Telefunken V72 DIY

I started building the V-72-S mic pre yesterday. It looks like it will fit in this small rack which will keep noise down. The rack is a surpus video router that has been gutted. Paid 10 bucks. Has a pretty heavy duty plating job which should help shielding. All the resistors are NOS German resistors from the old days. Scored them at a surplus store. Not carbon comp. That's all the parts you need for the PC board. Going to lay down the two tubes inside the chassis and solder the pins directly to the lead wires, so there will be no switches or contacts in the signal chain.

Using the Peerless K-241-D that I partially dissected a while back. Down 1 db from 3 hz to 30,000 hz without resistive termination. Has 90 db worth of shielding plus whatever I get by having it inside that compartment.:

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/72_1.jpg

I got this nice beefy permali turret board out of some stuff I scored at Andy's garage. Same stuff used in the original V-72:

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/72_5.jpg

I am using an external regulated pwr supply whcih will feed this additional pi filtering. Those are NOS Elkos like the original which will have to be brought up slowly. There's a NOS Phillips in there also:

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/72_3.jpg

I hand wound this three chamber bobbin for the plate choke. I squeezed on 8000 turns of #39 without a break. Going to lam it with those 80% Super Q lams. Had to fab the seperators as nobody makes a three chamber bobbin anymore. Probably will have less inductance than the original 300 Henries since I do not have Ollie's winding machine to wind the #44 hairwire, but it should still sound good.The bobbin is the same one used in the API output.

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/72_4.jpg

Going to use a Triad HS-50 output transformer.

Should sound pretty good. Based on the V-72-S for EMI studios. Only difference is an extra cathode resistor in stage one, and a differently wound input transformer. I will let you know how this thing sounds.

cj

CJ. You're nucking futs! Bless your bench my friend. :thumb:

I gotta take a picture of my winding machine. If you wanna see something nutty!

I got tired of spinning the wooden flywheel, so I took an old innertube from my ten speed bike and strung it between a hand drill and the winder. I taped the trigger down on the drill at medium speed and sat the couch on top of it to hold it in place. Worked like a charm! The tube ran right off the hand chuck. I controlled the tension by pulling the winder away from the drill moter. That way if I got a snag the bobbin would stop spinning. The funny thing was is that the couch was smashing the motor so that the further I leaned back on the couch, the faster the motor would run. Wanna slow it down, lean forward! Automatic speed control. Sure saved a hell of a lot of time. :razz:

looks cool man!

can you really get an old cap to perform as good as a new one?

dave

Well I had those Elko's laying around, there free, so WTH! Think Funk. If I know they are the originals, I can trick my brain into thinking they sound cool. :razz:

Hi CJ,

Very cool. I'll be curious to hear it. Ollie once loaned me a pair of his V72s, and I found them a bit noisy on quiet acoustic instruments. Or maybe I had a ground loop... I'm sure yours will exceed the standards and come out amazing, as your V76 did.

I gotta get some free time and give you a call...

cheers,
-tommypiper

CJ, you´re the guru!!!
:thumb: :thumb: :thumb:
Jens

Disaster on turn six: when I laced up that coil, the winding was to big. The E lams sliced thru one of the wires! Dang. So if you wind a coil, make sure you don't put too many turns on it. Leave a little clearance in between the bobbin edge and the windings.

OK, I just figured out a cheap DIY winder. Go down to the hardware store and get yourself a 20 dollar variable speed hand drill. Then take a 3/8 inch bolt and wrap some masking tape around the end til it fits snugly inside the bobbin you want to wind. Then put the bolt thru the bobbin and chuck it in the drill. Put the drill in a vice and you now have a coil winder. Works great! Tried it last night, wount the V-72 bobnbin in about 5 seconds.

Next I am going to run some wires out of the drill into a pot so I don't have to use the trigger. If you want, put an old tape recorder tape guide in the drill motor and run a fan belt out to a spindle. Then you will have a semi clutch in case the wire hangs up. The quicker the wire comes off the spool, the better, as the centrifugal force keeps the wire from touching the bobbin.

Great stuff CJ :thumb:

...Those are NOS Elkos like the original which will have to be brought up slowly...

Click to expand...

What d'ya mean by 'brought up slowly'? I've got some elkos like that which I was planning on using in the la2a psu - is there something to take into account when handling these?

Well, they have been sitting on the shelf for about 40 or more years, so you might have to reform the electrolyte.

What d'ya mean by 'brought up slowly'? I've got some elkos like that which I was planning on using in the la2a psu - is there something to take into account when handling these?

Click to expand...

