Anyone seen a 7 pin VACTROL?

I'm playing around with an old CBS model 450, which is a kind of dynamic EQ booster circuit, more details to come later after I've traced out a bit more of the circuit.

For now I'm trying to find out about the unusual VACTROL. It has 7 pins instead of the usual 4 and the code on it isn't coming up anywhere on the net.

Metal can printed with VACTEC, VACTROL, VT21L44. (Slightly blurry printing, the 2 could be a Z and the 1 could be an I)

Has anyone seen one of these before or have any ideas on where to find out more information?

Cheers!

Bottom of the PCB.

Apologies for the big blurry picture, not on a machine with editing capabilities just now, will try later.

I have a couple Vactec Vactrols in the metal cans like that.  One has the 7 pin base like a vacuum tube, the other has 4 wires coming through the seal.  I believe the 7 pin one came out of a CBS unit and I know I had a dynamic presence booster or whatever that thing is called.  Fortunately I pried the can off that one because it was dead.  Inside are a grain of wheat bulb and two photoresistors.  Each component has two leads, so that is why you see one unused pin on your PCB.  Mine is marked VTL7A1, the four pin model is 21L53A.

I may be wrong, but I think the Marti compressors use those too. 

Those cans were part of Vactec's standard product line for a number of years. You'll find them in the tremolo circuit of some Hammond and Conn organs from the 60s and 70s.  The 7 pin can is probably a dual. See the catalog page from Electronic Engineering Master Catalog from '73, attached.

I have a Marti CLA-40H in the closet. I think you're right, Doug, that they had one of these in them. Out of curiosity, I'll dig the manual out.

Quicker to pull the compression module off the back of the unit than to dig up the manual. Doug is correct. See photo.

Vactrol model number 21L53A. 4 pins.

The "21Lxxx" numbers were OEM part numbers. I've been trying for years to figure out how to cross-reference those with off the shelf "VTLxxx" numbers, but no luck. They were used in Altec 1600 series PA mixers in the limiter circuit, IIRC as well.

Never seen one with both "VT" and "21L" in the part number. Usually it's one or the other.

That's very handy knowing there's a bulb and 2 x LDR in there, I should be able to trace it out now. I'll have a scan through the catalogue and see if I can find a rough match.

Thanks for the info rackmonkey.  I don't remember scrapping a Marti CLA-40H but that must be where I got my 21L53A from.

mjrippe said:

Thanks for the info rackmonkey.  I don't remember scrapping a Marti CLA-40H but that must be where I got my 21L53A from.

Click to expand...

The CLA-40H and LA-20 (line amp that looks similar to the CLA-40H) both have screw-on modules in aluminum housings that contain the various sections of the amp (the input amp, the compression unit, the make-up gain stage/line driver) that are easily removed. Maybe you just had one of those modules show up at some point?

Funny, I didn’t even notice our part numbers were the same until after I posted

Finding all the right data now that I know the right search terms, thanks everyone. Part code on my VACTROL still doesn't match anything out there, but I have pinout and rough specs now.