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Ampex 351 Preamp Modification Info!

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Re: ampex 351 mod power supply

Postby greg on Sat Feb 03, 2007 10:20 pm

bigfatchampion wrote:hey greg,

i wanted to know if you have any schematics for the power supplies you built for EA 351's? i'm just getting started on the mod and i'm trying to gather as much info before i start.

thanks

We use the original design with bad parts replaced. If your talking phantom power, I assume you could monkey rig a regulated supply off the high voltage. I haven't done that though.
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ampex 351

Postby omaroski on Tue Jun 17, 2008 9:59 am

How much did you/would you pay for it? Not modified, in good conditions.

I've found one here in Italy which i would like to put my hands on but i don't want to end with an overpriced thing which i don't know when i will have the time to mod and probably i will use it for a long time unmodded.

It's 1K EU (1K550 USD).

Thanks!

edit: me fucked! I didn't want to reply to this thread, i wanted to start my own, sorry.
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Re: ampex 351

Postby otisroom on Tue Jun 17, 2008 12:23 pm

omaroski wrote:How much did you/would you pay for it? Not modified, in good conditions.

I've found one here in Italy which i would like to put my hands on but i don't want to end with an overpriced thing which i don't know when i will have the time to mod and probably i will use it for a long time unmodded.

It's 1K EU (1K550 USD).

Thanks!

edit: me fucked! I didn't want to reply to this thread, i wanted to start my own, sorry.


Wow that seems like a lot for a pre that's not that useful. I mean it has it's purposes but still it's a one trick pony.
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Postby steve on Tue Jun 17, 2008 12:56 pm

I don't think I'd pay more than $700 for an unmodified unit. To be honest, I bought the batch of them that started this whole project because I got them real cheap -- like $200 bucks cheap. If they weren't cheap I wouldn't have bothered. I'm glad I did, and they're great and everything, but unless you've tried them and liked them, I wouldn't stretch too far.
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Re: ampex 351

Postby greg on Tue Jun 17, 2008 4:23 pm

omaroski wrote:How much did you/would you pay for it? Not modified, in good conditions.

I've found one here in Italy which i would like to put my hands on but i don't want to end with an overpriced thing which i don't know when i will have the time to mod and probably i will use it for a long time unmodded.

It's 1K EU (1K550 USD).

Thanks!

edit: me fucked! I didn't want to reply to this thread, i wanted to start my own, sorry.

Is it a fully functioning tape machine, in wonderful condition?
If they are selling it as a "preamp" don't buy it. ...knowing what it takes to make them into reliable studio preamps, I wouldn't pay anything over $150US for one in great condition. They are selling hype right there.
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Postby greg on Tue Jun 17, 2008 4:27 pm

I will update the crappy schematic and description soon. There are a number of things I should note since recently reconditioning a few of these for people. First is a simple phantom power supply.
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Postby otisroom on Tue Jun 17, 2008 5:24 pm

steve wrote:I don't think I'd pay more than $700 for an unmodified unit. To be honest, I bought the batch of them that started this whole project because I got them real cheap -- like $200 bucks cheap. If they weren't cheap I wouldn't have bothered. I'm glad I did, and they're great and everything, but unless you've tried them and liked them, I wouldn't stretch too far.


Yet another trend you've started.

You trend setter.
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Re: ampex 351

Postby omaroski on Wed Jun 18, 2008 2:17 am

Is it a fully functioning tape machine, in wonderful condition?


Uh sorry i forgot to say it's just a single preamp there.


unless you've tried them and liked them, I wouldn't stretch too far.


I have the chance to try it but the seller would not lower the price so i guess i will not even go to test.

Wow that seems like a lot for a pre that's not that useful

I don't expect to get them at 150-200 USD. Here a bit of overpricing is allowed for such items, but ten times more is too much.

