Help with Amp schematic

[valdiorn]

Hey there, I know this is a bit off topic, but this place is so full of talented people that I thought it was the best place to ask

Anyway, thinking about utilizing this Engl schematic for a preamp project, but I'm having some trouble reading it.
See the schematic here: http://www.houseofenglamps.com/products.asp?sku=E645_V2
It's an Engl Powerball

I'm looking at the part after the first tube stage, where the Gain pots are. There are two small icons, one right after each Gain pot, labeled LD1/LT30 and LD2/LT30 ... anybody know what that is??

If you can comment on this, I'd really appreciate it


PS: thinking about J-FET'ing the preamp of this sucker and packaging it in a 2U 19" rack

[BrianJ]

Those are LDRs - Light Dependent Resistors (or something like that) - they're essentially a resistor that varies in resistance based on how much light it is exposed to.  The amount by which they can vary is ten or hundreds of ohms to as much as 50 meg making them useful as both variable resistors or, in extreme circumstances, switches.  By removing current from the light emitting part of the component (the light source LED is packaged with the LDR in a 4 lead package called a photocoupler) the resistive element rises dramatically effectively blocking the signal.   By increasing the current through the light emitting element the resistance drops until it becomes trivial.  The switch from "on" to "off" is somewhat smoother than relays so they have less tendency to "pop".  The pay off is that they are not perfect switches and sometimes bleed a little signal when you don't want them to depending on the impedances of the circuit around them.

In the scenario you present they are blocking the output of one gain pot at a time to switch between two settings.  Without a serious look I would guess there are others that will toggle in and out of the circuit to select between two EQ circuits and a master volume.

If you want to use them I recommend vactrols form perkin elmer.  I think Allied Electronics carries them.  One model, VTL5C1, is what I have used in the past for channel switches.

Good Luck

[valdiorn]

Wow, thanks for a great response!
I found the corresponding LEDs, they were lurking in the switching circuit.

I'm going to completely redesign the switching system, so I'll probably end up using a small relay there, or a 7400 series Digitally-controlled analog switch, should work. I'm guessing they used photocouplers because they make a more graceful (slower) switch between on and off, minimizing any clicks or noise that might occur during the change, but I'm not really sure... only one way to find out

[d95err]

One note of caution - Engl schematics are known to contain lots of errors (it seems Engl does this on purpose to confuse copycats). Missing components or wrong component values are common. I don't know about this particularschematic though, it could be accurate.

You can essentially see the optocouplers as SPST switches. They are useful in tube amps because they can take high voltages. In a FET preamp, there will be no high voltages, so it would be a lot cheaper and easier to use JFETs as switches instead. Just look at any BOSS schematic to see how it's done. If you don't need remote switching (using a footswitch) you can just replace the optos with mechanical switches instead.

[Roland]

yep all engl schematics are wrong and the powerball is wrong also. Interstage components responsible for the tone shaping and engl tone are missing and resistor values are wrong or outright missing..tone stacks are wrong.....etc. The power supplies are usually right tho.

There is hope however as they are reversed engl schematics out there...one of the better ones is the 530 preamp zip at the unofficial engl forum. You will have to do some digging there. There is a screamer at a russian site...some partials at blue guitar..and some decent efforts at sloclone.