I do understand the circuit. I was thinking of changing the original design to a plate follower. It's just a little cramped to mess with various tubes. Swapping one is a real PITA as it's soldered to the PCB.
What mods have you done on it. Mine currently has an M7 capsule but I don't much care for this particular one. I think it's defective. Mostly curious about circuit changes. I did the usual things that are fairly well documented. I think AA did a paper on the mods, which I did. Hard to remember, but I believe there was the zener changes in the supply and removing a few components from the mic PCB and swapping out some lower grade parts with other better parts.
This is what I did per Dave, though I did not change the transformer
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The following changes should be made to the 1050 circuit.
1) C9 & C10 should be removed. They can be cut out and are not required into today's modern world. C9 & C10 where meant as RF filter from the days of Taxi cab's operating their 2-way radios on the upper AM radio frequencies.
These capacitor values were spec'd to low in the 1050. So, C9 & C10 in parallel to ground causes the distortion at 11khz to increase 4 times because of the inherent resonance between these capacitors and the low buget output transformer.
SIMPLY REMOVE THESE CAPACITORS...DO NOT...STRAP THEM WITH A WIRE JUMPER AS THIS WILL SHORT THE OUTPUT OF THE TRANSFORMER TO GROUND.
2) C6 & C7 should be removed. These bypass the cathode resistor in the 1st stage. However, they are not necessary in the 1050 circuit and only necessary in a single stage C12 type circuit.
Once properly modified the 1050 circuit is superior to any single plate output circuit. With C6&C7 removed the circuit is more linear without any LF roll-off. The gain drops 4db but this increases the headroom and signal to noise ratio by 4db.
AGAIN SIMPLY REMOVE THESE CAPACITORS...DO NOT... STRAP THEM WITH WIRE JUMPERS AS YOU WILL SHORT OUT THE CATHODE BIAS RESISTOR.
3) Replace C8 the 1ufd outut electrolytic coupling capacitor with a 2.2ufd metal film. This will place any resonance between C8 and the output transformer down into the sub-sonics where it will not negatively effect the audio signal.
4) Replace the 12AX7 with a 6072a this lowers the gain another 2db but the output impedance on the first plate is much lower. A new Sovtek 6072a selling for around the $20 is more than adequate in this clever CCDA type circuit.
Now, you will have optimized the 1050 circuit which is a 2-stage CCDA (constant current draw amplifier) with an output impedance 10 times lower than the venerable original U47 circuit.
5) Replace the stock economy transformer which has a 10.5:1 turns ratio with our BV8 having a 6.5:1 ratio. The green wire goes to pin 6 and the black wire to pin 5 on the 7 pin connector. The white wire goes to pin 4&7 on the 7 pin.
The red wire is attached to the red wire that goes to the output of C8. Remember, the gain has dropped by nearly 6db in the front end but the BV8 has 5db less loss than the economy 10.5:1 transformer.
6) The economy 32mm (k67) type capsule with its 3-5 micron thick diaphragms is replaced by our AK47 capsule which is a single back-plate w/dual 6 micron diaphragms having identical specs to the original K47 used in the U47 & M49.
The K47 will fit onto the existing saddle but will mount either 2mm closer to the front or the rear. I prefer to move it back 2mm.
7) Next replace Z5 & Z6 sometimes noted D5 & D6 inside the power supply with two 62v Zener diodes. This will drop the power supply B+ voltage to 124v dc + and - 2%. These supplies are usually set for 144v dc and this will place about 70v dc on the capsule. The AK47 and the original K47 capsule seem to give optimum results when they are polarized between 55 and 60v which is what you will get from a 124v dc supply. These are located behind the pattern switch on the board.
That's it!!! If this is all done correctly then the microphone should fire up and sound like a WORLD CLASS MICROPHONE.
I will try and take some pictures of the next 1050 we get in for upgrade.
Cheers, Dave
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