Why are the wire leads on e.g. AC701 often coiled?

[rock soderstrom]

A picture is worth a thousand words.



What is this for? Thermal protection of the tube during soldering? Anti microphony measure?

 

[Rob Flinn]

If you cut them short I was led to believe the heat form the soldering iron can compromise the vacuum.

 

[RuudNL]

Glass and metal have different coefficients of expansion.

 

[rock soderstrom]

That's what I thought too, I think that's why early glass germanium diodes were also soldered with these coiled wires. I seem to remember seeing datasheets for such subminiature tubes with a minimum distance rule for the leads during soldering...my first teacher recommended that we use needle-nose pliers as a heat deflector for such jobs.

Do you also do this for other tubes in your mics, such as 6s6b, 5840 and so on?

 

[MagnetoSound]

Do you also do this for other tubes in your mics, such as 6s6b, 5840 and so on?

It’s only really necessary if space is severely limited and the leads would be so short as to conduct the heat immediately and without thermal resistance.

 

[Roman Beilharz]

Another advantage is, that if you are swapping parts regularly between builds, the coils help to adapt lengths nondestructively.

 

[rock soderstrom]

It’s only really necessary if space is severely limited and the leads would be so short as to conduct the heat immediately and without thermal resistance.

Makes sense, in microphones where there is more space this connection technique is less common.

What do you think, from what distance (in mm) is one on the safe side?

 

[Dark1]

Not only is excessive heat during soldering deadly to wire-ended tubes for obvious reasons, one must also take into account that such wires are made of multiple parts (part of the wire that goes directly through glass is usually made of special glass sealing alloy), so it is advisable not to bend the wires too close to the glass seal as it is easy to break them. So the coiling is not only good to dissipate some of the heat, it is also a precaution against breaking the wire since it relieves the strain from the glass seal end of the wire when manipulating with the tubes or soldering them in. Attached is a Neumann-issued sheet delivered with spare AC701s containing basic instructions how to perform the tube replacement...

 

[rock soderstrom]

Thanks Dark for info and re-posting this Neumann document. I saw this many years ago, but couldn't find it this morning.

 

[tomas.borgstrom]

That's what I thought too, I think that's why early glass germanium diodes were also soldered with these coiled wires. I seem to remember seeing datasheets for such subminiature tubes with a minimum distance rule for the leads during soldering...my first teacher recommended that we use needle-nose pliers as a heat deflector for such jobs.

Do you also do this for other tubes in your mics, such as 6s6b, 5840 and so on?

I always do it with ac701 but with cheaper tubes I’m sometimes sloppy and just use a plier or an alligator clip. It seems to work as well. I aim to solder as fast as I can.

 

[rock soderstrom]

I always do it with ac701 but with cheaper tubes I’m sometimes sloppy and just use a plier or an alligator clip. It seems to work as well. I aim to solder as fast as I can.

That was also my current decision. The 6S6B has survived well, thanks to thermal insulation via pliers (and fast soldering). But I would treat an AC701 strictly according to Neumann's instructions...

 

[soapfoot]

I like a locking pair of hemostats for heat-sinking when soldering fragile parts

Sometimes I’ll also just use an alligator clip test lead

On a valuable AC701k I’d probably take every reasonable protection measure I could think of

 

[Cqwet Dbdfte]

Tubes are microphonic and good at RF.
Most likely they ran out of ferrite beads ;-) .
Reduced mechanical coupling is another reason.
Heat sensitive tubes? That's a new one. Maybe they found out the hard way?
Cutting steel leads close to a glass envelope requires special care, with a supporting pliers or using shears.

 

[Dark1]

Ferrite Beads: Not really, Schoeps used a ferrite bead or two on the grid wire in their AC701-based pencil mics. Interestingly, they also used a screw connection for g1 instead of direct soldering (to keep the g1 connection as short as possible), in their early M221 mics (reverting to soldering all connections later on). Neumann opted for an additional (probably mu-metal?) internal screening for the tube inside KM54 et al. Both models can be prone to RF interference depending on many factors.

It's just too easy to damage the tube during soldering. No wonder Neumann also supplied the tubes with pre-cut/pre-coiled wires to minimize manipulation and heat damage in service centres (I think this was esp. early on: I have one such early-generation Neumann-selected AC701 and it even has a self-noise specification on the box indicating a tight selection process; later Neumann spares I have seen usually came with uncut wires and no noise specs.; no idea if this was due to better manufacturing at Telefunken -- although I think so, or if such spares had to be special-ordered?). I think Telefunken struggled making these early on, some early tubes even have the "pinch-waist" envelope to better control microphony, dropping this measure later, I assume after internal construction changes. So it is very likely they also addressed other problematic areas incl. glass-sealing during later development.

 

[GeorgeToledo]

Somewhere I picked up that it is to dampen the leads from excessively vibrating at some frequencies and passing that onto the tube. I think I probably got that idea from the old Neumann forum.