Thank you all of you
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Postby adrimusicien on Sat Oct 11, 2008 2:14 pm

hello everybody,
i'm french and i've an ampex 351, i'd link to modify it to keep the preamp and remove the rest of the electronic from the signal path. i've to replace the capacitors to...
i'd like to see the pdf shematic that is at the beginnig of the post but the link doesn't works, if someone could post it again it could help me very well! and if you have some photo of your modded electronics it could be cool to!

thanks!

adrian

http://www.myspace.com/artscenestudio
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Postby greg on Mon Oct 20, 2008 9:48 pm

greg wrote:I will update the crappy schematic and description soon. There are a number of things I should note since recently reconditioning a few of these for people. First is a simple phantom power supply.
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Postby madraso_ on Tue Oct 21, 2008 2:02 pm

a while ago when I was at the preliminary stage of taking on this project (with four 351's) for a friend, I found some old trace layouts from Ampex, photoshopped out all of the old components and then drew in the new ones (and jumpers) as per the mod.

Admittedly, this was over 1 yr ago and due to a busy life, I have not yet started assembling them (we have all the parts including what seemed like the last 4 NOS Daven attenuators on earth when we were trying to find them.)
If I remember/get motivated I can scan them and/or post links or something like that. It was not too difficult to do, and makes assembling the boards a breeze!

will update as time permits.


I guess my tense in the first sentence should be corrected to reflect the fact that I am still in the preliminary stage...
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Postby greg on Fri Nov 14, 2008 8:37 pm

Phantom power board, and picture update on first page.
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Postby greg on Sun Nov 16, 2008 11:13 pm

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Re: Ampex 351 Preamp Modification Info!

Postby recordinggeek on Tue Dec 16, 2008 2:15 pm

I have been working on the preamp mod for the Ampex 351, but with certain modifications:
1) A regulated 12V heater supply.
2) A built-in phantom power supply (using the old "DC heater" transformer winding).
3) Stepped resistor ladder for "record level" control.
4) Existing transfer switch used to provide input pad and a DI function.
5) FET buffer for DI input (input Z about 3Meg).
6) Homebrew stepped "U" pad at output (playback level).
7) Output transformer moved far away from power transformer to reduce hum.
8) "Record cal" pot removed.

I also plan to use the "EQ" switch to let me pad out the second stage gain.

One goal here is to provide as wide a gain adjustment range as possible, with max preferred gain being no more than 70 dB or so. Imagine my surprise when I found the "wide open" gain to be about 91 dB! :o At first I thought hum and noise levels were way too high (somewhere around -30 dBu, IIRC), but then I realized that should be expected with so much gain.

One part of the EA mod that I did not notice at first is a change to the plate load and cathode bias resistors in the phase inverter of the line amp. Does this change materially reduce the gain of the line amp, and if so, how much?
I note that your "changed" 39k feedback resistor from the output transformer is the value shown in the original Ampex schematic. So... how is that a change, exactly?

What is the maximum gain you normally expect from one of your complete modified 351's??
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Re: Ampex 351 Preamp Modification Info!

Postby recordinggeek on Wed Dec 17, 2008 8:26 am

About the "excessive gain" thing....

Last night I changed out the plate load and cathode bias resistors on the phase inverter in the line amp (per Greg's updated schematic) and then ran some more tests to see what changed.

Before the change the maximum gain was just over 93 dB, and after it was just under 90 dB. When I cut in the input attenuator, the gain drops to about 65.5 dB. Additionally cranking my stepped gain control to minimum gets the gain down to about 42 dB.

I also did some noise measurements. With a 150 ohm terminator at the mic input, at full gain the output noise is about -37 dBu, which gives an equivalent "referred to input" noise level of just below -126 dBu... pretty good actually, considering the crazy high gain here. Switching in the input attenuator actually RAISED the noise just a hair, and cranking the record level to minimum (24 dB attenuation) gave a reading of about -41. I scope and listen to the output as well as metering it, and it sounds like the hiss actually drops more than that. RFI may be a factor here, as I live only a couple of miles from an AM transmitter site (which I worked at some years ago). When I try to look at really low level stuff, I can often see modulated RF on the scope.

