Ampex 6445 tube preamp questions

Dubmaniac · Post by **Dubmaniac** » Sat Jan 06, 2007 2:46 pm

Hi gang. This is my first post, so please be gentle. I have five Ampex 6445 tube preamps that I rescued from an old movie theater that was being demolished. I also scored four Ampex 6516 tube power amps- that was a VERY lucky day for me! They were all going to be thrown out...can you believe it?

Lately I've been thinking about the 6445 preamps. Does anybody know anything about them, and could they be set up as either microphone preamps or direct boxes? They have some nice big Triad transformers on top, and the tubes they use are 6AK6, 5879 and 6SF5- one each per preamp. Each preamp also has one module only marked "equalizer"- it's a round metal cylinder that plugs into a tube socket, but I don't think it's a tube. I would really like to start using these things to warm up my sound. They're beautiful- point to point wiring...not a single transistor to be seen! I'm guessing they were made in the early sixties. It would be a shame to not use these if I can. Have any of you used these or can you tell me more about them? I'd sure appreciate any help I can get. Thanks.

Dubmaniac · Post by **Dubmaniac** » Tue Jan 16, 2007 3:55 pm

So nobody has tried these yet? Anyone...? Bueller...Bueller...?

Feedback · Post by **Feedback** » Tue Jan 16, 2007 5:30 pm

Nice score. I ran in to a situationlike that with 2 Altec 436C tube compressors and a couple of Altec tube preamps. They were headed for the dump, I couldn't believe it. I jumped in and said I would give them a good home.

The Altec stuff uses "accessory" cans, which are an octal metal plug in type deal. Here is one so you can see what they generally look like:

http://cgi.ebay.com/ALTEC-1588C-MIC-PRE ... dZViewItem

Maybe the Ampex uses something similar? Different ones do different things. Just a shot in the dark as I am not familiar with the Ampex you have discovered. Having found this one on Ebay:

http://cgi.ebay.com/AMPEX-6445-Tube-Pre ... dZViewItem

It looks exactly like the Altec accessory octal cans.

I'm Painting Again · Post by **I'm Painting Again** » Tue Jan 16, 2007 6:41 pm

I think those ampex are power amps..

Dubmaniac · Post by **Dubmaniac** » Tue Jan 16, 2007 10:13 pm

Toolshed of Death- actually, I do have four Ampex power amps, which were used in a P.A. system with these preamps. I've been able to get the power amps to work, but I really don't know where to start with the preamps. Looking back, I wonder why they had five preamps and only four power amps...hmm...

Feedback- you found it! That second eBay auction shows EXACTLY what I've got, except that mine have the equalizers in them. So...how hard would it be to turn these critters into microphone preamps or direct boxes? Were you able to do that with the Altec preamps, or were they already set up for mics?

What really makes me sick is that I had the chance to not only get these preamps but also the unit that powers them- I think it was called a power tray, or something like that, but I didn't. At the time (almost 15 years ago), I didn't know as much about audio as I do now, and I didn't think I'd need it. Oops...well, live and learn. Now I'll be learning about powering up my preamps, I guess!

Feedback · Post by **Feedback** » Wed Jan 17, 2007 4:04 am

The altecs I have were designed to be mic pres. I am not sure what you would need to get going with your stuff. Sorry i am of no help on that.

Dubmaniac · Post by **Dubmaniac** » Wed Jan 17, 2007 11:24 am

Hi Feedback. It's okay...I don't expect step by step instructions on how to use these as mic pres (although that would be nice!)...I'm just hoping someone can tell me whether or not they might work that way, or as direct boxes! With those big old transformers, I would think they might have some vintage mojo in there somewhere...anyway, thanks for the...ummm...feedback!

Sorry, I couldn't resist!

RodC · Post by **RodC** » Wed Jan 17, 2007 12:37 pm

Your prob going to have to start drawing out one or 2 of the tube circuits. You will prob need B+ around 220 to 250v, heater voltage ect.

