RGB LED light strip for Tubemeister 36

Hey all:

I love my Tubemeister 36, and have had it for a while now. Trying the mod/trick of changing the LED strip to an RGB strip with remote control. Works, but the power source I tap into from the old strip is not powerful enough. Anyone have any thoughts or amperage numbers for the output from this internal power source. I hate to have an AC adapter hanging off the amp. Any help, schematics or numbers would be great.

Thanks,

I don't know how you are connecting your RGB strip LEDs but it should be pretty simple. The feed to the existing LED strip is simply -22V through a 680R resistor. Take note of the fact that it is -22V and not +22V.

The idea is just that when the right number of LEDs are put in series with the supply voltage their forward voltages reduce the voltage across the resistor. The voltage that is left across it then limits the current through all of the LEDs to the specified value. It would be very easy to put some sort of active control in there, a transistor current source for example, to make this more solid.

What do you have set up at the moment?

I may be teaching granny to suck eggs here, you may be a very experienced engineer, but for everyone else who is not.  At 10mA current blue LEDs will have a forward voltage drop of somewhere around 3.2V.  This varies with the LED types and the current through them but it is reasonable for blue.  Red-Yellow types start at around 1.8-2V.

For the stock setup, the 5 blue LEDs will take around 5x3.2V=16V.  This leaves 6V of the 22V supply across the 680R resistor.  That means the current through resistor and all LEDs will be the same as they are in series and it will be 6V/680R=8.8mA.

As the LEDs in your RGB display will be Red/Green/Yellow types with a lower forward voltage I would have thought that the supply would have had enough current available to make them work ok.

Hey there bordonbert.  I am not an electronics guy at all to be honest.  I do have an amp tech that could do some of that for me though.  I am simply disconnecting the existing LED and connecting the small plug end of the PS that came with the strip into the existing cable for the blue strip.  There are 21 5050 LEDs on the new strip.  The strip power supply says it is 12V and 5 Amps.  I get a measurement of around 3v on the amps internal strip power source.  Hope this makes sense.  I can try to send you a couple of pics too.  Really appreciate your assistance on this.  I really would love to get full brightness out of the strip, and my amp guy mentioned perhaps changing the resistor.  He also mentioned he would not want to effect the amp sound or anything.  Let me know what you think.  The other one I have coming in this week is 2 amps instead of 5 amps.  I am getting the LED strips on Amazon.ca.

You cannot measure the internal voltage in the amp with anything attached to it and get any meaningful result. That 680R resistor is the key! It means that the supplied voltage is -22V from a source impedance of 680R. Your amp guy has spotted this. I would advise you to forget this project, it can't work. Let's see if I can explain.

You are measuring 3V on the supply from the amp. Well the amp supply is -22V inside. It just plain IS -22V! The reason you are seeing it as 3V is that you have something attached to it which is drawing current. That current is coming through the internal 680R resistor. Ohms Law (it trumps Mojo every time!) says that the resistor then MUST have a voltage across it. You have lost some of the -22V across the resistor. And the more current you try to draw out of it the more voltage you will lose in the same way. Remove all attached circuitry and just measure the voltage supplied from the amp with nothing attached to it. -22V! That is because now you have no current flowing through the resistor so it drops no voltage across it so you see the full -22V at the output.

The supply for the units you are buying shows this. The voltage it gives is 12V which is what the control section and the LED strips want to see. But this must be a solid 12V no matter what current is being drawn by the attached circuitry. It cannot be allowed to drop because current is being demanded, hence they give you up to 5A capability before it would start to drop noticeably. It will read around 12V with no current draw and around 12V with 2A draw. Your supply line in the GM36 cannot possibly do this. It is designed to do exactly the opposite in order to protect the blue LEDs.

Your tech is correct, you could remove the 680R resistor, but you will be demanding more current than that line was designed to supply. It also feeds the -ve supply to all of the amplifier's circuitry like its opamps used in the signal line. If you compromise the voltage supplied to them you will certainly suffer in sound terms.

It seems a very cool project at first sight but it is not possible to use the internal supply to make it happen with this unit. You would be better running a thin line from your wall wart supply specifically for the LEDs. Then they should work without any problem.

I just read what you said up there again and...

I am simply disconnecting the existing LED and connecting the small plug end of the PS that came with the strip into the existing cable for the blue strip.

Do you mean you have the original H&K LED strip powered by the 12V 5A PSU? If that is the case DON'T!!! Stop right away. You are trying to run a current controlled device from a fixed voltage. These are conditions which could blow the LEDs and will do if the voltage is only a tiny bit above their correct working level. They must have something in the circuitry to limit their current to a chosen value, that is what the internal 680R resistor is there for. You must not supply them with a voltage that is fixed no matter what current they take. The voltage needs to drop as more current is taken in order to settle at the correct point.

Despite what inexperienced people on the internet do or say, LEDs are not bulbs. You can't just attach them to any old supply and think they will work. You must learn how they are set up properly and why and stick to that. That isn't some form of engineering snobbery that is simple common sense protection of your expensive gear.

Nope not at all. I am attempting to power the new strip with the two wires that were connected to the blue strip. That is when the dimming is happening. I could mount the new strip power supply to the back of the amp, but that is my last resort. It is clunky and I am worried about the heat from the tubes as well.

