Channel Switching?

[Tubetastic]

Is there a 'better' system when it comes to Channel Switching?



Is there a benefit over LDR's to Relays or Vice Versa?

[jaysg]

Deftly sidestepping the question, what you'll see is that a lot of designers got used to relays, or LDRs, or Jfets and then they tend to stick with them.

LDRs are truly silent, but can fade away:
Fender
Rivera
Soldano
CAE
Crate

Relays:
Dumble

Maybe we can expand this list?

[Tubetastic]

I joined the Sloclone forum yesterday - curiosity - and noticed the LDR's on the schematic. I've seen them in Fenders and wondered, after seeing the relays in Dumbles....

I can't recall what was in the first issue Fender Prosonic, but recall a Mod for them as those things Popped like a Pistol Shot.

[paulster]

LDRs don't switch immediately (the delay depends on how much current you put through the LED and the choice of LDR in the first place), but that's not normally an issue. The potential drawback with them is they add their own capacitance and there is the potential for some slight bleed through depending on the type chosen (they don't go completely open or closed circuit, more to a very high and very low resistance).

You will get pop-free switching with them though.

[DonMoose]

There's the added fun that, since the R part contains Cadmium and RoHS rules forbid that particular metal, these devices are likely to get harder to find and more expensive.

RoHS rules don't _directly_ apply to US-only product, but if Europe and Asia are big parts of your component market, you wouldn't want to be putting resources into a part you can only sell Stateside.

I asked on another board for some chemistry expertise to think of a RoHS-compatible photoresistor. I also asked a retired Chem. PhD.

Anyone here got ideas?

[paulster]

That's interesting. I just checked the RoHS status on the stash I bought for a project and didn't use and it turns out they're non-compliant, as are all the other Vactrols from the same supplier. This is RS, who will always stock a RoHS part rather than a non-RoHS part so it looks like there aren't obvious replacements yet.

It'll be interesting to see what's going into new amps from the likes of Fender et. al. that have used Vacrols historically into Europe now.

Hasn't California implemented RoHS now as well, meaning that you can't (legally) sell an amp there unless it is compliant?

[Andy Le Blanc]

I was in a Marshall recently the used transistor switching, a JCM800 lead 100.
It seemed a simple solution, everything has a drawback, never was a fan of relay's.
RoHS is coming, it'll take time, but planning ahead for it shouldn't be to bad.
Although lead free solder is a pain.

[Structo]

Yes lead free solder sucks!

Andy were those Darlington's?

[solderstain]

Here's another view...

Mesa has been using LDRs for a LLLLLOOOONNNNGGGGG time. Look inside a TriAxis sometime. Or the Amp Switcher. Or any number of other of their late model amps. You can still buy the brand they used to use from Antique Electronics/CE Dist. They use a different brand now, and I'd guess it's got something to do with ROHS compliance so they can sell amps world-wide.

Here's one problem with LDRs: Logistics. There are are no DPDT versions (at least I haven't found them, but I haven't looked THAT hard...), and since many of the things we want to switch need DPDT capability, you need to use more than one... and a single LDR is about the same size as the relays we use now. Since you'd need more than one, you'd also need a more sophisticated switching system to manage the multiples it would take to accomplish DPDT functionality.

For me, the need for multiples of these large components and the need for more sophisticated switching to make a DPDT function pretty-much sent me back to relays...

My $.02...

EDIT: Since I have some to photograph, I snapped this: The LDR in the foreground accomplishes a SPST switching function. For about the same real estate the relay in the background accomplishes DPDT switching. Relays win!

[img:640:480]http://www.doctorvintage.com/amps/ldr_relay.jpg[/img]

[Tubetastic]

Thanks for taking the time to take the photo.

[Andy Le Blanc]

Structo...

They were BC 184 used in conjunction with an IC, CA 3046, that did the more
complex channel formating. It was a 2210 split channel 100. Its at schematic heaven.
The reverb on off was really simple, just a electrolytic cap, transistor tied to
the reverb volume, and couple resistors, and the power was derived from a
couple of diodes off the 6.3. I hate to put sand into a good amp but it looked
very practical.

[DonMoose]

Transistors work fine as shunt switches (send signal to GND), but when you try to use them as series elements, it gets hairy in a hurry.

Current fenders use some freaky looking circuit on a 2" x 3" card to replace the trem bug now.

Any leads on Mesa's current LDR maker? Maybe they have the answer, or maybe Mesa has an exemption based on the volume of Cadmium vs volume of the rest of the circuit - we know they pay some really good lawyers.

... and being California-based, they would be up on CA's RoHS requirements.

[drz400]

There's the added fun that, since the R part contains Cadmium and RoHS rules forbid that particular metal, these devices are likely to get harder to find and more expensive.

RoHS rules don't _directly_ apply to US-only product, but if Europe and Asia are big parts of your component market, you wouldn't want to be putting resources into a part you can only sell Stateside.

I asked on another board for some chemistry expertise to think of a RoHS-compatible photoresistor. I also asked a retired Chem. PhD.

Anyone here got ideas?

Vactrols are exempt as are tubes as are other opto isolators.
Capacitance in to out is .5pf on a 5C1 I dont see where that would be an issue?
They are transparent and do not have a finite life cycle like mechanical relays. There are Perkin Elmer, popular are the 5C1, there is another brand with lower current 20ma and close to 80 ohms or so and better specs I'll look it up. It is SO easy to turn them into A/B logic with a few transistor or 4049 CMOS which can drive them directly or TTL, 4049 are very easy and can give you a light in the foot switch. Soldano uses a Relay to toggle them but that is very unnecessary.
It can be an advantage having the elements separate when it comes to circuit isolation. Only drag on vactrols is you have to buy large qtys or you will pay $4 each for them but switching is sweet and silent.

[paulster]

Vactrols are exempt as are tubes as are other opto isolators.

I hope you're right but where did you get that from?

The suppliers I use in the UK that list them are reporting them as non-compliant. Tubes are listed as compliant because of the exemption.

[solderstain]

Any leads on Mesa's current LDR maker? Maybe they have the answer, or maybe Mesa has an exemption based on the volume of Cadmium vs volume of the rest of the circuit - we know they pay some really good lawyers.

... and being California-based, they would be up on CA's RoHS requirements.

I've been working through Tien Lawrence at Mesa for several years - he's helped me get parts from them for projects over the years... he told me that he's not allowed to disclose the current LDR source. I didn't press the issue, since I don't really intend to use them for anything anyway. My guess is that they're proprietary, but that's only a guess.