Mic Input Transformer (Ampex 600)

[El Fito]

Hi all,

After successfully recapping my newly acquired Ampex 600, I was thinking of fitting an input transformer to make it useable with modern mics. These beasts didn't come with input or output transformers inbuilt and from the manual I gather that the rated input and output is 10K.

Now for the technical bit: I looked at Sowter's website and was thinking that maybe a model 4935 (http://www.sowter.co.uk/specs/4935.htm) would suit :?:

What do you people think? Any help/ideas would be greatly appreciated.

El Fito

 

[MikoKensington]

Umm. If it's a tube model. There should be an input transformer. It's an octal plug-in.

 

[wilebee]

My 602-2's has small BEYERS input transformers in the front next to the meter about the of a dime in diamiter the socket was for a bridging input transformer Wil

Wilebee

 

[El Fito]

Guys, thanks for the answers. I just received an email back from Brian Sowter. He says that the 4935 is indeed the right trafo for the 600s input. I'll place an order and let you know how it works.

 

[rattleyour]

I have an Ampex 601, it is indeed transformer balanced at input.

EDIT: Bad Info

 

[gyraf]

While adding input transformers to tubed equipment is trivial, adding output transformers needs much more care.

You will need to determine your (often rather high) output impedance and find a transformer that will be able to transfer audio low-end at that source impedance.

As the output tube stage is optimized for no-trafo use, I think there will be little gained by adding output transformers.

Jakob E.

 

[CJ]

A lot of those Altec/Ampex plug in x-formers show up on evilbay from time to time.

 

[El Fito]

According to the manual, the Ampex 600 has an output of 10K ohms. Do you still think it's fine to leave it alone, or would it be preferable to mod it with an output trafo to lower that to 600 ohms?

Alternatively, do you think it'd yield better results if plugged into the mixer's hi z input?

 

[rattleyour]

Thanks Jakob--

Perhpas I'm getting a little transformer obsessed!

The 601 has certainly served me well as is, it can be very pleasing in combination with my (otherwise harsh) AKG C-1000.

Also, you get an interesting fuzz bass sound by overdriving the front end-- in my case I use a v-twin to accomplish this.

 

[aronaut]

Umm. If it's a tube model. There should be an input transformer. It's an octal plug-in.

the 600 model has no input transformer, the 601, 602 do.

 

[El Fito]

OK guys, an update has been due for some time now. I finally installed the Sowter 4935 trafo at the input stage. Fitting trafos and things inside the 600 is a joy, as there's plenty of space for tweaks.

Anyway, the 600's nominal input and output is rated at 10K and something tells me that even with a mic and a recorder to match, it would have been a bit noisy. None of my mics nor my tascam 2488 matched that impedance level, so the sound was pretty distorted and noisy for pro recording purposes. So I installed the Sowter trafo.

Since I had lifted the bonnet already, I decided to go one better: I installed an on/off switch to either route the signal through the trafo or leave as per the original. I also installed a 1/4" jack input next to the xlr input to use with bass etc, as the Ampex tube pres are good at handling bass.

Anyway, here's for the sound: I couldn't quite tell the difference at first. Then I fiddled with the grounding, made sure it was duly attached to the chassis, etc... and it started saturating less and the noise had noticeably diminished... but there was still a bit of a nagging hum (just a bit) and the output was still way too hot for moder equipment to handle decently.

So what did I do? well, I happened to have lying about a cheap mic to instrument Z transformer that I had snatched on evil bay for a fiver (those that come inside a long fat adaptor with a female xlr at one end a 1/4 plug at the other and a switch that selects Hi or Low Z).

I said: what if I used this as the output trafo? So I did, out of sheer curiosity. And guess what? What a difference! My old Ampex 600 now handles my Rode K2 like a breeze, no noise, no saturation, no hum but all the tube character of the Ampex pres! And it even handles my newly built Ribbon mic ( take a look at this: http://www.groupdiy.com/index.php?topic=7218&highlight=) like you wouldn't believe!... and you can imagine what a nice combination the ribbon mic and the Ampex make!

