Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Altec 1588B Microphone Transfomer Rebuild and Modification

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!

Anyway, I took Ned's original schematic and added in the original component values for those seeking to restore an original. That new version is shown on the left.  Hopefully this will help anyone contemplating work on one of these.

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.

On the right is a photo illustrating what I did with three of my 1588Bs.  I changed out the three electrolytics, and changed the 27K collector resistor on the first stage transistor to a metal film to reduce noise.  The electrolytics are Elna Silmic IIs.

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.  

The high gain mod calls for a 100 uF cap to replace the stock 47 uF cap on the pad line.  I found that the Silmic 100 uF/35v cap I had on hand was simply too tall to fit.  So I wound up using a Nichicon FG 100/25 instead.

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.

The picture on the right is a 1588B with the high gain mod.

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.


Post a Comment 2 comments:

  • Adracamas said...
    December 7, 2016 at 10:40 PM
    Hi there, I have a few of these and have been trying to figure out the best way to examine or test these for failure.. is there anything you suggest to do? Where to apply power, sound, output/lead test etc? I'm really new to this but really fascinated by how "old things" work.
  • Yr Fthfl Blggr said...
    December 19, 2016 at 2:54 PM
    Hi - I'm assuming you mean the mic transformers? They plug in to the Altec mixers. The best way to test them is in-circuit in one of those. Aside from testing to see if the primary or secondary of the transformer itself is open, that would be the way to test.

Post a Comment