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Electro Harmonix 12AY7 Mic Pre Amp Modifications
Old 16th February 2012
I assume this is the kind of thing I need.

At $85 am I spending way more then I need to for a solution?
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Electro Harmonix 12AY7 Mic Pre Amp Modifications-51-q7vr5myl._sl500_aa300_.jpg  
Old 16th February 2012 | Show parent
Here for the gear

that should do it, and come in handy for other stuff too...
Old 16th February 2012 | Show parent
Originally Posted by jc morrison View Post
that should do it, and come in handy for other stuff too...
Thanks - I do live sound a bit too, so I can think of situations where this box would have already been very beneficial - I'll pick it up....
Old 20th February 2012 | Show parent
Here for the gear

having re-read the first post of this thread and reviewed the content of the mod, i wanted to make a few more comments and suggestions. this is not in any way meant to discourage folks from modding! just corrections and a little direction.

the repair work the author did was necessary because of the failure of the power transformer. there is no doubt he was correct about that. a new power trans was in order! but there are several assumptions after that which need to be clarified. lack of documentation and the "instruction manual" would have made this difficult, hence the "educated guesses".

the first is that the B+ and the anode voltage are not the same thing... they are not. the ad copy for the pre that states "200 volt supply", refers to the B+ mains, and not the anode voltage, which should be 85 - 95 volts. the current source for the differential stage is adjusted (it has a trim pot) to accomplish this. adjusting this for 200 volts at the plates of the 12AY7 is absolutely incorrect. that means the tubes are completely cut off (little or no current going through them at all). they will definitely distort with any signal. as i mentioned before, setting the anode voltage for 150 volts is also not good for the 12AU7s. bad idea.

the next is an assumption that preamps normally supply unlimited amounts of phantom supply... that is grossly untrue. until recently, most pres and even mixers guesstimated well under 10mA per mic. there weren't any mics that needed that much. that "standard" doesn't fit today because of the proliferation of inexpensive direct coupled condenser mics from china. these typically draw more than 10mA. plug 5 of them into a mackie mixer made before 2006 and see what happens. most people wouldn't own more than 2... so that is unlikely to happen. i do take responsibility for this pre not having enough current for those mics, though. it doesn't, and no amount of changing the the zener string will help... a larger trans/wall wart could help, however. as would a dedicated phantom supply. but where to squeeze it in?

if the blue LED extinguishes upon turning on the phantom, you can be sure the mic needs more than the B+ in the preamp can supply.

the next assumption is that the 1/4" jack is unbalanced. it is not. it is a TRS jack and is balanced out. the stage associated with it has a small amount of gain and was never intended to be used as the primary output. the second pot controls that. if the gain on the pre is high (say with the clipping indicator blinking), and the monitor is also turned up, it will of course distort like crazy. it states this clearly in the instructions, but these could be lacking in a used ebay purchase, or if you throw them out when you take the thing out of the box.

i thought this second output would be useful as a rough monitor send, for headphone checks... old school. this was my biggest mistake, as way more users of the pre assumed it was a main out. i would never do that myself, and i assumed no one else would either. i am old and old fashioned... if you plug an unbalanced cable into that jack, of course hum can be coupled through. one has disabled the common mode rejection of the preamp.

so that brings me back to the mod... one thing that can be done to maximize the current available to the phantom supply AND the buffer, without altering the power supply much, is to do as the author suggests and move the power LED to the heater rail. grounding the 12AY7 grids removes them from association with the phantom supply, and without great penalty for the performance (there is a reduction of headroom and small increase of distortion, but the H pad in front of the 12AY7 provides a solution for that). the main reason for doing this would be because you own, or intend to, inexpensive modern condenser mics.

i cannot attempt to discourage you enough away from installing large value resistors in place of the 33K cathode resistors for the 12AU7 buffer. if there was more current available in the B+ mains, i would recommend reducing them! 22K or even 18K would make more sense. MORE current is what you want. here is what i would suggest as a compliment to this mod:

move the LED as suggested, ground the 12AY7 grids as suggested, readjust the current source so that the 12AY7 plates sat at 85-95 VDC... change the cathode resistors on the 12AU7 to 18K (from 33K). remove the 2 pin DIN connector and replace with a 4 or 5 pin DIN connector. remove the zener string that supplies the phantom supply completely, and carry a wire from the 50 volt pad on the string out to the new power connector. include a ground connection from the connector to power ground on the PC board. make a new "wall wart" with a 1.2 amp 12VAC toroid and a 48V @ 20mA minimum AC to DC converter module (these are made to supply vacuum fluorescent displays), and a new cable with a 4 or 5 pin DIN to match. a small "bud box" with room for an IEC connector, switch and fuse as well as the new trans and converter... done. perhaps a midi cable and 2 DIN connectors could be used as a ready made power umbilical? one could double up (use 4 wires) for the 12VAC, and 1 + shield ground for the 48V phantom... if you want to go 5 pin DIN.

