Off-topic but...
SY!
$2.88/gal. here yesterday.
Now don't go and get 'Manson' on somebody!
I did have a bag of around 500 (minus a few) T-3 green LED's, but I gave most of them to my computer geek brother. Ordered 300 of them from MCM quite a while back and almost 600 is what I got! Too lazy to count them I guess, so somebody just grabbed a few handfuls.
I've thought of using LED's as such and to use as zeners (V-refs) many years ago. I think you guys gave me the bug again!
Cheers
Wayne
edit: My wife tells me it's going for $2.59/gal across the river in KY!
SY!
$2.88/gal. here yesterday.
Now don't go and get 'Manson' on somebody!
I did have a bag of around 500 (minus a few) T-3 green LED's, but I gave most of them to my computer geek brother. Ordered 300 of them from MCM quite a while back and almost 600 is what I got! Too lazy to count them I guess, so somebody just grabbed a few handfuls.
I've thought of using LED's as such and to use as zeners (V-refs) many years ago. I think you guys gave me the bug again!
Cheers
Wayne
edit: My wife tells me it's going for $2.59/gal across the river in KY!
Hi there,
Since this thread is not to dead I thought I would ask SY a question concerning his CCS (again).
I am building a 5687 driver stage with 165V on the top. The cathode is biased with 2 red LEDs, giving a bias of 4.2V. The circuit settles at about 145V on the anode of the 5687. The plate current is variable between 18- 25mA. I have a 2K pot to adjust the CCS. I tried various things to increase the voltage drop across the CCS, but nothing helps. The interesting thing is that whatever position the pot is set in there is always just 0.6V between the collector and the base of the first transistor. There is a 9.0V drop from the base to the emitter. When the pot is at its lowest setting (ie highest current through the CCS) the voltage across the LED reference collapses and the LED's go out. At a slightly higher setting the LED's flash in time with the signal.
The CCS has a 22K resistor at the bottom of the reference chain, and 2K above that.
The question is do you think the CCS is working and i'am just not supplying adequate +B ?
The first transistor measures OK but if it failed would this cause the problems ?
Whats going on when the voltage reference is collapsing?
Would increasing the current thought the reference chain solve the problem?
Thanks
Shoog
Since this thread is not to dead I thought I would ask SY a question concerning his CCS (again).
I am building a 5687 driver stage with 165V on the top. The cathode is biased with 2 red LEDs, giving a bias of 4.2V. The circuit settles at about 145V on the anode of the 5687. The plate current is variable between 18- 25mA. I have a 2K pot to adjust the CCS. I tried various things to increase the voltage drop across the CCS, but nothing helps. The interesting thing is that whatever position the pot is set in there is always just 0.6V between the collector and the base of the first transistor. There is a 9.0V drop from the base to the emitter. When the pot is at its lowest setting (ie highest current through the CCS) the voltage across the LED reference collapses and the LED's go out. At a slightly higher setting the LED's flash in time with the signal.
The CCS has a 22K resistor at the bottom of the reference chain, and 2K above that.
The question is do you think the CCS is working and i'am just not supplying adequate +B ?
The first transistor measures OK but if it failed would this cause the problems ?
Whats going on when the voltage reference is collapsing?
Would increasing the current thought the reference chain solve the problem?
Thanks
Shoog
I think there's the source of your problem. That should be 0.7V. Check that transistor. Also, your reference string is carrying 8 mA or so. That's fine, but your 2k resistor is trying to drop 16V! Substitute another LED for it, or drop its value to 470R. I'd just play it safe and sub in the LED.
One other thing- the LEDs in the cathode circuit should drop about 1.7V each. If they're dropping something north of 2V, they may not be suitable; try some super-cheap Radio Shack or similar. The older, crummier, and cheaper, the better.
BTW, this is NOT "my" circuit, it's just one I've found useful and have popularized a bit. It's an old idea, but if one were to credit anyone, it would be the guy who set me down that path, Morgan Jones.
I worked out what was wrong. Boy am I stupid. I had installed the transistors as if they were NPN's (ie with the Emitter and the Collector reversed). Fortunately the transistors hadn't suffered. Everything is now working. Since the Red LED's have a voltage drop of 2.1V I took out one from the reference chain. I also replaced the 2K2 with a green LED.
The plate voltage now adjusts itself to the correct point for the current set.
Thanks for the help, it was your point that the base to emitter voltage should be 0.7V that got me thinking.
Shoog
The plate voltage now adjusts itself to the correct point for the current set.
