Troubleshooting FET

[philbard]

Hi All,

Hopefully the collective wisdom here can help out with this problem. After reading some posts earlier today that mentioned what normal V1 plate voltages should be, I realized that the readings on my recent 100w #102 build were much higher, up around 250v. I'm using the classic power supply, and after checking through my build I saw that instead of using a 22K resistor between the B+3 and B+4 filter caps, I had installed a 2.2K. Obviously this was not cutting voltage enough. I swapped it out and V1 plate voltages are now at 192 and 189. Put the amp back together and tested it, sounds fine but now the FET is very weak, even with the pot turned up all the way (I'm using the footswitchable version). Previously had been working fine. Before the resistor swap, B+5 was at 399v, now it is 310v. Have I caused some sort of problem running the FET at the higher voltage? At this point I don' feel I have enough experience to troubleshoot the FET, so any suggestions would be appreciated.

[martin manning]

I'm betting that the FET has been damaged. You could check the voltages at the top and bottom of the drain resistor to see if it is biasing as expected, but I think you will need to replace the FET.

[Charlie Wilson]

What was the voltage on the bottom of the 150k resistor with the 399v on the top? If it was around 20v and the FET worked fine then I don't think lowering the B+ would kill the FET.
I think maybe you have it biased too high. I noticed on my amp when I tried replacing the 24k(divider resistor) with Dumble's 8k2 and did not re bias, the FET was quieter and was cutting out.
CW

[philbard]

I'm betting that the FET has been damaged. You could check the voltages at the top and bottom of the drain resistor to see if it is biasing as expected, but I think you will need to replace the FET.

Hi Martin, I was hoping you would comment. If I have the drain resistor correct, the 10K, the voltage across it is 12.45. With respect to ground, 15.6v at the top and 3.1v at the bottom.

[philbard]

What was the voltage on the bottom of the 150k resistor with the 399v on the top? If it was around 20v and the FET worked fine then I don't think lowering the B+ would kill the FET.
I think maybe you have it biased too low. I noticed on my amp when I tried replacing the 24k(divider resistor) with Dumble's 8k2 and did not re bias, the FET was quieter and was cutting out.
CW

Hi CW, I don't have a value for any FET voltages prior to the resistor swap, and don't really want to go back. I just posted a response to Marin's suggestion regarding the drop across the drain.

[Charlie Wilson]

You want the voltage on the bottom of the 10k to be about half of what is on top. What is your divider resistor value 150k/?. Two ways you can set the bias. Read Martins set up using two 9v batteries or use Dumble's 8k2 divider resistor and a 10k or so trimmer in place of the bias resistor.
CW

[martin manning]

You can try adjusting the source resistor to get the bias back to where Vd is half of Vdd. Did you have to biased that way before you swapped the 2k2 to 22k? If so then it may just be way off now.

What is the value of your source resistor?

[philbard]

You can try adjusting the source resistor to get the bias back to where Vd is half of Vdd. Did you have to biased that way before you swapped the 2k2 to 22k? If so then it may just be way off now.

What is the value of your source resistor?

I built the board exactly as you drew it in the footswitch mod, I wasn't aware of it needing to be biased, just my inexperience talking there. I'm frankly a little lost with all the references to the labels, "source" "divider" Vd, etc. I've been looking across the forum, but can't find a schematic on which all of these are labeled for newbies like me.

[martin manning]

The top of the 10k is Vdd (the supply voltage), and the bottom (at the FET drain terminal) is Vd. Take a look at the write-up here:https://ampgarage.com/forum/viewtopic.p ... 91#p211891

[philbard]

The top of the 10k is Vdd (the supply voltage), and the bottom (at the FET drain terminal) is Vd. Take a look at the write-up here:https://ampgarage.com/forum/viewtopic.p ... 91#p211891

That's exactly what I need, thanks. Tied up on Zoom for the next hour but will test this as soon as I get off.

[philbard]

The top of the 10k is Vdd (the supply voltage), and the bottom (at the FET drain terminal) is Vd. Take a look at the write-up here:https://ampgarage.com/forum/viewtopic.p ... 91#p211891

Martin,

So, with a 3.3K resistor instead of the 2K I had in there (pot read 3.5K, close as I could get), my voltages are 21 and 9.9. Does that seem close enough?

[martin manning]

So, with a 3.3K resistor instead of the 2K I had in there (pot read 3.5K, close as I could get), my voltages are 21 and 9.9. Does that seem close enough?

That should work. How does it sound?

[philbard]

So, with a 3.3K resistor instead of the 2K I had in there (pot read 3.5K, close as I could get), my voltages are 21 and 9.9. Does that seem close enough?

That should work. How does it sound?

Really great, thank you for the help! I guess that would seem to indicate I avoided damage to the FET components, I was concerned that the transducer might have been compromised. In your opinion, since I didn't set up the FET when I finished building the amp and yet it sounded so good with FET engaged, was it likely just a coincidence that it was close enough with the 2K resistor at Rs to sound that way?

Not a day goes by when I don't learn something new about these amps...

[martin manning]

I guess that would seem to indicate I avoided damage to the FET components, I was concerned that the transducer might have been compromised. In your opinion, since I didn't set up the FET when I finished building the amp and yet it sounded so good with FET engaged, was it likely just a coincidence that it was close enough with the 2K resistor at Rs to sound that way?

Transistor? Me too. Anyway, yes it seems that it is ok, and yes the 2k Rs happened to be close enough with the higher voltage the first time around.

[Charlie Wilson]

Phil, the NTE 452 can take up to 30v at the drain. Even if your B+(5) goes up 100v from 300v to 390v the voltage at the divider would only go from 20v to 25v. The catch is what the voltage is when you first hit the stand by switch.
CW