Modding Tube Mixer Into DAW Front End

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HI, I'm new here and I just aquired a cool device. Its a Grommes / Precision Electronics / Western Electric G5M Tube Mixer. Its a lot like the Altec 1567 Tube mixer but has a lot of important improvements. It has 5 transformer balanced mic inputs, , a line input, tube EQ, and a master gain stage. Basically I want to refresh and modify it. I would like to install DI inputs bypassing the transformer and direct outs for each of the 5 mic channels. It uses both sides of a 12AX7 per channel so I assume there should be sufficient gain for the direct outs. I want to use this as the front end for my DAW. I want the individual outputs for use recording drums and I want to use it as a mixer in its natural state for overdubs, utilising multiple mics into one output. What I'm looking for is any help with this. I could use a schematic if there is one out there. Maybe one of you guys has one of these? How would I go about installing the mods on this? Thanks in advance guys.

Jesse Mesa-Savage
Is this mixer really THAT rare? None of you guys have heard of it? A company called TriTone digital even used the EQ section as the basis for a modeled plug-in called the Valvetone '62. If its cool enough to base a plug in on, the real thing should smoke for recording. I just don't want to be limited to one channel. I want direct outs and a DI input for each mic channel, but I also don't want to hack this thing apart improperly, I'd really like some assistance with the mod. I can't post pictures if that would help. Thanks in advance.

I looked through my WE archive and I have nothing on a G5M. I have no Grommes schematics to the best of my knowledge.

It has been my experience that adding direct outs from tube mixers is hard to do with out additional output transformers, 1 per channel. A smarter fella than I could probably make something usable via capacitor coupling. The best bet is to clone the summing amp then give each channel it's own output stage. IMHO.


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> Is this mixer really THAT rare?


> None of you guys have heard of it?

No, and I was buying audio stuff when it was made. I may have seen it in a Grommes ad or catalog, but it left no impression; I'm sure I've never seen one in the flesh. Many of these "professional" boxes were very limited producion, a few hundred at most.

> direct outs for each of the 5 mic channels

Problem there is: if it does not have channel outputs already, it probably mixes at ~270K impedance. You can do that when the whole mixbus is in one box and barely a foot long. It lightens the load on the preamps, saves a ton of money compared to low-Z mixing, money that went into better input transformers and nicer knobs. But it also means the preamp can't drive a length of cable, much less the typically 10K impedance of sound cards and other newfangled ana-digital gear.

The "proper" solution is to take the one existing output amplifier and duplicate it for each channel. You might be able to simplify/cheapen this amp, since it does not have to make-up mix loss and master fader headroom. Still it is likely to double the number of tubes and add a pile of iron.

Another approach, frowned-on here but practical, is to add a transistor or chip buffer to each channel. As Richard knows, you don't need monster tube power to smoke an ADC input. Fake-up a +/-15V supply, wire a TL074 (hi-Z input) as a follower; or derive a ~+30V 20mA supply and wire some 2N2222 emitter followers.

> I could use a schematic if there is one out there.

I hate to send you away when you just arrived, but you could try the gang at The Lab. I know a lot of the old pro stuff from living too long, but I am always amazed at what the kids find, both boxes and documentation.

However, a basic mixer is usually not mind-bending to reverse-engineer. Think what "should" happen: mike transformer to grid, plate to next grid, etc. Keep a 12AX7 pinout handy and start sketching what you see. If you get one preamp you got them all. If the EQ is bass-treble, there's only about two standard ways to do that (feedback and passive). The output amp will be a 12AU7-kin with a transformer. If it was made for small work, single-ended; if made to drive long telco lines or many bridged loads it may be push-pull.
Wow, thanks for the advice. Since it appears to be so rare, I think I will keep it as is and clone the circuit for experimetation. If its as rare as it appears, I don't want to screw it up. I think it will work great as a mic mixer for mono drums (kick, snare, room) and guitars (mic on each speaker of a 4x12 cab) and Bass (DI and mic). Pretty much anywhere I would gang together channels anyway in the DAW, now I can just record one track with multiple mics. I did find a fellow who has a schematic (from ValveTone the plug in guys who modeled this eq section) so as soon as I get that I will get to work on a clone of this circuit. The transformers will be the hairiest thing to copy. Are most of these octal based plug in transformers similar in their pinout? I have some Dukane and McGohan plus that I may use for this. Any further advice will be helpful, thanks.
I too have one of these. Unfortunately mine was already somewhat heavily modified before i got my hands on it. It sounds really fat and in your face, but mine's a little less 'hi-fi' than my 1567. The hi end in particular sounds a little murky, but I think mine's in less than perfect condition. You can still obtain the schematics from Grommes, believe it or not. The octal mic transformers are somewhat more difficult to find. If anyone has a source for those it'd be greatly appreciated.

So, what kind of mods does yours have? I am pretty sure I am leaving it stock, but I may install a switch and SS rectifier with the ability to switch between tube and SS rec. If I DO install individual outputs, I want to use period parts and will probably put direct outs on just one or two channels leaving the rest for the mixer section. I recently aquired a Grommes GT5A which looks like a transistor evolution of the same design. It has individual "remote" outs for each channel, unbalanced on RCA outputs with jumpers installed. You just remove the jumper and it pulls the channel out of the mixer section. While unbalanced outputs would be fine with me, I would prefer a transformer balanced out. Anyone have any more advice? Anyone else have one of these?

Here's an uploadable version. PM me if you need larger.


  • Grommes G5M SMALLER.jpg
    Grommes G5M SMALLER.jpg
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