variable gain with exp pedal

[Tahoebrian5]

What would be a successful way to control the gain of a tube stage remotely with an expression pedal? LDR or fets come to mind and seem more elegant than a huge array of resistors and relays. Has anyone tried this? Project I have in mind is a exp pedal controlled effects loop buffer/recovery with variable recovery gain.

[Enzo]

If it is the loop return, the simplest measure is to plug a volume pedal into the loop.

[Tahoebrian5]

While that would be simple, it would require running my signal 20' out to the front of the stage, then 20' back. But I also have other motives for wanting to figure out this circuit. My main problem, being a lead guitar player, is doing quick lead fills where I need to get a signal boost and have effects added quickly and smoothly for a short time, then smoothly back to rhythm. It's difficult to stomp a switch, play the fill, then stomp out smoothly. My concept is to use an expression pedal to do this. And also have a switch to "arm" the system. So exp pedal heel down equals zero boost, zero effects, toe down equals boost plus effects, a switch hard bypasses the whole system. I think a solid state fet loop might be fine for this too. I'm currently studying up on this. I'm having trouble finding a schem for the kleinulator which sounds like a good place to start. The tube version would be good also for my dumble clone. Anyway that's where I'm going with this.

[Mick Bailey]

I've built systems to include the (now disgraced) Vertex 'Clean Boost'. This has an expression pedal input that means the level can be controlled remotely and the audio loop confined to the immediate vicinity of the amp. The last one was housed in the back of a Custom Audio P-100 amp. The pedal is gooped internally so no clues as to operation.

However, If I was doing anything like this for myself I would use NSL32-SR3 opto isolators to control signal levels remotely. They're more than quick enough for the job, have a high on/off ratio and are easily controlled from an expression pedal. For a tube amp you need to make sure the recovery level is high enough to drive the following stage efficiently.

SS Kleinulator here:

[R.G.]

Craig Anderton: "Clean up your Volume Pedal", sometime in the 1970s. This is a version of the NSL-32 LED/LDR already mentioned by Mick and it works very well for volume as well as for a remote wah, which is how I used it. My variation is here: The Technology of Wah Pedals, the picture "remote wah" around the text "cure for pot scratch. A wah pot *is* a volume control, it just happens to make the circuit wah instead of changing the output volume.

Not running signal out on many feet of cable is a Real Good Idea.

I have to insert here that if I were implementing this today, I'd probably use digital pots though.

[Tahoebrian5]

Here is the kleinulator schem I believe. Would it work to place an LDR in series between the bottom of R10 and ground to use as a variable return output?

[Jazz P Bass]

Here is the kleinulator schem I believe. Would it work to place an LDR in series between the bottom of R10 and ground to use as a variable return output?

Depending on the unit it may not go low enough.

[Shadrock2]

While a Volume control does control the signal level, the poster did say control the gain of a tube stage.

I can think of three potential ways to do this and as it happens I've been thinking about this very thing.

Without providing any circuits, these methods are in broad terms these are

1. Using a remote cutoff pentode such as a 6SK7 and remotely controlling the bias point. Somewhat like an AGC circuit in old radio's.

2. My late friend wanted to control gain (for compression) using the extra grids of a pentagrid converter like a 6SA7. I somehow think that the same thing can be accomplished by controlling the supply voltage to the screen grid of a pentode.

3. While not strictly controlling the gain one could install a bounding circuit with a 12AX7 similar to a double Zener Diode that is shunts the signal to ground. A 12AX7 with one grid tied to the cathode and one grid tied to the plate would behave like a opposed diodes and you could control the conduction point by controlling the voltage to those grids. I bet judicious use resistors could control the cutoff characteristics.

I suggest these thing because I wouldn't mind hearing some discussion of the pro's and con's of these methods and also my pathological need to make the circuit all tube.

[Tahoebrian5]

I'm having trouble finding any specs on optocouplers. I'd like to know what is available.. What the resistance sweep is compared to voltage across the led. Can anyone point me to a supplier.

[Jazz P Bass]

http://www.thonk.co.uk/wp-content/up...8/vtl5c3c4.pdf

[Tahoebrian5]

Perfect! Thank you. I ordered a couple to play with

[Jazz P Bass]

Pay attention to the current requirements of the Vactrol.

That is what will determine the lowest resistance.

[Tahoebrian5]

Still kicking around ideas on the variable gain concept and came up with the following. What about running a vactrol in between a cathode bypass cap and ground of the input stage. Then have a supply voltage reference that varies with an exp pedal.. You could also have another pot which sets the range of adjustment.