Do you think the lower amperage led strip will make a difference? If not I will likely work on putting the original power adapter on the outside of the amp and running the cable inside to power the new strip.

The heat aspect is not a problem at all despite what people think.  This has been analysed pretty deeply to the extent of using an infra read thermometer gun to read temperatures inside the top case.  The reason it feels so hot is that it has a metal case acting to draw heat quickly out of the compartment and pass it to the outside air and hence feels very hot.  This is unlike a traditional amp which has a wooden insulating case which is always just warm to the touch.  It is a deliberate design feature on the part of the designers.  It keeps the temperatures inside the top valve compartment cool and draws heat out quickly so the lower compartment stays very cool too.  Compare it to some of the classic Fenders with their upside down amp fitting so the heat goes up to the metal chassis and is conducted through to the electronics compartment where it heats up a wooden insulated box.

Try looking at post #3 here:  Heat and Hiss

...and here:  Cooling

Without knowing the specifics of the unit you are trying to fit it would be difficult to say but I don't think you are going to get any joy out of this without butchering the TM's circuitry. You are trying to fit a device into a system which is specifically designed to do something opposite. The units you are fitting want to see a fixed voltage which will give them whatever current they demand. The existing setup is for a unit which wants a specific current and the voltage is being automatically reduced until nothing more that that current can be supplied.

I've got your LED unit up klegger. amazon.ca - ALED LIGHT Led Strip 16.4FT/5M 5050 RGB 150 SMD Waterproof+44 Key IR Remote Controller+2A Power Adapter+Receiver for Home Lighting & Kitchen and Indoor Decorative.

But this is 5m long! What are you actually doing with it?

Cutting it to size .. same length as the existing one. You can cut them on the cut lines.

Can I send you an email with a link to a youtube video of what I am working with? I have it connected to it's own PS right now, but the connection from the amp is shown in the video too. Cannot send links in my posts or personal messages right now. Send me your email via PM if you want.

Yes, there is a lot of info on YouTube about these.  Here is one: LED strip usage  The white blocks are the LEDs and the smaller black blocks are the limiting resistors.  Note how some strips have the same number of each and some have three LEDs to one resistor.

I can see what you are hoping to do now.  It is exactly how I imagined it would be.  It won't work without a bit of modding of the TM and the results could cause problems with the other systems in the TM which share the same supply line.

The strips you have bought work by having a power line and a ground line running all the way along them.  There are then points where LEDs tap off their current from that power line.  Some of the bigger ones have each LED with its own connections to the power via its own resistor, the smaller ones have 3 LEDs in series connected blocks with each block having its own resistor.  When you cut them you are cutting each block as a whole.  Where there are three LEDs and one resistor, the LEDs are in series with each other and the resistor in a chain.  This way they each share the same current one after the other which means only one current stream for the three.  That means the voltage they need to see is much bigger.  Where each LED has its own resistor it can be fed from a lower voltage supply but there will be 3x as much current needed as they are no longer sharing it.

Each individual LED/chain is then a separate device on the power line with its own personal limiting resistor and is controlled just as I've described for the TM's LED action.  The resistor acts to restrict the current to the correct value for the LEDs to work given a particular voltage.  The problem you have is that these LED chains in the strip are all in parallel which means their individual currents through the controller will add up.  In the original setup the LEDs are all in series meaning they each share the current one after the other.  As they all need roughly the same current whether they are in series or parallel this means that your new units will multiply their current demand by the number of LEDs you choose to use.

Assuming you want to use only two blocks of LEDs from one of the smaller strips it would be possible to work out the current needed if we knew the value of the voltage supplied by the controller unit to the LED strip, that's not the same as the bigger dumb power supply voltage, and the value of the resistor on the strip.  Can you see any writing on the tiny black resistors?  It's likely to just be something like "330" not something complicated.  Can you also measure the voltage supplied by your controller when your short length of LED strip is being driven from its supplied power supply?

Pic attached .. hope you can see it.

That's odd. 2 of those are 150R and one is 330R. Is that all of the components in that whole section or is there another resistor off the right hand edge which is still part of it? Do all of the LEDs work the same doing the same job colourwise?

This is what seems to be one full section.  Each light does full color which is really nice.

No, it makes sense. You have 1 +12V line and a 0V line for each of the colours. The LEDs have each of their colours all in series with each colour having a resistor. The different values are to balance up the intensities.

That's bad for you because it means that you will have a varying amount of current supplied depending on what colour you are running. Red, green or blue will use only one colour chain. Yellow, cyan or magenta will double up as they require two colours and white will need all three running.

And they each require 12V. The LEDs will be a little tolerant of their voltage, the resistors will reduce the current change to a point, but the controller won't like it if it see anything other than its stated 12V. The TM36 can't supply that. You have to forget your idea of using its internal supply. With the variation in current to give you all of the colours you can't even use a resistor to drop the -22V to 12V as it would be the wrong value as the current through it changed.

The only way is to build a 12V regulator, not difficult for your Tech and the parts are pence but not cheap for you for his time, and use that between the TM's -22V line after the resistor is taken out to supply the controller.

This is an interesting topic. It's one of the reasons I found this forum. I was thinking of doing a similar mod to my little Tubemeister 5. I think you're making this more complicated than it needs be, but I'll reserve further comments 'til "I put my money where my mouth is", and do it myself.