KEL84 Bi-coloured LED

[Mark]

Hi,

I took the bottom off my KEL84 recently for the first time in 12 years to try a stepped attenuator. At the same time I thought I would sort out the bi-colour LED which has always irritated me by starting green and going to orange. Being a simple soul I thought simply swapping the wires over might change it round to start orange and then go to green - which is more logical. Well the attenuator is fine, the amp behaves normally, but the LED now starts red and then goes orange ? How do you get red from a green and orange LED ?! Clearly it's not as simple as I thought. Any ideas anyone ?

Thanks

Mark

[bikerhifinut]

That's because its a red and green bi colour LED. Red plus Green = orange.
simply swaping the legs over only alters the sequence.
One LED is wired into the cathode circuit of one output Valve and the other (red) in the heater circuit. Thus as the amp warms up the colour changes from red to Orange as the valve begins to cunduct and volts appear on the green LED. In fact the colour should be closer to yellow than orange. Also as the voltages are different, 12V or so on the cathode and 6V or so on the heater then the dropper resistor for each leg is different and you would need to swap those over too, depending on where you made the switch of course.The fact that it started green and went orange suggests also that the LED had been reversed originally anyway so it looks like you have restored the default condition.
The idea is that if a fault occurs then the colour of the LED changes. It's of limited use really.
Referring to the build instructions and circuit diagram will show you this if you read them, assuming you built it yourself of course and obtained the full build CD.

Andy.

[bob orbell]

Hi Mark, Andy's comments on limited use is correct, on my WD88 I just used a nice single blue LED, that really is all that is needed. BOB

[bikerhifinut]

It did diagnose an open circuit KT88 bob, but that was more good luck due to the duff valve going in the socket where the LED was wired from.
IMHO to be of any diagnostic use there should be an individual bi Colour LED on each valve, or a single LED on the cathode of each valve to diagnose open circuits as any daft lad can see if the heaters glowing or not. Then again it would be more use to have a meter on the front panel that could be switched across each cathode resistor and then you'll get a measurement that might tell you something about the health of the device.
On Balance i like your idea of a blue LED just to indicate that its switched on. (or choose a colour of your own liking)

A.

[bikerhifinut]

A quick thought.
If you simply disconnect the red LED wire, then it will only ever show green on switch on. If you leave it connected to the cathode circuit then it will slowly come on to full brightness as the amp warms up. Up to you whether you leave it like that or connect it into the heater supply. If you do this just make sure you use the correct value dropper resistor.

In the same vein, you can choose to have it showing red constantly, or by connecting both in parallel you'll get a constant yellow light which to me is the least annoying/distracting colour of the lot and that includes blue! Sorry Bob.
The comment about making sure the dropper resistors are correct values applies in all cases.

Andy.

[Mark]

Andy,

Thanks for your help. I have no problem with the bi-colour LED in principle, so I would like to keep it. I understand its limitations, but it reminds me, and gives me time, to turn the volume knob down to avoid speaker damage and complaints from the wife !

I built the amp myself and I have the instructions, but they were limited to a single separate sheet of A4, which was all that was available at the time.

As-built it started green and then went orange. The central lead goes to the star earth as-built. I have only swapped the outer leads over on the LED. From your respone it now looks like I should have swapped over the connections to the valve bases instead, or as well - but I am not clear about that.

I would like it to start orange (i.e. red and green together and then go green (i.e. red off ?). Is that possible in principle ? If not I will simply put the leads back where they were.

Thanks

Mark

[bikerhifinut]

Originally Posted by Mark

I would like it to start orange (i.e. red and green together and then go green (i.e. red off ?). Is that possible in principle ? If not I will simply put the leads back where they were.

Thanks

Mark

Er, no.
Have a wee think about it mark. In order to get orange (actually should be yellow if both leds are at the correct brightness) then BOTH LEDs have to be powered up, so in order to switch the Red one off you'd need some sort of relay or electronic switch that energised as the power valves come up to temperature and pass sufficient current. Quite possible to achieve but requires a bit of complicated circuitry to sense then switch so why make an easy thing hard?
I still think the default state is Red turning to yellow as the valve warms up if wired correctly.
What's important here is the the dropper resistors are the correct value as they come off the heater supply and off the cathode of whichever valve its connected to.
For myself i'd just get a LED of the colour I liked best and connect in to the DC heater 6.3V with an appropriate dropper resistor that you calculate using good old Ohms law and the data provided from the diode manufacturer (Vf and current draw in mA) subtract Vf from 6.3 to get the drop required and then its R= V/I. Simples. Or just leave it be and accept it for what it is. Or disconnect the Red LED and leave it green. Lets face it you'll soon know if the HT or LT fails cos it'll just go silent.
A.

[VantheMan]

When my E.E. mate rebuilt my OTT Puresound P10 valve phono into 2 boxes he somehow put a delay on the switching on of the full circuit so that the heaters had time to reach their operating temp before the anode (or was it cathode) started in. My led starts yellow but I should wait for it to change to green before placing needle on disc. Something to do with valve life. Maybe someone can explain.