I'm sort of glad I butted in here with my no light/one light bias proposals, as it has led to a lot of very fruitful simulation work regarding shunt regulators that should be useful for active cathode bias circuits as well as discrete shunt regulators in general. When all my PSpice twiddling dies down, I'll present my results here, but start a couple of other threads or add to others I have going to detail the results with the circuits that I actually use.
My bathroom circuits usually smell bad.....one way or another. My best circuits come from those ADD moments when my mind wanders into a zone somewhere. Meetings at work involving managers and Powerpoint slides, or boring tasks like mowing the yard are good fertilizer for ideas. If I could only enter that zone when I really wanted to, I would be a freaking genius!
It's just too simple. I got to try it. I decided to ditch the LED's on my latest project because the amp lights up the whole room (8 output tubes). The active circuits being proposed, have no glow, but I could just stick in some random LED's just for effect, but they wouldn't dance to the music when cranked like the white ones do.
Wrenchone, if I was to build up your cct in post #666 for an EL84 PP, aiming for around 11.5V and 40mA on the cathodes, as an experiment, would the following component values be a reasonable place to start? It would be interesting to try this and compare it to the LED's I am running now, but being a newbie, design work is not my forte. Hence the question.
R1 = 50K trimpot (10 turn)
R2 = 2K7
R3 = 510R
Omit R4 or is it required?
Would it be feasible to connect the 2 cathodes of the PP output section tubes with say a 2K2 resistor to keep each CVS functioning when its tube is in cut-off? What is the purpose of R3/R2, to dampen the response a little?
R1 = 50K trimpot (10 turn)
R2 = 2K7
R3 = 510R
Omit R4 or is it required?
Would it be feasible to connect the 2 cathodes of the PP output section tubes with say a 2K2 resistor to keep each CVS functioning when its tube is in cut-off? What is the purpose of R3/R2, to dampen the response a little?
I don't think you want to replace R1 with a pot for fear of damaging the ref input of U1, instead, why not just reduce the value of R1 and add a smaller trim pot between R1 and R6 to adjust the voltage.
Vref = (Vo/(R1+R2))*R2
So, for A simple divider, R1=Vo/Vref*R2-R2, so for 11.5V, R1=(11.5/2.485*10K)-10K = 36277.7 ohms. 36K5 being the closest fixed value(1%).
If you want to make it adjustable over a small range, use a 33K (5%) resistor for R1, a 10K pot, and 4K7 resistor for R6.
For safety, add a 20K resistor from the adjustment leg (to the adj pin o fthe TL432) to the bottom of R1 so that it will pull the adjustment pin up if the wiper goes open.
Vref = (Vo/(R1+R2))*R2
So, for A simple divider, R1=Vo/Vref*R2-R2, so for 11.5V, R1=(11.5/2.485*10K)-10K = 36277.7 ohms. 36K5 being the closest fixed value(1%).
If you want to make it adjustable over a small range, use a 33K (5%) resistor for R1, a 10K pot, and 4K7 resistor for R6.
For safety, add a 20K resistor from the adjustment leg (to the adj pin o fthe TL432) to the bottom of R1 so that it will pull the adjustment pin up if the wiper goes open.
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Or inside a shiny metal box.
If you are not in a hurry, I will try this and other circuits on my amp when I return from a 3000 mile road trip which begins today.
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I just realized that tying the safety resistor that way will result in Vo going to zero, which normally would be the safe way, however since it is providing bias to the tube that will bias the tube on hard. Not a good idea. Taking the 20K from the wiper to the lower pot to the top of R6 junction will result in Vo going to max and it will shut down the bias current thorugh the tube. However, what will that voltage be? 30V? This should be safe for the pass transistor so it should not be a problem for it, but the TL431 has a max of 37V K-A.
Dual 20K resistors from teh wiper to both sides would probably be the safest way to wire it.
Dual 20K resistors from teh wiper to both sides would probably be the safest way to wire it.
Would these Nice Edcors work in this amp? I've heard good things about the SE transformers and was wondering if anyone has used them in this amplifier.
EDCOR - XPP15-8-8K
EDCOR - XPP15-8-8K
You may have to twiddle the compensation network a bit, and these won't be as nice as, say, James, but for the price, they look nice.
The bitty Edcors are nice for the price, but you can get better for not too much more - the next size in the Edcor P-P lineup, for example. I've also been grabbing a lot of EL84 and 6l6 transformers parted out of organ amps. They don't look too bad, and they're cheap. I'm using a pair in a push-pull project I've got going in another thread.
I tend to agree. I got a pair of the XPP10-8-8K OPT's and was not impressed. I got them for a 6AQ5 P-P experiment and they worked OK in that amp. I have since wired them into a Simple P-P which is similar to the RLD. The little Edcors are overwhelmed. Granted I am running them above their ratings. I could have bought the XPP15's for $3 more. What was I thinking.
Next time I will go for the XPP25's for about $30 each.
I was actually thinking of the GXPP series. If they are anything like the SE series, the core stack will be much larger, with better low end extension.
It seems that the product weight correlates well with the price across both the XPP and GXPP lines. So it seems that price VS performance may be close to a constant. One thing to note, the GXPP's have end bells, while the XPP's are open framed.
I have plenty of the "rusty transformers" without rust still in their original 8 per box packaging. I am using in all of my P-P experiments where the impedance comes close.
Edcor does list lower operating frequency for the GXPP series. I wonder if they're tinkering with both the turns and the stack height? A measurement of primary inductance would be instructive.
Speculations and actual circuits for active biasing schemes have been branched off to a new thread called the "No light District". Cheeky....
Glow, you want glow.....we don't need no steeenkin LED's to make glow.
My version of the NLD makes plenty of glow, cheerful and even excessively tacky glow. And despite my reputation none of it is coming from the plates of any tubes.
Not to dirt on SY's deal at all - one day I may do an LED-biased amp in a context where I can put the excess light to a tasteful use. I was thinking of a cube-style amp with marble sides (translucent) and a bunch of bias LEDs inside. Who needs a pilot light? It so happens that I have a slab of rough-cut marble in my basement looking for a purpose in life.
I have constructed a "Tube Cube" amp. I was digging through my warehouse looking for some sweep tubes to feed to the red board when I found my old Carver M-400. It was the first Carver branded amp and it was cube shaped about 8 inches on a side.
I decided that I would build a cube shaped tube amp with all the guts on the inside. The insides are all visible since the front is mostly clear Lexan. To add to the "bling" There is plenty of shiny stuff. I even went to one of those shops that caters to the "sport compact car" crowd for some parts. Glow is provided by the VR tubes and maybe some "lightning wire".
The amp has been built and working, but is currently dissassembled for modifications to the screen regulator ans the No Light circuit (LED eliminator for cathode bias). I have squeezed 50 WPC into a 12 inch cube.