LDRs as pots?

[Koreth]

Wow. What an interesting thread. A lot of cool ideas in this one. Please forgive this act of necromancy, but I figured the question would be better appeneded to this thread than it would be to start a a new one.

It seems that most of the thread is concerned with using LDRs to make voltage dividers to be used as controls, (gain, tonestack, etc) but has anyone thought of using the LDRs to form a simple variable resistor, that one could then use as the plate and cathode resistors of a tube amp stage?

Think about it. You could have all the gain stages in your amp optimized for a beatuiful warm, sparkly clean, then with a MIDI comand to the control circuitry, the load lines and bias points change to what would be ideal for an dirty clean that's dancing on the line between edge of breakup and a full crunch. Combine that with the LDR pots that people in this thread are trying to implement, and you could have the most awesome super versatile programmable preamp that could go from tweed or blackface cleans to a Marshall roar to singing Boogie or anywhere between. Well, maybe not that versatile, as I can't think of a solution to the different cap values in various preamp flavors, but you'd still have a programmable amp that's far more versatile than the current crop of amps.

Now, I know there's people out there who are much smarter than me, and know more about electronics than I, and more than a few haunt these boards. Beyond what's beeen discussed in this thread thus far, what would be the considerations that would make using LDR variable resistors as digitally controllable plate and cathode resistors impractical?

[Steve Conner]

It would be quite practical, but load lines and bias points just don't make much of a difference to the tone, compared to the usual controls provided on a tube amp.

[Robert M. Martinelli]

Originally Posted by Steve Conner

It would be quite practical, but load lines and bias points just don't make much of a difference to the tone, compared to the usual controls provided on a tube amp.

Hmmm.....Steve, with all due respect, allow me to disagree on this one....
load lines and bias points DO have a great degree of influence on tone, because they affect a stage's gain and headroom and the way that stage might "clip", ( think about grid current limiting, cutoff, the possibility to introduce symmetric/asymmetric clipping which in turn can enhance odd/even harmonics etc. etc. ), so they allow the signal's waveform to be manipulated.

The actual point, IMHO, is that, of all the "possible" bias/load line settings, only a few are viable/effective/good sounding, and I think this is the main reason why no one offers the possibility to render these parameters variable by users....it's simply not worth the cost...

JM2CW

Cheers

Bob

[Steve Conner]

Yeah, I agree. I suppose they do influence the tone, but not in a very musical way, that's all.

I've built amps with adjustable bias points for preamp tubes, and there seemed to be one spot on the bias control that sounded right, and outside of that it sounded like an old transistor radio with a flat battery.

The cold biased stage in a SLO etc. would be one exception, I guess. But I'd simply have a switching arrangement to bypass that stage (and maybe a few others) for cleans.

[Koreth]

Originally Posted by Steve Conner

I've built amps with adjustable bias points for preamp tubes, and there seemed to be one spot on the bias control that sounded right, and outside of that it sounded like an old transistor radio with a flat battery.

The cold biased stage in a SLO etc. would be one exception, I guess. But I'd simply have a switching arrangement to bypass that stage (and maybe a few others) for cleans.

Which is why I was thinking of affecting load lines as well, there may very well only be a few bias points that sound ideal with each load line. Change the load line however, be it through a different plate resistor value or a different voltage to the plate from the supply and you'll probably have to change the bias point too to get it back to that point where that stage (and all others it effects) sounds ideal.

This, combined with a switching arrangement as you suggest to bypass gain stages would give you a way to stuff a lot of voicings into a single amp.

[Gregg]

It seems that most of the thread is concerned with using LDRs to make voltage dividers to be used as controls, (gain, tonestack, etc) but has anyone thought of using the LDRs to form a simple variable resistor, that one could then use as the plate and cathode resistors of a tube amp stage?

I think it's a better idea to use regular resistors and switch them when necessary. There are only several values that are most common.

By the way I already have a working programmable tube preamp where the pot I built (picture is on previous page) is implemented as Gain pot. The whole thing is in a 1U rack unit but inside doesn't look good because of the numerous experiments conducted.
I'm currently drawing a PCB for the new version but it will take time. I can record some samples if anyone is interested.

In the mean time the infamous Hamamatsu HTV-P873 is now available from Banzai:

Hamamatsu HTV P873 :: Photocells :: Optoelectronics :: Electronic Parts :: Banzai Music

[loudthud]

That Hamamatsu part is Ouch! Expensive! I've read that the Vactrol photo resistor cells are non-linear when there is large voltages across them. Is this true of the Hamamatsu parts?

[Steve Conner]

Gregg: Samples plz!

Yes, LDRs are non-linear, but so were carbon comp resistors, so it all adds to the mojo.

They're also now banned in Europe because of the cadmium content, as far as I know.

[Gregg]

Gregg: Samples plz!

I'll try to record some this weekend although I'm not very good at recording.

They're also now banned in Europe because of the cadmium content, as far as I know.

