[Thursday] Mullard Tubes

[Robert Vincent]

For those interested, Mullard tubes (12AU7, 12AX7 etc) were widely used in
Heathkit test equipment (especially Oscilloscopes) Beginning in the 1960's
through 70's and maybe longer. Most of them in later dates have the IEC
(International Electronic Components) logo on them along with Mullard. (IEC
was the authorized American Distributor of Mullard Tubes in the USA)  Now
as of late there are a large amount of frauds offered on Ebay and elsewhere
(Chins, Russia rtc.) as Mullard tubes. Mullard Tubes are very good and
spectral analysis using a FFT analyzer over a frequency range of 20 to
20Khz and as high as 100 Khz clearly demonstrates that they are very good
tubes compared to many American and Japanese and other European
manufacturers.  However, the knockoffs cannot hold a candle to original
Mullard vacuum tubes, which is why many test equipment mfg.s. in years back
used them.
So how do I know a fake from an original.. Date Codes,  The mullard tubes
had Manufacturer sites and date codes. etched into the glass at the very
bottom of the tubes.
See attached document.

The most sought after mfg. site is. Blackburn, England,  These tubes
command the highest prices when sold on various sites because of their very
low noise and especially microphonics (mechanical noise within the tube
from vibration etc.

Telefunken uses a similar identification but only identifies them as
"Telefunken". Look at the bottom of the tube within the pin circle and
there will be diamond etched in the glass in the bottom of the tube.
One more thing about European tube mfg's... Do not wash tubes or handle
them excessively as the finish containing the mfg, and logo will be wiped
out, then you do not know what the tube is and can only be verified using a
observations in internal structure or by testing and observing the
indicated (as explained above)  markings, Amperex bugle Boys are Notorious
for this logo wipeout.. Usually (99% of the time). Ribs in the glass at the
top indicate European Mfg. and more often than not, specifically Amperex,
made in Holland. What a lot of people do not know is that Hewlett Packard
tubes made under their logo by Amperex, are  Amperex Bugle Boys, Not all of
them however, only the ones marked "Made In Holland". (Sylvania also made
tubes for HP, and spectral analysis indicates they are also very good and
most likely hand picked from Manufactured lots to meet HP specifications)
 These premium vacuum tubes can be verified using spectral analysis as
Amperex BB have noise figures exceeding greater than -120 to -140 dbv for a
30 hz bandwidth. ( Also Note, some of the Telefunken and Mullards can go
down to >-140dbv 30 hz bandwidth).  My analyzers, with the exception of a
Tektronix 7L5 bottom out at -140dbv 30 hz BW... The 7L5 exceeds -170dbv.
Yes, that is right, the level is in 10to -9 volts or Nano volts.  (This is
why 7L5's command such a high price even though they are over 40 years old.)

-- 
Robert Vincent
DLM Antenna Technology
401-487-3933

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