My 10 cent Vactrols

[LaBelleAurore]

Made myself some VTL5C3 replacements for 10 cents a piece using readily available LDR’s and LED’s. I dropped these into one of my 259’s for testing and there is no distinguishable difference from my other 259’s. I don’t know if these have the necessary specs for LPG applications. Will have to test that out.

[shoegazer86]

I had made a bunch of these for a 291 build back before I built up a 291R. They worked surprisingly well, but when using them on my 208 LPG they were very unpredictable.

Even Buchla used LEDs and LDRs in some of their e series modules so they should work in theory

[LaBelleAurore]

I had made a bunch of these for a 291 build back before I built up a 291R. They worked surprisingly well, but when using them on my 208 LPG they were very unpredictable.

Even Buchla used LEDs and LDRs in some of their e series modules so they should work in theory

Yep, whether these have the right specs for LPG use is an open question. In 259, 265, 266, 285 builds these should be fine.

[elmerfudd]

Have you tried experimenting with green leds? Couple of years ago was doing a test circuit with old cl 505 cells got much faster response and a more consistent result between cells. Was going to try a test rig with a tri color led and see if I could tune the response via color balance. One of those back burner things that fell off the stove….

[LaBelleAurore]

Have you tried experimenting with green leds? Couple of years ago was doing a test circuit with old cl 505 cells got much faster response and a more consistent result between cells. Was going to try a test rig with a tri color led and see if I could tune the response via color balance. One of those back burner things that fell off the stove….

I don’t have any green LED’s on hand, but thats something I can try. So far i experimented with yellow and red LED’s of various mcd values. For use in my 259, I find that the brighter red LED works the best.

[elmerfudd]

A 40 year old memory of a spec sheet is that cadmium cells are green sensitive. Why Urei opto cells had the phosphor glow screens in them.

[LaBelleAurore]

It turns out that dual vactrols are not much more difficult. Made a VTL5C3/2 replacement. The package is definitely ugly. Need to figure out a more elegant package, but I replaced the VTL5C3/2 in the MOD INDEX of my 259. Absolutely no difference in sound or performance.

[fallout]

Nice! Who cares how it looks once you button things up?

What LDRs did you use?

[LaBelleAurore]

Nice! Who cares how it looks once you button things up? ;)

What LDRs did you use?

I found that there are not easily identifiable brands of LDR’s. Instead, I just hunted down LDR’s that have similar dark and light resistance to the specs in the data sheets for the VTL5C3 & VTL5C3/2. The data sheets list dark/light resistance as 10M/30K depending on current. For the dual version I placed two LDR’s side to side, set the LED in the middle between the two, then joined two legs to create the center tap between the photo cells.

[fallout]

Thanks. I just ordered a kit of GL series LDRs. I'll be curious to see the different responses between the types.

[LaBelleAurore]

Thanks. I just ordered a kit of GL series LDRs. I'll be curious to see the different responses between the types.

Checking GL codes, the GL5549 has a dark resistance of 10M.

[shoegazer86]

Keep in mind you also do not want to use ultrabright LEDs. Old vactrols have LEDs with a much lower lux than modern ones. You don't want to oversaturate the CDS cell too early. We disassembled a vintage can style VTL from the era Buchla was using them. The LED is pretty dim, kind of surprising.

[KSS]

Keep in mind you also do not want to use ultrabright LEDs. Old vactrols have LEDs with a much lower lux than modern ones. You don't want to oversaturate the CDS cell too early. We disassembled a vintage can style VTL from the era Buchla was using them. The LED is pretty dim, kind of surprising.

I've made a similar point about replicating old 'vactrols' and optos. And completely agree.

But it's also true that LEDs grow dim with age-use and the older type did this more than the newer types. <--Or it only hasn't been long enough for the newer types to show the dims-with-use degradation.

[LaBelleAurore]

I replaced the channal A dual vactrol in my rev 1.1 292c. This build used Silonix vactrols that sound great. Lopass mode sounds identical, but on combo and gate modes decay is too fast. I need to try different LEDs, also I accidentally found that the LED distance from the photocell impacts performance.

[dougcl]

Keep in mind you also do not want to use ultrabright LEDs. Old vactrols have LEDs with a much lower lux than modern ones. You don't want to oversaturate the CDS cell too early. We disassembled a vintage can style VTL from the era Buchla was using them. The LED is pretty dim, kind of surprising.

I wonder if anyone has tried a trim pot to adjust LED brightness?