Just to do things properly and not to skimp on those really important details, now I've determined that I have plans to buy another Sowter trafo for the output stage :grin: .

 

[bradzatitagain]

It's built to output to tape heads, so...

Have you ever tried the headphone output? It's at about 2K iirc and the trnsformer while kinda small is definitely bigger than the one in the hi-to-lo z converter. You'd probably like a UTC A-25 or equivalent if you're using this as a mic pre.

 

[bradzatitagain]

[quote author="wilebee"]My 602-2's has small BEYERS input transformers in the front next to the meter about the of a dime in diamiter the socket was for a bridging input transformer Wil

Wilebee[/quote]

Hey Wil, the 602 is the same circuit as the PR-10 recorder. A UTC A-10 is essentially what the input for the 600 and 601 would call for. Your Beyer was the same thing found in the MX-10 and 35 mixers, saturates if you look at it the wrong way. Nobody mic'd close back then.

E.F.: you pick off the mic pre at the record level pot, eliminate the head amplifier, wire in an output like the UTC a-25. If you're humming at the output jack to your board you might try terminating the line. But the siganl going to what would have been the heads is the signal you want, not the monitor signal--the output jack's supposed to plug into a 620 suitcase amp, that what it was built to do. Use the play head signal as DI, etc etc. Already mentioned the headphone jack which you can also succesfully use as an output to a consumer hi-fi input. Have you recapped the power supply in this thing? Trash those multisection caps and check the dropping resistors for spec. If you're pulling tubes and chopping out circuits you're going to have to rework the psu to provide correct plate voltages at the tubes that are left over.

That 6F5 can be noisy too, that's why Ampex stopped making these, they couldn't get low noise ones from RCA anymore, jammed the PR10 circuit in the chassis and called it the 602, last gasp before the AG600.

Personally I think it's a waste of space to use that chassis if you're just going to run one circuit. Yank all that crap out of there, put the tube sockets on the back of a 2U box; if you ever get hold of another one you could probably run two pre's out of one 2U box. You probably have enough juice to roll two circuits from that psu anyway. Sell the meter on eBay, make some money, pull the Ampex tag off the front and make a tie clip.

I'm kind of sick of all these ancient rack whores, they take up too much room. I just gutted my remaining 960's and I'm trying to get 4 circuits in a 3U box times 2. Better circuit than the 600's anyway, same as the 351 without the iron, built for dynamic mikes and consumer hifi line levels. Get CJ to spin a bunch of HA100X's and get the output iron from Sowter, ta-da. Nobody's supposed to be rummaging around in the control room anyway so who cares if there's a wall of Ampex tape amplifiers in there or not.

 

[GregL]

I have a pair of 601 preamps and they have output trafos (as well as being shown on the schematic in the manual). They however are not particularly good ones. Since there isn't space inside the box (yet) to put another one I have run these with some old Altec 15095 connected in reverse,plugged into the output jacks. Initial listening results sounded pretty good. These came with some very nice Altec inputs (the real nice ones, not the octal ones) but I appropriated these for another project and am installing UTC A11's which should be perfect.
Running the trafo on the output also dumps some of the excessive gain.
Will post when I finish the project on measurements and results.

 

[bradzatitagain]

[quote author="GregL"]I have a pair of 601 preamps and they have output trafos (as well as being shown on the schematic in the manual). They however are not particularly good ones. Since there isn't space inside the box (yet) to put another one I have run these with some old Altec 15095 connected in reverse,plugged into the output jacks. Initial listening results sounded pretty good. These came with some very nice Altec inputs (the real nice ones, not the octal ones) but I appropriated these for another project and am installing UTC A11's which should be perfect.
Running the trafo on the output also dumps some of the excessive gain.
Will post when I finish the project on measurements and results.[/quote]

A-24 backwards going in is good for free gain, around 5dB. I'd be curious to know what the cheapo Edcor would sound like going out.

I still think somebody who's currently fiddling with one of these should check out the signal that goes to the heads, not the line out signal. Pick it off at the record level pot and insert you favorite hi-z to line level transformer.