostensibly so as to be "portable"). Anyway (local) pro techs don't wanna touch them, they will do the bare min to get them kinda working but they need more love than that. At the time I didn't know enough to work on them, frankly they still kinda scare me, but It's time to put on the big boy pants.
So anyway, this first one I got is a bit of a weirdo in 2 ways:
1) it has no octal sockets for the plug in "input accessories" (so a dummy plug for unbalanced line in, a bridging transformer for balanced line in, or a nuvistor based mic preamp module), and
2) the output stage is based on 6BK7 vs a 6DJ8 tubes as on all the schematics I have ever seen online (I had the original paper docs for this guy including a wonderful centrefold schem, but somehow lost it - I think some tech at some point kept it - honestly can't rememeber).
So when this came to me it was only good for unbalanced line level input. There was a pre-existing mod - an output pad (simple in-line resistor) that was documented on that fold out schematic I lost. As an eager kid I researched the snot out of these, and eventually tracked down a pair of Beyer "peanut" mic input transformers (as used in the plug in preamps) and had them installed, along with 1/4" jacks and a switch (XLR-mic to Beyer tx and 1/4" for the stock line connection), and also put the pad mod on a switch. I had already removed the tape transport control jack and replaced it with a standard IEC power jack (the transport supplied AC to the preamp, on pins 1 - hot, and 2 - neutral, of the 6 or 8 pin connector).
Obviously, even with the mic input transformers stepping up the signal this thing was missing a gain stage so it was not quite enough gain for all situations. Condensors, using an external phantom power supply, and most dynamics were generally fine, but ribbons or something like a Shure SM7b that need an s-tonne of gain, or a quiet source like breathy vocals would require me to daisy chain the channels (mic in to channel A, and channel A out into channel B line in) to get enough gain, which was fine and sounded absolutely divine, just meant I had one less preamp channel to work with if recording live off the floor (and at the time that was a problem).
But then both of my units got the gremlins; I could not rely on them being functional. Took them to various techs and they'd work for a bit and then down again. I had other preamps and left them alone for a few years, figuring they needed a complete overhaul. Eventually, I did bring one unit home (the weirdo described above) thinking I'd get around to seeing what I could do about it. It has been a year or 2 at least.
A very old pic; the rack has seen some turnover and that console is gone (replaced).
![preamp rack.jpg preamp rack.jpg](https://cdn.imagearchive.com/groupdiy/data/attachments/84/84310-b06f83ce7ee51ebe810960ebe2e7d660.jpg)
Incidentally the thing labelled Altec was my first attempt at a rack job of some preamp cards/modules (used a PSU kit for the +/-15V and phantom from JLM Audio, added Jensen output transformers which can be used as passive stand alone unbalanced -10 to balanced +4 line level interface when the preamp is off) it's a horrible mess in there (built with all reclaimed wire; I was young OK) but it works. I should use it more, but I digress. ...
So I have had this late 60s Ampex PR-10-2 preamp for ages. Made some good records with it. Jumped on a second one when I had the chance. But now neither of them are reliable. Put off doing anything because they are such a pain to work on (you'll see in the pics later; very cramped - it's the same circuit as an Ampex 354 but in a chassis 1/2 as big, ostensibly so as to be "portable"). Anyway (local) pro techs don't wanna touch them, they will do the bare min to get them kinda working but they need more love than that. At the time I didn't know enough to work on them, frankly they still kinda scare me, but It's time to put on the big boy pants.
So anyway, this first one I got is a bit of a weirdo in 2 ways:
1) it has no octal sockets for the plug in "input accessories" (so a dummy plug for unbalanced line in, a bridging transformer for balanced line in, or a nuvistor based mic preamp module), and
2) the output stage is based on 6BK7 vs a 6DJ8 tubes as on all the schematics I have ever seen online (I had the original paper docs for this guy including a wonderful centrefold schem, but somehow lost it - I think some tech at some point kept it - honestly can't rememeber).
So when this came to me it was only good for unbalanced line level input. There was a pre-existing mod - an output pad (simple in-line resistor) that was documented on that fold out schematic I lost. As an eager kid I researched the snot out of these, and eventually tracked down a pair of Beyer "peanut" mic input transformers (as used in the plug in preamps) and had them installed, along with 1/4" jacks and a switch (XLR-mic to Beyer tx and 1/4" for the stock line connection), and also put the pad mod on a switch. I had already removed the tape transport control jack and replaced it with a standard IEC power jack (the transport supplied AC to the preamp, on pins 1 - hot, and 2 - neutral, of the 6 or 8 pin connector).
Obviously, even with the mic input transformers stepping up the signal this thing was missing a gain stage so it was not quite enough gain for all situations. Condensors, using an external phantom power supply, and most dynamics were generally fine, but ribbons or something like a Shure SM7b that need an s-tonne of gain, or a quiet source like breathy vocals would require me to daisy chain the channels (mic in to channel A, and channel A out into channel B line in) to get enough gain, which was fine and sounded absolutely divine, just meant I had one less preamp channel to work with if recording live off the floor (and at the time that was a problem).
But then both of my units got the gremlins; I could not rely on them being functional. Took them to various techs and they'd work for a bit and then down again. I had other preamps and left them alone for a few years, figuring they needed a complete overhaul. Eventually, I did bring one unit home (the weirdo described above) thinking I'd get around to seeing what I could do about it. It has been a year or 2 at least.
A very old pic; the rack has seen some turnover and that console is gone (replaced).
![preamp rack.jpg preamp rack.jpg](https://cdn.imagearchive.com/groupdiy/data/attachments/84/84310-b06f83ce7ee51ebe810960ebe2e7d660.jpg)
Incidentally the thing labelled Altec was my first attempt at a rack job of some preamp cards/modules (used a PSU kit for the +/-15V and phantom from JLM Audio, added Jensen output transformers which can be used as passive stand alone unbalanced -10 to balanced +4 line level interface when the preamp is off) it's a horrible mess in there (built with all reclaimed wire; I was young OK) but it works. I should use it more, but I digress. ...
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