vactrol for compressor circuit

[The Groke]

Hi.

I have a Marti CLA-40H limiter/compressor (OK, not a stompbox, but I'll be using it to process bass guitar). The unit is missing the compressor circuit, but it is just a plug-in module (CA-40) that attaches to the back of the chassis, and I have the schematic, so I thought maybe I would try to build a new one.

It seems entirely doable, even with my limited skills, but I have no idea what to use for the Vactrol. The original was a Vactec 21L53A. There is no info on this online.

Can anyone suggest a possible replacement to try from SmallBear? Some earlier threads on this forum suggest using the VTL5C2 for compressor circuits, but I'm not sure what other components I would need to change in the circuit to make it work (if any). Again, I'm approaching this somewhat blindly.

I've attached a link to the circuit. The Vactrol is marked VVR (I guess that's obvious though). If anyone has any thoughts on this, please let me know...

http://www-personal.umich.edu/~damont/MartiCA-40.pdf

Thanks - DT

[armdnrdy]

I would imagine that the 21L53A number is a Marti factory number.

I don't know of any other Vactrols that Vactec (Perkin Elmer) manufactured besides the VTL series.

I would venture to believe that in this circuit it would be a VTL5C2. (slower decay, higher "off" resistance)

If not, maybe a VTL5C3 (slightly faster decay, lower "off" resistance)

Either Vactrol will work in this circuit.

[PRR]

By the drawing-style, this pre-dates LEDs. The opto would be an incandescent lamp. With very slow attack! Which is why it has an FET: to band-aid the poor response.

IMHO, there are better plans.

[The Groke]

Hi,

Thanks very much to both of you for your comments. I will try the VTL5C2.

I should check the pot codes, but the Marti CLA-40H units were made throughout the 70s. I think it could have LEDs instead of incandescents, especially given the very small size of the vactrol package. It is metal though - so there could be a bulb in there... but it would have to be smaller than those EICO lamps used for indicator lights.

I have used working Martis, and I liked the sound - though my positive opinion on them could stem from the fact that they have decent input and output transformers.

I'll let everyone know how the VTL5C2 works out when I get them.

Thanks!  DT

[The Groke]

Pics of this mystery Vactrol from a friend's working unit (for those interested):

Marti has a different part number listed in the schematics, so this is probably a Vactec number. 1975 date code stamped on it?

http://www-personal.umich.edu/~damont/MartiVactrol1.JPG
http://www-personal.umich.edu/~damont/MartiVactrol2.JPG

[gritz]

Pics of this mystery Vactrol from a friend's working unit (for those interested):

Marti has a different part number listed in the schematics, so this is probably a Vactec number. 1975 date code stamped on it?

http://www-personal.umich.edu/~damont/MartiVactrol1.JPG
http://www-personal.umich.edu/~damont/MartiVactrol2.JPG

I like mystery vintage components. The encapsulation and printing suggest that it's more than some tiny run by a backyard manufacturer, so it's odd that there's no info about it on the interwebz. The schematic suggests that even with no signal  the control current thru the device is significant and the maximum possible current will be quite large - R15 is 15 Ohms. Have a bit of a think before connecting up your new vactrol to the unmodified circuit! Maybe there's some info out there somewhere. Perhaps it is just a grain o' wheat bulb and an LDR...

'Tis intriguing though.

[The Groke]

Hi Gritz,

OK, you got me feeling a bit nervous (since my skills are pretty limited) and adventurous...  I called Broadcast Electronics (they acquired Marti) and spoke with one of the old Marti Employees (Kim). She looked up the two part numbers (21L53A and 451-001) in their database and pulled up a replacement part that was sold by Marti right up to 1998: VTL3A47.

The folks at Perkin Elmer told me that any old part starting with "21" was a custom product, but that the other numbers don't reveal any further info. So, that explains the odd number on the original Marti Vactrol.

These are the specs  for the VTL3A47 (it uses an incandescent!):

VTL3A47 Specifications
Maximum Isolation Voltage (V) : 1.5k!
# Input Channels : 1
# Output Channels : 1
Output Breakdown V Min : 100
t(r) (s) : 25m
Maximum Decay Time (S) : 900m
Type of Input AC/DC : Incandescent
Resistance(on) (Ohms) : 40
Resistance(off) (Ohms) : 1M
Min. On Current (Controlled) : 40mA
Min. On Voltage (Controlled) : 12V
P(D) Max. (W) : 550m
Minimum Operating Temp (!C) : -40
Maximum Operating Temp (!C) : 70
Package Style : Axial
Mounting Style : T

So, as suggested above, this is quite a bit slower in rise time and decay than the modern versions... not sure what to do.... looking through the Perkin Elmer catalog now - not that I'd be able to order any of the special items....  Maybe I should just make my own Opto-coupler?

best,

D

[PRR]

This lamp:
http://www.digikey.com/product-detail/en/7219/CM7219-ND/242927

This LDR:
http://www.digikey.com/product-detail/en/PDV-P9001/PDV-P9001-ND/480615

This won't be as aggressive as the original; but it may not matter.

[The Groke]

Hi PRR - thanks for suggesting some parts for me. Could you please explain what you mean by not as aggressive? Do you mean slower, since the LDR you referenced has a slower rise time than the overall rise time of the original opto-coupler? Could I not just pick an LDR with a faster rise, or i that easier said than done? Or maybe you meant something else....

Thanks - DT