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post #211 of 269

is your power supply rated for 110vac input or 220vac?

 

post #212 of 269

If he's using the normal 48volt supply, its a switcher rated 100-240volt

 

cheers

FRED

post #213 of 269
Thread Starter 

Yes, the Cisco PSU I am using is rated 120-240AC

post #214 of 269
Thread Starter 

Ho my what do we have here?

 

CIMG1193.JPG

 

CIMG1192.JPG

 

CIMG1197.JPG

 

 

Warning Ugly! (Click to show)

Form VERY hot to warm:

 
CIMG1200.JPG

 

 

 


Edited by TestSubject - 1/14/12 at 11:42am
post #215 of 269

^Looking good there Pierre.

 

The volume knob turned out looking pretty nice, I think.

 

Did you get the high power supply voltage down, or is it still running about 68v or so?

 

I would like to see about 12v to 13v across the heaters. I'm not sure how they will last with almost double the rated voltage on them.

post #216 of 269

 

Quote:
Originally Posted by digger945 View Post

I would like to see about 12v to 13v across the heaters. I'm not sure how they will last with almost double the rated voltage on them.


This. 


Edited by nikongod - 12/29/11 at 2:54pm
post #217 of 269
Thread Starter 

I get about 16V at the source of the Mosfets. Also, why are they so hot? Aren't my heatsinks big enough?


Edited by TestSubject - 12/29/11 at 3:26pm
post #218 of 269

16V is too much. 

 

Why so hot....

What is your B+ voltage? it should be 48V, so lets go with that. 

you said you have 16V at the source of the mosfet, so OK. 

I Will ASSUME that you only have 150mA=0.15A going through the tube. This is the rated current at 12V, so it is probably more at 16V.

P=V*I

(48V-16V)*0.15A=4.8W

heatsinks that size will get hot burning off ~5W. 

 

If you still have 60V or whatever it was for B+ you have even more power being burnt off in the mosfets. 

If you have more than 150mA going through the mosfets (and you almost definitely do) you have even more power burning off in the mosfets...

 

At this point you have options. 

Tweak the voltage divider resistors so that they get you the correct voltage at the source of the mosfet. 

Add a series resistor between the tube heater and ground to take up some of the voltage. A 27ohm, 2W resistor should do it. 

There is a backdoor option #3, but I'm not totally sure it will work. I think I will call it the "hungry hungry hippo".

 

Ignoring the mysterious third option, I'd personally add the resistors on the heater supply. I would reccomend that the resistor be on the ground side rather than the mosfet side, and yes there is a difference. Having the heater at a higher voltage than the cathode reduces the odds of some funky "diode effect" where AC on the heater leaks back into the cathode. Its probably just me being anal retentive here, but it doesnt cost anything more to maybe be better or at worst the same... I'l post a schematic in a sec. 

 

I think I need to build the Hungry Hungry Hippo now. How could it be bad with a name that awesome? 

post #219 of 269

DSC_3746.JPG

 

Use at your own risk, Im pretty sure its good though. 

post #220 of 269
Thread Starter 

I have really limited acces to parts, 27ohms 2W, I wont be able to get them soon...

 

Why on earth is my Power supply rated 48 volts and outputting way more like this? I mean it is for Cisco equipement, you would think it is quality? Or maybe it as nothing to do with the PSU?

post #221 of 269

Whats the model number of your Cisco power supply? do you have the correct one?

 

Mine is rated 100-240volts output 48volts, unloaded its putting out 49.5V

 

cheers

FRED

post #222 of 269
Thread Starter 

My cisco PSU  is model PSA18U-480C Output: 48V AT 0.38A

 

It as the Cisco Logo but I did got it off eBay

post #223 of 269

Disconnect the power supply from the amp and measure the voltage at the power supply plug.

post #224 of 269
Thread Starter 

I had noticed this before but i though i was turning crazy... When you plug the power supply, it start at 50 something Volts and keep increasing. After a minute or two its at 62Volts and still going up.

 

Tried Input at 220 and 120 using a transformer.


Edited by TestSubject - 12/30/11 at 5:36am
post #225 of 269

Hook up your voltmeter to the power and measure the voltage with the power supply connected to the amp, then slowly and carefully move your hand close to the mosfets and observe the voltage. DO NOT TOUCH ANY EXPOSED WIRES!!!

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