Accelerator I don't want to disagree with you but I just got off the phone with 2 of the old engineers at AKG who checked their records and verified to me that the original SM204 was produced in 1951 and changed slightly over a couple of years until the final version SM204/23 was produced in 1953. I only have this by phone but I can try to get them to send me copies. I have no idea why your schematic is stamp dated 1960. I have worked on 4 SM204s and they all had shallow resonator CK12 capsules which were manufactured in the middle 1950s. I understand the biasing differences but the greatest difference is the transformer. Supposedly there were some manufactured with a Henry transformer but those I have seen had Rode and Schwarz.
I am not sure I would call these completely different microphones but there are differences.
Actually the photo you posted of the pair of SM204/23 internals is a photo that I took
Tim, I totally agree with you that the transition from the AKG flat-shallow dish resonator CK12 capsules which were manufactured in the middle 1950s to the AKG deep dish resonator CK12 capsules at the early 1960’s and the transition from the Henry V-3044 transformer to the Henry V-2148 Transformer (AKG C-12 Microphone System at middle 1950s) and then to the Rohde & Schwarz V-2545 (AKG C-12 Microphone System at late 1950s, AKG - Siemens SM203/4 - C23 Tube Microphone at late 1950s/ early 1960’s) to the Haufe T14-1 (AKG C-12 Microphone System at early 1960s, AKG C-24 Microphone System at early 1960s, AKG-Telefunken ELA-M 250/251 (non E with the AC701K tube) and AKG-Telefunken ELA-M 251E (with the 6072A tube) were the “huuugeeee steps” in the transformation to what we all know today as “AKG C-12 sound-like”...
AKG was the small Austrian “newcomer boutique” company back in time at middle 1950s with non the “big history and the clear records” neither the “Big Status” that both Neumann & Schoeps had, so they had to did everything just to “make their fit” in the antagonistic microphones market, and logically there were several “back and forth” decisions about their microphones
So, as we can see today that they used the AKG flat-shallow dish resonator CK12 capsules which were manufactured in the middle 1950s anachronistically in the AKG C-414 EB microphone at 1976, they keep as "urban legeng" the
uncorrected “Ol’ wife’s tale” value of biasing the 6072A tube at
Grid Bias Voltage (V) -1 Volts for the AKG C-12 Microphone System and the
uncorrected value of
2k7 Cathode Resistors at C-24 Microphone System with the 6072A tube which are clear indicators of a previous one designing and implementation of C-12 / C-24 microphone system(s) running at 120 DC Volts Vb with the ECC81 – 12AT7 valve, as the
correct values for the 6072A – 12AY7 tube running at 120 DC Volts Vb is
Grid Bias Voltage -1.23 Volts for the
AKG C-12 microphone system and the
correct values are
2K05 Cathode Resistors for the
C-24 microphone system(s)…
The only “Unofficial” report that I have for the usage of
6072A - 12AY7 tube in the
AKG C-12 Microphone System comes from the American magazine
“Electronic Design News” and it was dated at
April 1957…
Before this from 1953 to 1957 there was nothing in the Official and Unofficial record about the tubes for the AKG C-12 Microphone System…
Returning to the topic as
Matador has involve both of these options of the usage of
ECC81 – 12AT7 valve with
47KΩ Anode Resistor at
120 DC Volts Vb I can say that the correct Grid Bias Voltage (V) is
-1.01 Volts and if someone would like to use the
6072A - 12AY7 tube with
100KΩ Anode Resistor at
120 DC Volts Vb the correct Grid Bias Voltage (V) is
-1.23 Volts…
