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

- ----- Original Message ----- 
From: "Eric Goldstein" <egoldste  >
To: <rollei  
Sent: Friday, May 02, 2003 6:56 PM
Subject: [Rollei] Re: Why the 6-element lens for 3.5Fs

> Richard Knoppow wrote:
> > Neumann microphones show some of the problems of
> > parts in early post-war Germany. The famous U-47
> > for instance, uses an odd tube. In fact, it was a tube
> > designed for five-tube AC-DC radios and was used because
> > was relatively easy to get and made the design simpler.
> > are awful tubes (heresy!!) and have been impossible to
> > for decades.
> I just keep rebuilding the one in mine... that is pretty
much what everyone
> does who did not FETize theirs (take about heresy!)...
> There is quite a bit of lore surrounding this particular
tube... to say that
> it is awful is heresy indeed but no mark of courage if
only posted to the
> rollei list... <g>
> Eric Goldstein

 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 :-)
- ---
Richard Knoppow
Los Angeles, CA, USA