Building FETZILLA - Questions and Answers

Status
Not open for further replies.
Jump to Latest
#102
I'd say you will probably be OK, but take note that the way to tell quiescent current through the output devices by measuring the voltage drop across these resistors changes.

With 0.1R: 1mV = 10mA so aim for 30-40mV across the resistor.
With 0.15R: 1mV = 7.5mA so aim for 40-53mV across(corresponding to 300-400mA quiescent).

U=I*R so I=U/R. Volts, Amps and Ohms.

The resistors will also dissipate more heat, hopefully still within safety margins. Also depends if you intend to play full blast for hours on end.

Take everything i say as pure guesses, i have only minutes of experience with my own Fetzilla so far.

Good luck!
Kris
 
Last edited:
#105
Pre-solder the pad for the tab.
Place the 223 in position.
Heat the pad not the sot223 tab.
The heat will eventually melt the solder under the tab and the Thermal connection is then made, without overheating the 223.
Let it cool a bit and then solder the small pads/pins.

You could solder the small pads/pins before the final soldering of the tab.
The slight disadvantage of this order is that a thick layer of pre-solder lifts the 223 up at an angle and that angle remains while the small pads/pins are soldered.
Then when the Tab is soldered the 223 falls down to it's correct angle straining the pins. These should then be re-heated to sit them flat.
 
#107
The way i soldered the SMD VAS was i "tack soldered" with one of the small pins first while keeping the package pressed down and in position. I used a very small amount of solder on the pad. Remove the solder pen and keep holding it in position for a few seconds. Then i soldered the larger tab, trying to get it to flow nicely under the package but i think i chickened out as you shouldn't keep heat applied for very long with semis. Then i soldered the smaller pins. In the end i was pleased with the results.
 
#109
resistor values.
The FETZilla uses exclusively FET devices.
The manufacturing tolerances of FET is decades worse than what we find with BJTs.
The effect of this is that currents can be very far away from what the designer has assumed at the voltages applied to the various nodes.

The only way to overcome this is to experiment with resistor values to get the node voltages and currents back to what the designer intended plus or minus a tolerance.

Don't go guessing, Try to work through from known points where tolerance has not upset the expected values.

In the first one I built up I had to change (I think but not sure, it was a couple of years ago) 4 resistor values. I posted a sch for the NFetZilla with the adjusted resistor values. That might give a clue to what I had to change.
 
#112
Thanks Andrew for your response.

I admit that i'm not experienced with circuit analysis and troubleshooting so I did the following:

Assembled the second channel (this works as expected) and adjusted according to instructions (no problems).
I compared (measured off line) all resistors of both channels (except those that cannot be measured in circuit due to near by capacitors) and found they are almost identical.

What troubles me is that I measured the voltage (referenced to earth) near the gate of 2sk170 (just before r6):

The working channel has "gate voltage" about 3.04V and offset at output near 0 (+-10mV). (R12 is adjusted at 24V, and bias at 37mV)

In order to adjust the non working channel i need to have more "gate voltage" and then i run out of P2 range, having 3.26V near the gate and the best offset at the output is around -630 mV (again R12 is adjusted at 24V, and bias at 37mV)

Do you have any more suggestions on what to measure so that i can troubleshoot the problem?
Is there a possibility that there is a failure of an active component?

Thanks in advance for you time and guidance

regards

Theodore
 
Last edited:
#115
So, at last, my fist ss amp build is a reality, thanks to you good people.

Thank you all for your help and support.

Here is a pic:
The box is totally diy, the torroids are overkill (500VA) but i had those at hand.
It still sounds wonderfull though....

fetzilla.jpg

regards Theodore
 
#116
Thanks to Andrew, Kris, and particularly Patrick for helping Theodore and other constructors to get their FetZilla up and running........

I'm somewhat ashamed that I don't help much, but to be honest my energies are limited by my health and I have to concentrate on sales and supporting my NAKSA modules, which take most of my time these days.....

Cheers,

Hugh
 
Status
Not open for further replies.
Top Bottom