That's true. Farnell for example does't offer them in Europe anymore.
But weren't there GaAs photocells as well?

All this cadmium, RoHS and "energy saving lightbulb" thing is complete crap initiated and lobbied by the biggest corporations as Philips and Siemens. EU banned the 100W lightbulb but the new ones have lead gas inside them which the mentioned companies say is OK...

[Merlinb]

Originally Posted by Steve Conner

They're also now banned in Europe because of the cadmium content, as far as I know.

What, LDRs are banned? Or do you mean a particular kind?

[Steve Conner]

I mean all of them, since they're all made of cadmium disulphide.

[Koreth]

Maybe that's why the Mesa Mark V used all relays and JFETs for its switching instead of LDRs like the Marks have always done; Smith didn't want to sacrifice sales in Europe.

[Gregg]

I mean all of them, since they're all made of cadmium disulphide.

Is this the only material that LDR are made from or others can be used as well?

Maybe that's why the Mesa Mark V used all relays and JFETs for its switching instead of LDRs like the Marks have always done; Smith didn't want to sacrifice sales in Europe.

I think Lonestar and Roadking are on relays as well and they came out some time ago, so maybe there's another reason for that. Like realizing that too many LDRs influence the sound in a let's say nondesirable way.

[Merlinb]

Originally Posted by Steve Conner

I mean all of them, since they're all made of cadmium disulphide.

There are still plenty for sale on Farnell UK. I googled about a bit, and it looks like there are still non-cadmium LDRs, so no worries. (I think?)
Cadmium LDRs for audio are still exempt from the ROHS legislation until the end of this year. I guess there will be a review then.

[Gregg]

There are still plenty for sale on Farnell UK.

Sorry, I misread non-RoHS as not available.

[Don Moose]

Tboy -

Thanks for digging this thread back out. I found the bit that I needed.
Have a great day!

- Don

[gtrdude]

Hi there, I'd like to say hi as this is my first post on this board! I see also that this thread has know just turned 3 years old Sorry in advance for the long post but I have to get some ideas out and ask some questions

I have been dreaming of modifying my 5150mkII for MIDI automation for a while now. I have already built a PIC based MIDI receiver which has been tested and works quite well. It was actually built for a couple of other projects but will easily adapt to this one. I don't find assembler programming particularly difficult as I have a background in IT. The real challenge (for me anyway) lies in the digitally controlled potentiometer.

As I understand the Hamamatsu HTV-P873 units are available from Banzai in Berlin (as previously mentioned). They are reasonably expensive at over 10 Euro a piece. There are 12 pots on the 5150mkII (not all requiring 2 ldrs each) so that makes this project worth somewhere around $250-300 Australian for me to undertake but I reckon it's probably worth it given that you would probably pay a lot more than that if this amp had full MIDI control straight off the shelf.

Can anybody confirm that the P873's are the actual models used in the Triaxis and also where the heck can I find a proper datasheet for them?? Much googling has not yielded any results.

On the other hand I cant help but wonder whether there may be a solution that does not require a center tapped ldr. I mean, does the resistive element in the ldr wander around that much that it needs to be "servo"ed and error corrected many many times per second?? I'm guessing maybe not.

Our main problems with the ldr approach are that the resistivity tends to vary with age and also with changes in temperature. One idea that I had was to perform a "calibration" at startup by testing the ldr over its full resistive range (I'm guessing with an ADC) and storing the key values in the memory of a microcontroller. This is basically the same idea as when you calibrate an optical pedal and should all but take care of the "aging" problem. The process would probably have to take place when the amp is on standby with a relay perhaps conecting the ADC to ldr's resistor legs. The only real problem I can see is that as the amps many valves heat up and reach operating temperature this could send our calibration values out of whack. But by how much??? Also if the amp is left sitting for many hours the outside temperature may change depending on the environment. Outdoor gig anyone?? If the amp is unused and is left on standby this would not be a problem because our microcontroller would be continually looping calibration tests.

Am i crazy?? Could this work? Thoughts???

[Gregg]

Can anybody confirm that the P873's are the actual models used in the Triaxis and also where the heck can I find a proper datasheet for them?? Much googling has not yielded any results.

Check out the picture below. It's the Triaxis tone stack:

On the other hand I cant help but wonder whether there may be a solution that does not require a center tapped ldr. I mean, does the resistive element in the ldr wander around that much that it needs to be "servo"ed and error corrected many many times per second?? I'm guessing maybe not.

You can use a shunt volume and gain control instead of the "regular" volume control and then you'll need only one center tapped LDR such as VTL5C3/2, 4/2 etc.

Shunt Pot Volume Control - World-Designs-Forum

This is what Mesa does in their amps as well, but in the tone stack and couple of other places you'll need the double cell LDR. If you have a capacitor across your gain control pot you'll need that one as well.
If you don't want to pay for P873's go ahead and build yourself some.

By the way the samples (along with some other experiments) I promised previously are coming up very soon.