Thanks for the help, it was your point that the base to emitter voltage should be 0.7V that got me thinking.
Shoog
This LED biasing topic has got me interested. Years ago I bought a bag of 1000 rectangular-shaped red LEDs for $3. Talk about CHEAD red LEDs! I wondered what I would ever do with that many LEDs, but I couldn’t pass up the bargain. 1000 pilot lights for a 1000 projects? Not in my lifetime. Maybe now I know…
Surface Mount Small red LED
Hi,
I have measured the V/I characteristic of the small surface mount RED LED. Attached pls find the measured datas.
It seems to me that the linear region of such small LED is good from above 1mA. It should be good for use in the circuits with smaller idle current.
Johnny
Hi,
I have measured the V/I characteristic of the small surface mount RED LED. Attached pls find the measured datas.
It seems to me that the linear region of such small LED is good from above 1mA. It should be good for use in the circuits with smaller idle current.
Johnny
Attachments
what about regular diodes for lower BIAS voltage?
Hi everybody,
I was just wandering... what about a few (2 or 3) "regular" diodes (e.g. 1N4148)
in series for tubes requiring lower BIAS voltage/current ?
And what about Zeners for higher BIAS voltages? (well, I guess they would
require good bypass at least for noise, so maybe their use is pointless... or
maybe not?)
Somewhere I've even seen vacuum diodes used for this purpose...
Hi everybody,
I was just wandering... what about a few (2 or 3) "regular" diodes (e.g. 1N4148)
in series for tubes requiring lower BIAS voltage/current ?
And what about Zeners for higher BIAS voltages? (well, I guess they would
require good bypass at least for noise, so maybe their use is pointless... or
maybe not?)
Somewhere I've even seen vacuum diodes used for this purpose...
I use glass signal diodes to bias the fets (fet/triode cascode) in my phono stage currently. I chose these as I needed only 0.6V. My phono stage is not perfectly quiet but I do not suspect the diodes at this point.
I use zeners to bias my output stage. I use two identical strings of 5 x 5.6V zeners, in parallel. This is for power handling, and it works better than I thought it would. The zeners were a little unstable/unpredictable at higher frequencies but a film capacitor took care of that (2u2F MKP).
I use zeners to bias my output stage. I use two identical strings of 5 x 5.6V zeners, in parallel. This is for power handling, and it works better than I thought it would. The zeners were a little unstable/unpredictable at higher frequencies but a film capacitor took care of that (2u2F MKP).
If I'm not mistaken I read somewhere in the pages of this forum that zeners may tend to be noisy? And this could be amplified by the transistor that uses that zener as voltage reference.
That's why I stick with LEDs and the good old 4148s.
EDIT:
I read an article in tubecad that zeners with lower brekdown voltages are muych quieter than higher ones.
That's why I stick with LEDs and the good old 4148s.
EDIT:
I read an article in tubecad that zeners with lower brekdown voltages are muych quieter than higher ones.
Re: Surface Mount Small red LED
Nice work, thanks. At 2-3mA, the resistance does look fairly linear for small current swings, but the dynamic impedance is extremely high, roughly 30 ohms. That's typical for high efficiency devices, great for lighting, less so for reference or cathode use.
There's an optimal point where noise is minimized. I can't remember, but it was somewhere around 6V. But it's still very noisy compared with an LED or a reference diode like LM329.
kmtang said:
Nice work, thanks. At 2-3mA, the resistance does look fairly linear for small current swings, but the dynamic impedance is extremely high, roughly 30 ohms. That's typical for high efficiency devices, great for lighting, less so for reference or cathode use.
JojoD818 said:
There's an optimal point where noise is minimized. I can't remember, but it was somewhere around 6V. But it's still very noisy compared with an LED or a reference diode like LM329.
that is probably the same argument I saw in that article. and the very reason why I always play around with an LED instead of zeners, not to mention that LEDs are much much cheaper.
btw, SY, this is all your fault. ever since you showed me a variation of Morgan Jones ccs I have been busy reading his book and playing around with ccs, the difference is just - it's different and I like it.
only problem is now, everytime I see a tube circuit, I have this undeniable itch and ask myself how can I install a ccs in that circuit.
cheers
As I mentioned, I used 5.6V zeners. This solution was proposed for an output stage, a good choice I think due to the lack of further gain in the amp.SY said:
Quality film bypassing is necessary, but with this, the most noise my amp makes is clearly the residual hum I can hear when listening at the speakers.
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