[digger945]

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

 

[Fred_fred2004]

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

 

[TestSubject]

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

 

[TestSubject]

Ho my what do we have here?
 

 

 

 
 

Spoiler: Warning%20Ugly!

Form VERY hot to warm:
 

 

 
 

 

[digger945]

^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.

 

[nikongod]

 
Quote:

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. 

 

[TestSubject]

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

 

[nikongod]

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? 

 

[nikongod]

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

 

[TestSubject]

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?

 

[Fred_fred2004]

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

 

[TestSubject]

My cisco PSU  is model PSA18U-480C Output: 48V AT 0.38A
 
It as the Cisco Logo but I did got it off eBay

 

[digger945]

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

 

[TestSubject]

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.

 

[digger945]

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!!!