last and very importantly, only use the 1/4" TRS jack with a balanced cable, or not at all.

my 2 cents...
Old 21st February 2012 | Show parent
Here for the gear

ahh, one last mod thought... having just looked at a midi cable, forget that idea. it can't carry the current. you'll have to make your own umbilical.

Old 1st October 2012 | Show parent
Gear Nut

You have any idea why my 12AY7 mic pre sounds like this (high frequency distortion)? The recording is not clipped in any way and the pre's clip led does not light up when I play. Trim pot is full but the distortion occurs even if the trim is half way. This has bothered me for years. Only solution is to turn the trim pot inside the pre but that lowers the volume a lot which is not something I want from an already low gain pre. Regards.
Old 1st October 2012
Gear Nut

Here is the attached mp3.
Attached Files
Old 2nd October 2012 | Show parent
Gear Nut

I have tried several other tubes. Some with the same specifications (12au7/ecc82 and 12ay7) and some others. Still the distortion is there. Can it be capacitor or something that is not up to par anymore? Regards.
Old 6th October 2012 | Show parent
Here for the gear

Hello ickefes. Are you running your guitar through a direct box? Impedance issue? I know some inputs do not like guitar signals.
Old 12th December 2012
Here for the gear

Just got my moded EHX 12AY7 Pre in mail from Steve Hunter (The Electric Church. Athens, GA). Steve used this mod for my unit. It sounds great and silent. Thank you!
Here is his page:
Old 18th February 2013
Gear Addict

man im getting two new ones pretty quick her.I hope i dont have any problems with noise.I also would love to do mods but when i read this stuff its like trying to read chinese or german, i cant understand or even know where to begin.Are there any of these steps that a first time modder could do to help the unit out?
Old 26th April 2018 | Show parent
Lives for gear
Originally Posted by Antoloco View Post
Hows things man thanks very much for taking the time to post this. I have been using this pre for around a year now as an upgrade from a presonus tube pre and have found it to be a worthwhile purchase. Definitely more life, body and dimension and worth the extra cash paid. It gets used quite regularly and has not let me down....yet, touch wood!... but one thing that I have found is that it is extremely sensitive to what it is connected to, and if everything is not perfectly balanced there will be a very disturbing hum that renders it nearly unusable. Fortunately I have cracked the code and found what it needs to be happy by experimenting with different connections. I would say this will be unique to each users own particular rig.

Anyway Im upgrading now to a GAP pre 73 in the "studio" and will attempt to use the ehx unit for live vocals as it seems to make my nd 767 mic sound almost condenser like and I think it might make a decent foundation for a modular channel strip on our live rig. God only knows how "upset" this pre will get in a live environment in the hum and reliability departments so I am strongly considering your mods as an ounce of prevention if I am happy with its sonic performance. That is of course if my basic electronics skills are up to the challenge Thanks again and I will post any updates on my experience(S) trying out these mods. All the best, Anto

Why are you trying to bring a studio mic pre, known to be troublesome to a show?

Unless your headlining large venues and have a team of dedicated roadies and soundmen (Maybe you do?) You have NO BUSINESS making EVERYONE'S life difficult by being so finicky about what you THINK your voice sounds like, that you are willing to cause the trouble bringing that mic pre to a show will cause, not for you, but for your band other bands (People might leave the venue when they see you screwing around with it mid-set)

If you can't get the job done with the 58 the soundman hands you, then you have no business playing shows.
Old 11th December 2018 | Show parent
Lives for gear
Theo Stobbe's Avatar


Originally Posted by terrible.dee View Post
You have NO BUSINESS making EVERYONE'S life difficult by being so...

If you can't get the job done with the 58 the soundman hands you, then you have no business playing shows.
Soundmen Running Business! :-)
Old 23rd December 2019 | Show parent
Gear Head

Originally Posted by Dr F. View Post
The missing "picture"

Dr F.
HI Dr. F, quick question please, are you sure the 12ay7 tube is supposed to go on the right side? I'm just confused, some say it's supposed to go on the left? Would be happy to have someone confirm with certainty. Also, could you damage the tubes or the device if they are reversed?

Thank you,
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