One other thing I notice is a tendency toward microphonics. Tapping on any tube or either amp card "rings" a bit. A rap on the front panel will do this also (but not as much). Again, I am not certain whether I am seeing a real problem, or if it is just a case of "well yeah, crank up 90 dB of gain and you WILL get stuff like that".

I would still like to know whether my results are in line with what should be expected when you feed the second stage directly to the input of the line amp.

So far, I am considering 3 alternatives to drop the maximum gain to reasonable levels:
1) Bypass the second gain stage, feeding the output of the record gain pot directly to the line amp.
2) Change the plate load and cathode bias resistors of the first two stages to reduce the gain of each stage.
3) Use a lower gain tube (with changes to resistor values if needed) to reduce gain of the first two stages.

Any one of these options, though, might materially affect the sound of the preamp, which might reduce its value (I will eventually sell this thing, as I have a hard time with the idea of using up 4U of rack space for a single channel mic preamp, no matter how "cool" it is).

Greg, (or anyone else) any advice would be appreciated.
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Re: Ampex 351 Preamp Modification Info!

Postby mixedupsteve on Wed Dec 17, 2008 2:17 pm

Just a quick question about the 20dB pad. First off, could this circuit be used on any mic or line level source? Also the schematic shows that when it's applied it leaves pin 2 floating with the signal from pin 3 routed back through the resistors to make it balanced again(hope that's right and makes sense). Is R101 only acting like an inverter there, or what?
Thanks
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Wed Dec 17, 2008 4:18 pm

mixedupsteve wrote:Just a quick question about the 20dB pad. First off, could this circuit be used on any mic or line level source? Also the schematic shows that when it's applied it leaves pin 2 floating with the signal from pin 3 routed back through the resistors to make it balanced again(hope that's right and makes sense). Is R101 only acting like an inverter there, or what?
Thanks

Nah,
I screwed that up. Thanks for pointing that out.
I will correct it when I get out of session.

The overall gain of our 351s is set to 65 dB. I either set it with the "cal" pot, or mimic the "cal" pot with a voltage divider of the correct value.

I need to double double check the schematics again.
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Re: Ampex 351 Preamp Modification Info!

Postby recordinggeek on Wed Dec 17, 2008 9:40 pm

greg wrote:The overall gain of our 351s is set to 65 dB. I either set it with the "cal" pot, or mimic the "cal" pot with a voltage divider of the correct value.
Woof! By my measurements, the correct pad would be 25 dB, which is more than the gain of the second stage! Before I did my proper gain tests, my plan was to put a switchable pad (about 24 dB) between the second stage and the line amp (in place of the cal pot).

Somehow having a permanent pad in place that is equivalent to removing one whole stage of gain seems a bit silly, and I cannot think of any reason why the total gain of the unit (without such a pad) should EVER be more than 70 dB, and it is quite possible that 65 dB would be a more sensible upper limit. Would it not make more sense, then, to simply drop the second stage and feed the wiper of the record level control straight to the line amp input? Or is there some other important bit that I am missing??
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Thu Dec 18, 2008 12:48 am

I don't think you're missing anything. After playing with more gain, we settled on 65. It is rare that we ever need more than that from any preamp. We kept that second stage because it is part of the circuit (I'm guessing. That decision was made when I was a lowly intern here). I suppose if we had problems with noise, we would have looked at it closer.
You should try bypassing that second stage and see what you get.
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Thu Dec 18, 2008 1:04 am

recordinggeek wrote:One part of the EA mod that I did not notice at first is a change to the plate load and cathode bias resistors in the phase inverter of the line amp. Does this change materially reduce the gain of the line amp, and if so, how much?
I note that your "changed" 39k feedback resistor from the output transformer is the value shown in the original Ampex schematic. So... how is that a change, exactly?

Where do I say that? I need to clean up this thread now that the newer schematic is up.
The new plate resistors do lower the gain, but not by much. That was done by a previous tech, why? I don't know for sure. I believe it might have to do with the new B+ voltages (less load from missing tubes, higher voltage).
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