rebuilding/modding an old Ampex 351 for a research grant

I recently recieved funding to do a research grant on valve electronics where I am able to rebuild and modify the electronics of the Ampex 351 into a mic pre. I've done a couple of small guitar effects projects and general amp and equipment repair but this will be by far the most in depth and challenging project for me yet. As of now, I have about $375 to spend on equipment and whatnot. The catch is that I already have the 351, with the original PCBs and big ol transformers.

I have alligator clips, a soldering iron, various tools, but no multimeter. So my questions are this:

1. where should I spend my funds?
2. I've seen the Electrical Audio schematics for their 351's, has anyone tried this project and had good results?
3. are there other tube preamp designs (like the V72) worth looking at as a model for my design?

there are no real rules with this grant. The purpose is for me to understand the signal flow and interworkings of a tube preamplifier.
Thanks in advance!

kevin-

you dont need to redesign anything to use a 351 as a mic pre. its all there, just plug a microphone in. NYDave has posted frequently over the years about replacing the selenium rectifier if your unit has that, change it. Another thing to do is address the VU meter, either disconnect it from the circuit or put it on a switch so you can shut it off. The level you'll be outputing in most mic pre circumstances will be off scale for what that meter is designed to read.

if you are looking at redesigning the thing, you should just build something from scratch, the circuit is already complete for you. I would disconnect the bias circuit as you dont need it, redo all the wiring inside, just really strip the whole thing down, but keep the circuit.

before you start are you aware that working on that 351 can kill you? like, real death? There is high voltage running throughout that chasis so just make sure you know what not to touch if you are new to working on tube electronics. Not a big deal, but kind of a big deal.

dave

thanks for the advice. I forgot to mention that the 351 was already dissassembled. I know about the high voltages because I've worked on my tweed deluxe before, but nevertheless, do you think it's good to work with a variac?

Hmmm. I don't know anything about any VU meter problems.
All of mine are dead on. 0VU = +4dBM out.

You should disconnect the VU anyway unless its buffered, which its not in the 351, right?

M

[quote author="mattmoogus"]You should disconnect the VU anyway unless its buffered, which its not in the 351, right?[/quote]

Right.

As for other things to do...the 351 was never designed as a mic preamp; its mic circuit was strictly a utility job for emergencies. The machine was intended to be a line-level tape recorder.

If you're intent on turning it into a mic preamp, my suggestion is to redesign the preamp circuitry, which right now is a 12AX7 with way the hell too much gain and high distortion. Perhaps this would be a good place for something like an EF86 wired as a triode, perhaps as a feedback pair with a 12AU7.

Peace,
Paul

what tubes did the V72 use? can I still use the same boards (with some jumpers) if I want to modify it for different tubes? I've seen the schematics for the V72 before, and it seems pretty simple and compact, is it really the design that everyone makes it out to be, or is it because the Beatles used it at Abbey Road?

On a different thought here, what is the best multimeter in the $100 range to get for a project like this?

The V72 is all about the input transformer. The circuit isnt hard to make but getting the transformers, especially that input with its massive step up is the problem. Appart from that I think the tubes you need to make a copy are around $100 each, at least they were last time we had to buy some.

There are heeeaps of tube modules of various kinds on ebay all the time. Often for pretty cheap too, if they arent well known. Thats a good place to look for other designs to become familiar with and restore/mod.


M@

This whole deal with the "grant" sounds a little fishy to me--or maybe I've just been hanging around with the wrong people! But if your goal is truly to learn about how a tube microphone preamps works--and not just to pimp your 351 to make it "kewl and phat" for your studio--then you should hit the books before you worry about the particulars of 351s, V72s, Steve Albini or whatever.

Take some of that money and sign up for an associate or student membership in the AES. You'll have access to a wealth of information, downloadable technical papers, at a discount rate. There are several papers from the '50s and early '60s that are concerned with the design of tube preamplifiers. And of course, there's a number of worthy textbooks out there, some of them even available for free off the web. Here are some links to get you started:

http://geek.scorpiorising.ca/RDH4.html
http://headfonz.rutgers.edu/RDH4/
http://www.pmillett.com/tecnical_books_online.htm

I'm certainly not suggesting that you need to read and absorb all these books and papers before you pick up your soldering iron. But book-larnin' in conjunction with solderin' and tweakin' makes for a more meaningful learning experience than, say, simply swapping caps, copying a circuit or mod or following instructions from a bunch of strangers on the Internet. :wink:

I would like to take this opportunity to debunk the v72.

I have never known a single engineer who has used them with any regularity because they are not useful mic pres. They are line amps with a very small amount of available gain, somewhere in the 30dB area. The people who have made these popular are gear slutz types that have never heard them, never used them and dont understand what they are. They are absolutely NOT usable mic pres, you can maybe get away with one on a guitar cab or on a drum kit depending upon the mic you have plugged into it. There are people that have modded them by increasing the feedback to get more gain but to my ear the sound of them changes radically when you do that. V72s most certainly sound nice, there's no doubt about that but maybe some one in the universe can explain to me how a 30dB mic pre is practical, if it was solid state with that amount of limited gain, what kind of reputation would the unit have?

The v77 or V76 are the mic pres to look at. How either would relate to the idea of a 351 is anybodys guess.

The meter is unbuffered and it does operate just fine at +4dBu, but all the color in the unit is at the top of the gain structure, whenever I use mine, which is rare, I use it off scale to find the sweet spot of the amp. The meter is IMO worth more than the thing as a mic pre as a whole, it woudl be a shame to break it using it like that.

if you really want to learn something, I would start from scratch and build something cool, at least then you'll have the potential at making something awesome instead of starting with a platform that is already a total compromise. The wiring in those things is a real PITA, if you build from scratch you'll be able to neatly layout something that wont be a mess when you are done.

there are probably somewhere around a gazillion quadra trillion designs for tube amps out there that will be better than that 351. What many people leave out of todays 351 lore is the fact that they were considered "cool" because back in the day they were either free or $3. Nobody got exicted about the sound of the things... There are wayyy cooler mic preamps to build, some very very easy, lots of them discussed on the board regularly. Here's a thread on one such pre-

http://www.groupdiy.com/index.php?topic=14341

dont mean to discourage you, Ive worked on many 351's and never enjoyed either the process or the end result. The fact that you talk about redesigning it is enough to suggest just abondoning it and starting with a clean empty chasis so you can work forward instead of constantly having to trace the mess that is inside that 351 to get anywhere.

dave

NewYorkDave said:

This whole deal with the "grant" sounds a little fishy to me--or maybe I've just been hanging around with the wrong people! But if your goal is truly to learn about how a tube microphone preamps works--and not just to pimp your 351 to make it "kewl and phat" for your studio--then you should hit the books before you worry about the particulars of 351s, V72s, Steve Albini or whatever.

Click to expand...

It's all good man, but thanks for your concern. I'm sort of one step ahead of you. I'm already a member of AES, I've found a great deal of reading material from this forum, and my current read is "Valve Amplifiers" by Morgan Jones. I also did not mention that $275 of the grant money is going to research and reading materials. I only listed the $375 that is allocated toward equipment and electronic materials. When it comes down to it, I'm just a student who wants to learn more than what they teach at my university. I have 1 year to do my research and present my findings which will culminate in the construction of a tube pre.