Telefunken v41a / b DIY

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shoonbassjp

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May 25, 2014
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Hi if anyone can help me, really appreciate

I am looking at telefunken v41a / b  schematics to build EF12 mic pre
and see the 2nd 3rd gain stage has choke coil used +B supply to plate

wondering whats the spec I should been using alternative local brand DC choke

I looked for the META page and WEB but seems no luck  finding the data of this choke
 

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moamps

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I have no specs for CKV41 chokes, but believe you can use there chokes for TFK V76: BV514 and 515.
Or you can design it for yourself; you know DC anode currents (2 and 7mA) and Xl should be around 100kohm at LF.
 

shoonbassjp

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May 25, 2014
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thanks moamp!
I thought sowter makes replacement choke for v76, I will looking into those
I am still studying calc for choke spec but I figure out
 

shoonbassjp

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WOW ! Michael Tibes
What a great offer thanks
But I have also dont know how to measure choke coil....
 

CJ

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that choke is probably about 250 Henries like the V72.  Value is not critical as long as you are over what you need,

Frequency response is critical, since there are perhaps 12,000 to 22,000 turns of wire, you have a problem with capacitance. So you use a three chamber choke to reduce C by 3^2 = 1/9 th the normal coil value. That is how the Telefunken chokes are wound.  Why do we need low C?

If we have a large C, then the  hi freq coming off the tube plate gets shunted to ground via the filter cap. We want to transfer that hi freq thru the cap and down the audio chain.

Why so many turns? That choke carries DC so we have to use a gapped core. (less inductance per turn).

Sowter uses a two chamber choke since nobody makes a three chamber bobbin (Tab/Funkenwerk had a custom bobbin made, big $$$).  You can put tape in between layers every so often to get a little less C.

Henries is not an exact science, it varies dramatically with frequency and level, since the choke determines low end response, (and high end by way of stray C), we  should probably measure Henries at around 20 Hz.

Measure the DCR, that will tell you a lot more than an inductance meter will tell you.

you can also use a resistor in place of the choke, most ears would be hard pressed to tell the difference.

if you want to measure inductance and have a signal generator and AC volt meter, you can do it like this>

 

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shoonbassjp

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thank you CJ for always sharing your knowledge !
I understood now why right choke is needed for Telefunkens plate
I learned a lot from your post in past and this time too
 

shoonbassjp

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and few years passed I made v41 clone
 

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shoonbassjp

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sound pretty bold, fat and fast
different from RCA, Gates those I used to
 

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nashkato

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you should use these :
i always use these
https://www.tube-town.net/ttstore/Kabel-Leitungen/Kabel-Befestigung/Kabeldurchfuehrung-10-mm::1548.html
 

PRR

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> does it make sound better?

Nashkato is worried about the plastic insulation running through a metal hole.

It will not "sound better", until the day that the jagged metal cuts-through the insulation and shorts-out something. No sound, or erratic sound, or smoke/fire.

Telefunken sure would have protected those wires.

Yes, if you just set it in a rack and don't move it, it may not give trouble in your lifetime.

On the other hand I have seen road-tour equipment fail after a week of bad roads and constant motion.

Ideally you put the grommet in before the wires. However you can cut a grommet and work it around the wires and onto the hole.
 

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