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[Rollei] Re: Why the 6-element lens for 3.5Fs

Richard Knoppow wrote:

> Somewhere in my archives I have an article from a German
> electronics magazine of about 1947 announcing the set of
> tubes. They were intended for the usual five tube superhet
> table radio so familiar to pre transistor people. The
> filaments were set up for series string service on a 240V
> line. I don't remember what the filiment voltage is on this
> thing, about 50v I think, at some low current. Its dropped
> off the same supply which biases the diaphragm. The U-47 was
> an economy model, the deluxe model being the M-49, which had
> some more standard tube in it (6AU6 or something like it).
> The tube in the U-47 had problems with being microphonic and
> noisy. Who built them? My memory is gone now, AEG-Telefunken
> I think. In any case at some point they built a special run
> of tubes for Neumann to keep the old guys running.
> One problem with M-49 and U-47 mics is that the plastic
> diaphragms of the original elements harden up with time
> causing all sorts of parasitic resonances. Later mics used
> Mylar which is very long lived.
> The M-50 used a metallic diaphragm, which does not age.
> Another interesting mic of the time is the old
> Altec-Lansing M-21 which had a glass diaphragm.
> So much for antiques. BTW, I am one of the few left who
> knows how to re-ribbon old RCA and Western Electric mics.
> Talk about your useless skills :-)

The U-47 sound is generally regarded as more desirable than the M-49 sound,
and the tube has a great deal to do with this. Modern M-49s are nicer
microphones than the vintage ones. These microphones are to recorded sound
what Rolleiflex TLRs are to recorded images...

There are specialty firms who replace the old diaphragms with lower mass
modern ones. 

Re-ribboning is not a useless skill. I had my 77DX and BK-5b re-ribboned by
the one remaining RCA specialist in NJ about 10 years ago. Ribbon mic
designs are currently experiencing quite the renaissance...

We better stop now...

Eric Goldstein