Start with the heaters and B+, start looking at the specs of the tubes that are in it. A lot of old data sheets will give you some example circuits and operating voltages.

Dubmaniac · Post by **Dubmaniac** » Thu Mar 08, 2007 11:35 am

Good news- I got schematics for the preamps last week, thanks to Vern Sauer at TSI in Arlington Heights, Illinois. He's a former Ampex service guy and an excellent resource for Ampex schematics. I also got schematics for the power trays- they're called the #6449 preamplifier tray, to be exact. All this gear was part of the Ampex Academy Theater Sound System. Here's what the little blurb said about it...

"The Ampex Academy Theater Sound System is designed for use with four track magnetic or optical films. It offers maximum compatibility with existing Ampex combined Optical-Magnetic Theater systems in use at Army and Air Force Motion Picture Theaters.

Not only may the Academy Theater Sound System be used for motion picture applications, but it may also be used to provide intermission material through a tape recorder or phonograph pickup, or for similar purposes."

Hmm...so it sounds like they were mostly made for military use, although mine came out of the old Cooper Cameo movie theater here in Denver. Anyway, I can scan and email the schematics to anyone who wants to see if these would be viable as microphone preamps or direct boxes, and how easy or difficult it would be to build power supplies for them. I do know that the schematics show a dual power supply. I think this was so that, if one power supply quit working, the other could be switched on. They make reference to that capability in the general description sheet that came with the schematics. Since they were designed for military use, I guess they wanted maximum reliability. Being a definite non-military kind of guy, I would only need a single power supply!

So...who would like to take a look at the schematics? Anybody...anybody? Bueller...Bueller?

RodC · Post by **RodC** » Thu Mar 08, 2007 6:30 pm

Scan them and I will host them. I'll take a look, I'm sure you should be able to get a PS to work.

punkerdubh · Post by **punkerdubh** » Thu Mar 08, 2007 7:20 pm

Well, the 5879 is a low microphonics signal pentode from RCA that can give the EF86 a run for the money. This tube is often used as the first stage in microphone preamps.

The 6SF5 is single high mu triode (like half of a 12AX7) and most likely sits in the middle for additional gain.

The 6AK6 is a power pentode good for about a Watt of output. This is your line driver (probably through a 10K to 600 transformer). I'd guess that these preamps are good for 60 dB or so.

Should be a fun project -- I'd love to see the schematics.

emrr · Post by **emrr** » Fri Mar 09, 2007 10:30 pm

I believe you can jumper the EQ slot and get flat response. Like to see the info when it's ready.

RodC · Post by **RodC** » Sat Mar 10, 2007 6:36 am

Dubmaniac sent me the schematics. Looks like that will be a fun power supply to recreate. Lots o taps on that tranny. The schematic shows a dual power supply, I guess incase something fails during a show the operator can just flip to the backup.

Also there are no indication of what the input transformers are, hopefully the cans are marked. They will prob have to be changed because they are for the head of the film projector.

I ftped them to my site here is a zip file of the PDF files:

Ampex6445.zip

emrr · Post by **emrr** » Sat Mar 10, 2007 8:26 pm

cool, thanks for hosting those. Looks like you mod as necessary to get that 105 VDc from the existing 300VDC and you're good to go. I'm sure an off the shelf International Power 265 VDC supply would work fine for the B+.

I'd put money on it being flat response transformer also. Pull the EQ out; I bet it's flat.

I wouldn't assume they were specifically for military use, just that they sold a lot to the military.

I dug through my pics, and found these with all UTC iron, all Triad iron, and both mixes of Triad and UTC. All in the same rack. So I'd guess it was whatever they had in stock on a given assembly day, and the winding is basically the same.

RodC · Post by **RodC** » Sun Mar 11, 2007 6:35 am

Yep, after I looked at it a bit closer I bet you could just mod a 300V supply to get the 105.

Triodeelectronics.com has a 320 that looks promising.

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Ampex 6445 tube preamp questions

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