Received my General Electric B-46 photoconductive cells yesterday. After struggling with the 40 year old blister pack I stuck this part across the leads of a meter..

1.26-1.71 K under ambient light in the living room (no idea what angstrom I'm working with) and 160-200K after 5 seconds of cover, which puts it right in line with the Clairex CL505L that is the heart of the early T4 optos.

13th week of 1969 would be my guess. With the octet I concluded hastily that, should all fall within spec I can roll four T4 clones; I'm now leaning on slapping a range similar modern CdS cell to drive the meter, since that leg is out of the audio path and there is no benefit to a period correct part there. I tend to ignore what the meter is doing anyway..

Now to source the electroluminescent part, I'm pretty sure I can find a 2.2 meg resistor in a bin. There's also the matter of enclosure, but I do have some things I can cannibalize.

Continuing contemplation of the Teletronix LA2A as I shift and shuffle the new workbench into a useful state. Here's an early revision schematic I found at The Bottle Garden studio site, the absence of a LIM/COMP switch points to a first revision so far as I can tell.

Signal path: From the input transformer signal passes through cascading halves of a 12AX7 and into a cathode follower built on stacked halves of the 12BH7 feeding the output transformer (through a DC blocking cap).

Supporting electronics: Input signal is split into a strapped 12AX7 voltage gain stage which feeds the 6AQ5 which drives the luminescent panel of the T4 through a DC blocking cap. The T4 then, when fired, illuminates the CdS cell which decreases resistance, effectively draining more signal to ground before it hits the grid of the very first gain stage of the signal path.

So, the resistive drain of the CdS cell will always be in circuit, before any signal amplification. I'm making the leap to the conclusion that the CdS element has a tremendous impact on overall voicing and characteristic of the device. I'm also shooting down the statement of relief in my previous post regarding mica dominoes in circuit. They are not, unless you believe subtle frequency nuances of the side chain amplifier driving what amounts to a light bulb are going to play much of a sonic role. Which brings me to the electroluminescent element, it makes sense that the characteristics of this are going to play a large part in actually driving the CdS cells into their lower resistance state.

I bought this octet of NOS General Electric CdS cells at auction, I'm hoping that with the ability to play with series/parallel configurations I can come somewhat close to the static response range of the Clairex CL505. Since there was no data sheet I could find, this does amount to a shot in the dark.

LA2A

Been breaking my back lately, since one of my current primary focuses is to set up the studio at the new abode. I think the last time I had it close to properly set up was in 2006. This, coupled with the fact that I am planning to record the band Stalagmite sooner rather than later, prompted me to end a long term loan of my Teletronix LA2A to my friend Ty at Sublevel9. So what follows is essentially a refamiliarization as opposed to sheer speculation, and for the record, this unit is probably cleaner now than when I initially lent it out, so no laying blame for any seams of crud perceived in the following pics.

Spartan good looks, I defy you to make a grab for the wrong knob on this device.

It was built with a hinging front that will allow access to the guts even while powered on. I love gear designed with an inherent trust of the operator such that they don't automatically conclude that the operator is an idiot that will likely kill themselves due to such a servicing convenience. You simply can't get away with that today... perhaps the people are actually honestly less intelligent?

Moving to the rear, we see your typical back mounted tube and transformer set. Note pre-UREI label of manufacture. I do believe "Studio Electronics" was a Bill Putnam endeavor however, this unit appears to be a rev 2, with HA-100X input transformer and compressor/limit switch and T4A designator. As you can plainly see, the T4 opto slot is populated by something decidedly homebrew (thanks to my uncle for the opto build, and the unit). I have fuzzed out the serial number, which is low four digits.

I've always preferred everything in duplicate, being fixated on stereo. Having kept a loose eye on the going rate of complimentary UTC HA-100X transformers, I am planning instead to build a clone utilizing something I already have in the input iron pile, hopefully I can come close, I'm sure I can achieve 'useful'. Looks like a 50th week of 1964 to me, indicating they were running through existing stock on their way to rev #3, A-10 inputs.

Here's a shot of what's going on inside the current opto element, simply a switched VTL5C4/2 allowing for fast/slow decay. I'm planning on rolling a closer to stock arrangement with luminescent panel coupled with photoconductive cells when I get a chance. I did have to assert focus to avoid trying to dig up the box with old stock photocells in it while moving stuff from the old storage set-up to the new room. They are there, somewhere, and perhaps some of them will come close to operating within the range of the original Clairex CL505L elements used in the T4A & B (so far as I can tell).

Spec attached, for what it's worth.

I do plan on recapping this, as it appears that everything is original. However, my replacement frenzy will NOT include these, only the electrolytics. Frankly, having recalled mica "dominoes" inside, I was a bit dismayed that they are not in the signal path. Oh well, at least it will contain enough carbon comp resistors to convince most electrical engineers that I am a mindless savage.

Pretty neat layout, and the relatively low parts count makes for a nice wide open layout. I believe the part at left is a variable mica capacitor, the Aerovox center appear to be date coded 20th week of 1966, the domino cap at right is across the "FLAT" trimpot, indicating it may have a say on overall frequency curves and as such is probably carrying a bit of signal. Well, I can rest easier.

Continuing on the datecode front, the UTC A-24 output transformer looks coded 44th week of 1966.

The Sprague 40-40-30-30@450 can cap is dated 4th week of 1967, pretty sure that one needs to go. I'll probably slap a new production CE Manufacturing twist-lock at 40-40-20-20 and make up the 10uf internally if the 20s cause problems, which I doubt.

28th week of 1968 seems as close to build date (without going over) as I can get. I'd say late 1968 origin, since I haven't been able to find a serial number to date of manufacture cross reference.

This is for you, Nigel Tufnel.