You have to recreate the small Al-oxide dielectricum between Aluminium and electrolyte by powering up the cap very slowly since this process takes time. I use for powering up everything new an adjustable transformer which goes from 0 to 230V (in fact for controlling current with a meter to see if things go wrong). Such a device can be used in this case for ultraslow powering up your la2a with those old caps.
Jens

Nice work CJ! Would it be difficult to convert a V74 line amp to V72? I have a V74 which I want to convert to the preamp?

hey how about a variable speed control "foot pedal"?
ya know like a sewing machine uses.
BTW I like your preamp and all the info here
thanks for posting it.
mebbee i'll try this lil Toobie Snack project too.
ts

First off, there is a lot of good V info here:

http://recforums.prosoundweb.com/index.php/f/39/6831/?SQ=42bd4ec2397cdbcbacda940236ac4e62

And here: http://www.tab-funkenwerk.com/

The V74 is basically a buffer line amp with low gain, but you could probably use a lot of parts from it to make a V72. You could use the chassis of course, the pwr supply if it's wired for your voltage, the tubes, the plate choke, and the output transformer. You would need to use input iron with a higher ratio. Original V72S was 1:20 I believe, where as the 74 is 1:2. So you would lose a lot of voltage gain from the get go, and input impedance might not be right for some mics.

So, get a new input, use the 300 hy first stage choke as the output choke, put in a plate resistor and coupling cap on the first stage, re-work the feedback loops, remove the cathode choke on the last stage, in short, just rewire it to the V72S circuit.

One neat thing would be that you have multiple outputs for splitting the signal.

I am tearing apart my twist drill as we speak to mod it into a winding machine.

cj

OK, making some more progress, drilled and tapped some phenolic blocks to hold the transformer in place. A couple of 6-32's used to grab the lip by the leadplate. Will shim it in with some foam glued to the top and side. QAlso have the xlr and 1/4 inch roughed in:

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/box_1.jpg

Nice short run from the jack to the input iron:

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/box_3.jpg

Phenolic standoff's used to mount the turret board:

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/box_2.jpg

Gettin there.

Here's how I am winding the plate choke. Just some 1/20 hardware, some tape, a bobbin, some wire, a drill motor is all you need.

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/wind_1.jpg

If you make your own three chamber bobbin like the original V-72 choke, use a nice stiff material and don't just tape it to the bobbin. (One of the seperators is .030 nomex. The other one is made cut out of one of those black dvd movie cases.) Glue them down good. I used 3-M weatherstrip adhesive from the autoparts store, which will glue anything together. The reason for the stiff chabers is that as you wind, the turns will gradually dam up on the seperator and push it over, which loosens up the coils and causes problems. You can see that one of the sections in my first attempt is bigger than the other two. (first post)
Here is the mandrel and bobbin ready to go:


http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/wind_2.jpg

Spinning on some turns:

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/wind_3.jpg

OK, this is gettin easier by the second. I remember back to my transfomer days and noticed that the small coil winders did not need any hands to wind once they got set up. So I figured why hack up my hand drill to put in a foot controler.. So I came up with this setup:

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/wind_4.jpg

Drill in the vice, and this DIY wire guide/tensioner mad out of c clamps, a nut driver, some coreprene pads and some tape:

http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/wind_5.jpg

I can dial in the wire tension with the small c clamp, and set the wire angle by rotatating the vice. Now all I have to do is squeeze the trigger and spin on the turns. The long distance to the tensioner means that the wire will wind itself back and forth without having to guide it.


http://vacuumbrain.com/The_Lab/TA/Telefunken/V-72/wind_6.jpg

CJ, inspiring stuff as always :thumb:

I use for powering up everything new an adjustable transformer which goes from 0 to 230V

Click to expand...

Where can I get an psu like this to reform these old caps then? Is it easy to make one with that voltage range?

Ta

It´s not a PSU, it´s just a transformer with adjustable secondary. You get AC! I power up the Device under test including its own PSU. So if you have a tranny like this (what´s the name in english therefore?) built your diy and do a very slow powering up. But I have no idea how long it might take to reform the dielectricum. It´s a chemical process so give it maybe some minutes.
:sam:
Jens

if it smokes turn it down :wink:

Hey,

I've never seen/used or know where to get transformers with adjustable secondaries which vary from 0-230V...is it just a regular transformer with some extra shit added or...?

It'd be handy if anyone is up for reforming these caps i got... You can keep a few if so - If not and i cant find this adjustable transformer thingy I'll just power up as normal otherwise :guinness:

[quote author="daArry"]If not and i cant find this adjustable transformer thingy[/quote]

The adjustable transformer thingy is called a variac and it acts just like a regular old transformer, except that you can adjust the output voltage. In other words, it spews AC, so you can't reform caps with it - you need a DC source.

Peace,
Al.