The Red Light District - another PP EL84 amp

[SY]

No problem, Larry. I'm talking about the output stage- that's the only place where the timing is important. The input stage can come up as lazily as it wishes.

 

[Doug in Indy]

Thanks SY. One more question. I have fairly long heater wires to the tubes. I assume I should put a 0.01 ceramic cap from tube heater pin to chassis ground?

 

[SY]

Yes, that's always a good idea. Much of the noise in heater lines is common-mode, and that's an effective way of killing it, so use a cap from each heater pin to ground.

 

[Doug in Indy]

Thanks again. I will have more Q's I'm sure. I appreciate the help.
Doug

 

[SY]

Never a problem. Trust me, you'll end up paying the price.

 

[jackinnj]

I was getting 450 volts, so I put a 3300 ohm resistor in after the bridge. This brought the voltage down to 390.

Belt and suspenders needed (perhaps) here -- i've found some bulging electrolytic capacitors -- what i did not appreciate is that the line voltage here in NJ, while typically low in the summer months will go over 127, and for long periods of time -- over-voltage on the filter caps causes heating.

I now use a pair of 350V in series with load balancing resistors (220k).

I became aware of the voltage problem when I used to print B&W -- the color temperature of an incandescent bulb changes with current, and as it shifted red the exposure times got longer and longer!

 

[Doug in Indy]

I'm baaaccckk:

Well, it has occurred to me that I may have done a poor job selecting the OPT. I chose the Edcor CXPP21-MS-8.6K. 21W, push-pull tube output transformer for 8.6K Ohms to 4, 8 and 16 Ohms. Freq Resp 20-20k.
My speakers are Lynn Olsen's Ariels. He advises 4 ohm taps.
Well, I have been trying to figure out the effect of different impedances, and I THINK I ghouls have chosen a lower impedance. I have been trying to find info on the effect of this for a pentode, but not having any luck, and my brain is starting to hurt.

So, gurus, before I wire up these OPTs, are they OK to use? I don't mind losing a couple of watts.

Thanks again.
Doug

 

[Doug in Indy]

Uhhh... autocorrect seems to have replaced "chosen" with "ghouls."
I THINK I should have chosen a lower impedance.

 

[SY]

That impedance is perfectly good. Don't sweat it though, your speakers' impedance probably varies 4 or 5 : 1, and that impedance is reflected back to the primary.

 

[Doug in Indy]

Thanks SY!

 

[SY]

Oh, don't worry, you'll pay.

 

[Doug in Indy]

I'm stocking up on Masonite and shelf paper for you

 

[SY]

My wife will be soooo pleased.

 

[Doug in Indy]

Well, I seem to be unable to make a Maida reg that regs. First, I am checking it out on the breadboard. I have a 120/240 volt transformer into a bridge, then 3k3 resistor followed by CLC (100u/9H/100u). This gives about 260 volts into a 25k dummy load (about 10 mA.)

When I connect the Maida I get the same voltage whether the pot is set to 100k or 0k, about (220 volts.) The TIP50 checks ok as a diode from pin to 2 and pin 2 to 3. The zener checks Ok. I have replaced the LM317.

Also, there is something funny about the pot. I put a new one in, and it goes from 100k to 0k. Then I put it under power, recheck the pot value (without adjusting it) and it reads 24k. Then it won't go above 75k in the circuit. I checked the pot I took out (because it was doing something similar) and it now goes from 100k to 0k. So, it acts weird in the circuit.

I have re-soldered everything several times. No regulation.

Just to have another pair of eyes on this, this is what I made on perfboard:



I am stumped.

 

[SY]

Is that a top view or bottom view with respect to the transistor and regulator?

 

[Doug in Indy]

top view

 

[SY]

OK, you have 260V as the raw supply? If so, that's your problem right there- you want a higher voltage than that. It has to be at least the regulator voltage plus the zener voltage plus some padding. 325-350V is more appropriate.

 

[Doug in Indy]

I was just testing it with the isolation TXR because it was handy and not quite so scary to play with. I have another for the actual build. Are you saying that this reg won't work at this voltage? I know it's too low for the actual screens.

 

[SY]

Yes, at this point, you're under the dropout voltage. You can confirm this by lowering the voltage setting resistor and seeing if the regulator functions at a lower voltage. A 25k set resistor will give you a nominal output of 150-155V or so. Does that happen?

 

[Doug in Indy]

Is that the 56K? I can just use the real TXR and see what happens. I'll report back.
Thanks again SY.
Doug