The Red Light District - another PP EL84 amp

[wrenchone]

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.

[tubelab.com]

another one of those bathroom circuit scribblings that panned out.

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!

How about an emitter follower with LED(s) as the ref?

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.

[rknize]

Could always put them behind a faceted crystal disco ball.

[Ian444]

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?

[TheGimp]

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.

[tubelab.com]

Could always put them behind a faceted crystal disco ball.

Or inside a shiny metal box.

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.

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.

[TheGimp]

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.

[Lingwendil]

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

[SY]

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.

[wrenchone]

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.