I mentioned previously that I have 5 Altec 1588B microphone transformers that came with my 1592B. I wanted to rebuild them and run them in with the mixer after I rebuilt that. So here we go!
Here's one of the transformers with the can removed, on the 'helping hands' stand. The cans are easy to remove - there are small tabs on the can that are bent down to hold the chassis in place. Just bend them back and use a small screwdriver to prise the can off.
I'm going to simply rebuild 3 of my 1588B modules to the stock values with new caps. And I'm going to modify a fourth one. (Keeping the fifth one stock for now to compare tonal/noise differences).
If you search the interwebs for anything related to 'Altec 1588B,' you'll find the famous Ned Clayton modification schematic. His mod is to enable the use of a 1588B as a standalone preamp by raising the output gain by decreasing negative feedback.
A stock 1588B has a gain of about 33.5 dB. His mod puts it up to about 52 dB, making it viable as a preamp for condenser mics without using the rest of a 1592B's output circuitry (if that's the Altec mixer you're using). A 1592B has a gain of 87 dB (!) with a stock 1588B. I wanted to try the standalone mod, but I also figured it might be interesting to try a ribbon mic with the extra gain - theoretically 107 dB - yikes!
The 1588B works in virtually any older Altec mixer/preamp, not just the 1592B. For example, it works in the 1589B. The 1589B has an output gain of 66 dB I believe, so the high gain module would make that one suited to ribbons as well. I'm speaking in the theoretical at this moment; I haven't tested any of this. In fact, I don't own a ribbon mic yet!
I included the transistor types as well - there might be some advantage to be had by changing these to more modern transistors. My head is still spinning a bit from the rebuilds, so changing transistors is something I can't contemplate right now.
I also changed the 390pf ceramic cap across the output of the input transformer to a silver mica. In the process of removing that cap, I managed to break the original 22K emitter resistor in half (!), so I changed that to a metal film also. I hadn't planned that as part of the rebuild.
You can see the size difference on the left. I honestly don't think there will be much tonal difference - the FG is a quality cap.
A 25 volt rated cap is all that's needed here, by the way. I just didn't have any 25 volt Silmics (Silmi?) on hand - they may be smaller than the 35 volt cap I had, and they may fit in this application.
I changed the input and output caps for Silmics, and changed the 27K carbon for a metal film as in the 'stock' rebuild.
The high gain mod calls for replacing the 47 uF pad cap with a 100 uF and the 3.3K carbon resistor on that part of the circuit with a 499 ohm metal film. I only had a 470 ohm resistor on hand and used that. I wouldn't think the 29 ohm difference will matter that much.
I burned in in my rebuilt 1592B mixer with the newly rebuilt mic transformers installed. I couldn't judge the gain difference in the modded transformer all that much - there is more gain for sure. But I won't be able to do any critical listening tests until I can record with it.