Kofi Annan in: "Cascodin' with Steve Bench's RIAA!"

[Salas]

As you face it legs down, flat face with letters in front of you, it is CBE from left to right.

 

[Salas]

Are you making this HV shunt?

 

[Kofi Annan]

Yes! Thanks!

 

[Kofi Annan]

Yes, although I have a CLC after rectification.

 

[Salas]

In your case, the 33R R1, can be 56R and you can have around 40mA constant current in the shunt, since 2 bench channels need 20mA. You can save a lot on heat this way.

 

[Kofi Annan]

Here are some progress photos. First, the filament supply. This has been tested and works fine.

 

[Kofi Annan]

Then the shunt regulated supply that Salas created. This has been assembled, but I won't be able to test it until tomorrow. Note that I did not anticipate that the heat sink would be so big that I would not be able to get it in the MOTHERFATHER BOX!

So, I'll spend some of tomorrow re-cobbling the box. No big deal.

Kofi

 

[Kofi Annan]

--SETBACK ALERT--

The good news: So, I tested the circuit this morning and got about 175VDC with the trimpot set at 50R and a paralleled set of 20K 3W resistors as the load. Pretty green LEDs! Yes!

The bad news: After about five seconds or so, I heard a sizzle, then smelled a resistor, then saw a small pop / flash from the 33R / 1W resistor feeding the IRPF9240. After unplugging and letting the caps drain, I noted that the 33R is toast and that the heatsink on the IRPF9240 was hot as a mother!

I'm guessing that there's a little too much voltage / current flowing through the circuit right now. I'm investigating, but a recommendation would be appreciated.

Some details:

I full-wave rectified a 200-0-200 (i.e., center tapped) transformer. I did not ground the center tap, but simply attached the 200V secondaries to the rectifier inputs.

The 175V was initially stable, but then started to fall as time went on-- probably due to the 33R resistor failing, but maybe not.

I have the smaller heatsink on the IRFP9240 and the huge one on the IRF840. I think this is correct, but...

Any help would be appreciated. I'm headed back out to investigate.

Thanks,
Kofi

 

[Kofi Annan]

I just fed this thing 500VAC, rectified to 700VDC, didn't I?

Damn, Kofi.

 

[Salas]

You are lucky if you walk out of this with just one dead 33R and a dead IRFP9240 only. Start with the resistor substitution.😉

 

[Kofi Annan]

DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY

 

[Salas]

P.S. Make sure you have the right input voltage before you connect it. When all works OK again (maybe you have a dead IRFP too or more semiconductors, it worked in Amps) leave it settle for 15mins and trim the output voltage then. The ring of two CCS that sets the current through the Vref resistor, the resistor and the Mosfets will show thermal effects that need to stabilize. Its normal to have slide down until it settles.

 

[Salas]

DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY DUMMY

Come on! I do it all the time! No worries.

 

[Kofi Annan]

--STATUS UPDATE--

I replaced the 33R and the IRFP9240 and it's workin' and regulatin'! Hot damn!

A couple of things to note:

1. I waited about 10 minutes or so for the voltage slide to stop and it was still dropping very slightly (about .1 volts every 5 seconds or so). I will continue to run it in and I'll hopefully get to a dead stable voltage after a while like you mentioned, Salas.

2. The heat sink for the IRF840 was nice and cool after about 10 minutes, but the IRFP9240 was still very, very hot. If this is par for the course, then I'm OK, but I wonder if I shouldn't have opted for a bigger heatsink.

Any comments on this?

Thanks so much for all your help. It's really satisfying to have the supply up and working.

Kofi

 

[Salas]

What is your Vin - Vout right now?

 

[Kofi Annan]

I think it's around 280V rectified before smoothing / filtering and about 180V out.

 

[Salas]

That is why your sink is hotter that you might have thought. 100V across and 65mA. I.e. 6.5W. For your needs, I have posted on another time, your R1 can be 56R 1/4W, so that drops you down to about 41mA. Try it. It should relax the little sink appreciably and you will still be running twice what the Bench stereo channels will need.

 

[Kofi Annan]

I replaced the 33R with the 56R (had to series / parallel a few resistors to get that value) and tested. The heatsink is significantly cooler now and I'm ready to move on to the audio circuit.

Thanks again for all your help and guidance.

Phase II Status: --COMPLETE--

 

[Kofi Annan]

I had some time to wire one of the 12AY7 sockets today. I may have the audio circuit completed by tomorrow. There's also an even chance that I'll have a hangover by tomorrow.

Who will win?

Stay tuned for tomorrow's update.

Same Kofi-time. Same Kofi-channel.

Kofi

 

[Salas]