Pigtronix philosophers tone germanium ltd Vactrol question?

[JebemMajke]

Schematic


My question is will vtl5c10 be any good in this pedal? Is it going to work as good as original, or at very least pretty close?
According to datasheets they are somewhat close.
Vtl5c6
http://www.excelitas.com/downloads/DTS_vtl5c6c7.pdf
Vtl5c10
http://www.excelitas.com/downloads/DTS_vtl5c9c10.pdf
C6 1ma ( there are 10mA and 40mA as well ), C10 2ma; in dark C6 is 75k Ohm ( 10k ohm, 2k ohm ), c10 is 400 ohm; Light ( at 10 sec ) c6 is 10M ohm and c10 is 400k ohm; dynamic range c6 is 88db, c10 is 75db; Turn on to 63% c6 is 3.5ms, c10 is 1ms; Turn off decay is ( for c6 there is only 1M ohm measurement ) 50ms, c10 ( there is only 100k ohm measurement ) 1.5 seconds.
The way i see it VTL5C10 is faster to turn on ( 1ms ) but it's decay is slow ( a whopping 1.5 seconds!!! ). So, since I am still a newbie ( especially with vactrols ) can someone smarter than me explain if VTL5C10 any good for this pedal.

[PRR]

Probably work fine.

[armdnrdy]

The crew from the other site that reverse engineered the Philosopher's Tone tried different VTL5C vactrols as well as "roll your own" LDR/LED combos. with less than stellar results.

The gentleman with the original Philosopher's Tone wasn't satisfied with his build until he used a VTL5C6.

I recently built one using the VTL5C6 and it sounds great! No noise and a ton of sustain even with a "clean" tone. The blend control can be used to mix in clean signal to offset the effects attack. Overall great compressor/sustainer.

[JebemMajke]

I'm aware of that. That's the reason i'm posting this thread. It has faster response time than other VTL5C versions, but slower decay. And the range is from 400ohm > 400k ohm. I've been thinking of getting one and trying it in this circuit. If it doesn't give me satisfying results i can use it in demeter compulator.

[armdnrdy]

Okay....

How about something you don't know.

When I was working on this project I tried to add an "Attack" control to this circuit. Using a typical R/C - potentiometer "attack" circuit, I tried inserting it in several places using different values and the results were dismal at best!

The areas I tried were the rectifier/vactrol LED and the LDR side of the vactrol. Introducing ANY change in the vactrol area brought less than stellar results and distortion.

Looking at the VTL5C10 and VTL5C6 data sheets.....they are two different animals. The VTL5C10's 400Ω in parallel with the gain stage 2.2M would equal 399Ω compared to the VTL5C6 which in parallel would equal 72KΩ.

I believe that this particular Vactrol was chosen for it's properties and the gain circuit was designed around it.

If you want to build the Philosopher's Tone because you like the "sound" of it.....use a VTL5C6.
If a VTL5C10 works at all.....it will be a different animal......probably not for the better.

[R O Tiree]

To carry on from Larry's excellent analysis, it gets worse... the 5C6 has a turn-on time of 3.5ms, compared with just 1ms for the 5C10. That is going to mean a very fast attack time, which leads to one of the most common criticisms of fast-attack compressors, in that they make your initial pick-strike sound like hitting the strings with a teaspoon. Not only that, but the turn-off times are radically different... 50ms for the 5C6 vs 1500ms (1.5s) for the 5C10. A 5C10, therefore, is going to "breathe" vey badly.

So, even if you changed the other aspects of the gain cell to cater for the changes in ON and OFF resistances, the 5C10 would still be a bad choice. It would work as a compressor, but it would be a bit sh!t, really. Here's why...

Designing the gain cell of a Vactrol-based compressor is not a trivial exercise... you have to factor in the rate at which the gain cell recovers not simply by choosing this or that Vactrol recovery time, but also plot that against the parallel resistance of the Vactrol and the "gain" resistor, in this case 2M2. The 5C6 has a dark-adapted resistance of 100M, according to the datasheet linked to above, but it only achieves that after 24 hours. It reaches only 1M after 50ms or so and, during playing, this would be a typical working maximum figure. Thus, the parallel resistance would end up at around 700k when you pause in your playing. It takes the 5C10 over a half a second to get to a measly 100k. To get to a 50ms reaction time, we're looking at a resistance of about 2k. Compare that with the minimum ON resistance @2mA of 400 ohms, and you can see that the amount of gain control (5:1) you would end up with would be pitiful. Looking at the 5C6, on the other hand, at 2mA we have somewhere in the region of 40k ON resistance. Compare that with 1M OFF resistance and we have 25:1... a massive improvement and about right for a guitar compressor. Why? Because the timing and gain figures that have been arrived at over a massive number of experiments over a great many years have proved to be most pleasing to the ear. That's the only reason that counts, in the end.

So, when someone designs a compressor around a certain Vactrol, there are really good reasons for it.

[JebemMajke]

Thanks for clarifying that.



I hate the fact that I can not find VTL5C6 in my country, and i don't like the idea of ordering from abroad. Additional expense is a bitch. 

Again thank you for your time.

[R O Tiree]

Try Banzai Electronics.  Yes, they are pricey but they are the go-to Vactrol for a lot of the circuits we tend to build. They are based in Germany. Postage to where you are in Europe (?) should not be too much?

[JebemMajke]

Been looking at banzai's offer and it's ok. Vtl5c6 is a bit pricey, 18 euros. Plus 5-10 euros for shipping. My guess is i will have to do it once. Just wanted to avoid that if possible. Obviously not.