Fet sounds like a blown speaker??

[Frankie]

My ODS 80`s fet is clipping hard, and sounds like a blown speaker. Even with single coils, and the level set low on the fet circuit.
Anyone who has a mod that will keep it sound tight, but with some dirt?

[ChrisM]

It is misbiased. The symptoms you describe a misbiased transistor very well. First check the 3.3M is indeed going from gate to ground.

[Frankie]

It is misbiased. The symptoms you describe a misbiased transistor very well. First check the 3.3M is indeed going from gate to ground.

The 3.3 M is grounded. So it was not the issue. :(

[UltraHookedOnPhonix]

Frankie, what is the Voltage on the Drain of your FET?

[Frankie]

Frankie, what is the Voltage on the Drain of your FET?

Where is the drain on the board, sorry for not knowing.

F

[Frankie]

Frankie, what is the Voltage on the Drain of your FET?

Measured on the 150k- 10,4 Vdc, 10k- 3,1Vdc, 3,3k- 2,3Vdc.
Changed the 8k going to ground to16 k.
Voltages are now 150k-19,37. 10k- 11.64 and 3,3k 2,51

[UltraHookedOnPhonix]

Frankie, what is the Voltage on the Drain of your FET?

Measured on the 150k- 10,4 Vdc, 10k- 3,1Vdc, 3,3k- 2,3Vdc.
Changed the 8k going to ground to16 k.
Voltages are now 150k-19,37. 10k- 11.64 and 3,3k 2,51

Ok, good. Now the FET’s Source and Drain are both inline with 2.51V and 11.64V respectively.

Now, you shouldn’t have to change the 150K/8.2K voltage divider to get in range as long as you maintain a B+5 of approximately 300V-325V. This junction should read about 15.77V to 16.82V.

Question is, what is your B+5 supply voltage? Also, is this a 100W build? And finally, what kind of power transformer are you using?

FWIW, here are some useful tidbits concerning the FET circuit:

1. If the voltage at the Drain of the NTE452 falls below 2.45V, signal will be cut off.
2. The lower the Source resistance, the lower the Drain voltage.
3. If the voltage increases much above 11V at the Drain, the volume will drop slightly. Therefore, try to maintain a Drain voltage of about 10V-11V. That’s the sweet spot in my experience.

[Frankie]

Frankie, what is the Voltage on the Drain of your FET?

Measured on the 150k- 10,4 Vdc, 10k- 3,1Vdc, 3,3k- 2,3Vdc.
Changed the 8k going to ground to16 k.
Voltages are now 150k-19,37. 10k- 11.64 and 3,3k 2,51

Ok, good. Now the FET’s Source and Drain are both inline with 2.51V and 11.64V respectively.

Now, you shouldn’t have to change the 150K/8.2K voltage divider to get in range as long as you maintain a B+5 of approximately 300V-325V. This junction should read about 15.77V to 16.82V.

Question is, what is your B+5 supply voltage? Also, is this a 100W build?
And finally, what kind of power transformer are you using?

FWIW, here are some useful tidbits concerning the FET circuit:

1. If the voltage at the Drain of the NTE452 falls below 2.45V, signal will be cut off.
2. The lower the Source resistance, the higher the Drain voltage.
3. If the voltage increases much above 11V at the Drain, the volume will drop slightly. Therefore, try to maintain a Drain voltage of about 10V-11V. That’s the sweet spot in my experience.

100w Ods, Hammond PT (Twin dual 100).
B+5, 305 volt.
Fet is a 2N4416A.

[Frankie]

This is a new build #0124 Standard High plate in preamp.
When I set the FET volume 12 O`clock the volume is twice as loud as the normal volume, when I engage the od the volum is also twice as loud as the clean, with od trimmer set to very low nine o`clock, and gain 4, level at 5.
This is not how my ODS 50w will behave. That one will be at the same volume between clean and od when knobs turned to 5 all three, and od trimmer set to 10 O`clock. :(
Can`find the issue here either, and everything is wired correct, Have triple checked the wiring.
Hm. Strange.
If I raise the value of the resistor for the source, you say that the drain will be lower. Correct? If so maybe I should try 3k7 then?

[UltraHookedOnPhonix]

Measured on the 150k- 10,4 Vdc, 10k- 3,1Vdc, 3,3k- 2,3Vdc.
Changed the 8k going to ground to16 k.
Voltages are now 150k-19,37. 10k- 11.64 and 3,3k 2,51

Ok, good. Now the FET’s Source and Drain are both inline with 2.51V and 11.64V respectively.

Now, you shouldn’t have to change the 150K/8.2K voltage divider to get in range as long as you maintain a B+5 of approximately 300V-325V. This junction should read about 15.77V to 16.82V.

Question is, what is your B+5 supply voltage? Also, is this a 100W build?
And finally, what kind of power transformer are you using?

FWIW, here are some useful tidbits concerning the FET circuit:

1. If the voltage at the Drain of the NTE452 falls below 2.45V, signal will be cut off.
2. The lower the Source resistance, the higher the Drain voltage.
3. If the voltage increases much above 11V at the Drain, the volume will drop slightly. Therefore, try to maintain a Drain voltage of about 10V-11V. That’s the sweet spot in my experience.

100w Ods, Hammond PT (Twin dual 100).
B+5, 305 volt.
Fet is a 2N4416A.

Great, so with that in mind, you should be seeing 15.88V with the 150K/8.2K voltage divider.

Somethings not right. You said that when you changed the 8.2K resistor to 16K, you got 19.37K at the voltage divider junction. That can't be, 305V going into a 150K/16K voltage divider should give you close to 29.5V.

But then again, if you're reading 11.64V on the Drain and 2.51V on the Source of the FET, you should be having a nice sounding FET circuit.

******Correction from previous post*******

The lower the Source resistance, the lower the Drain voltage.

[ayan]

If so maybe I should try 3k7 then?

Let me offer a simpified approach to solve your problem. I agree that the FET is most likely misbiased. Leave the power supply alone, and leave the drain 10K resistor the way it is The source resistor is commonly 3.9K or so, bypassed with a 4.7uF cap, leave the cap alone... You can do one of two things: connect a 10K pot there instead of the fxed resistor (using the wiper and one of the other inputs) and turn it from 0 to 10K to find the sweet spot. Otherwise, use 6.8K resistor and you will probably be fine... that value has worked in every single amp I have built.

Gil

[Frankie]

If so maybe I should try 3k7 then?

Let me offer a simpified approach to solve your problem. I agree that the FET is most likely misbiased. Leave the power supply alone, and leave the drain 10K resistor the way it is The source resistor is commonly 3.9K or so, bypassed with a 4.7uF cap, leave the cap alone... You can do one of two things: connect a 10K pot there instead of the fxed resistor (using the wiper and one of the other inputs) and turn it from 0 to 10K to find the sweet spot. Otherwise, use 6.8K resistor and you will probably be fine... that value has worked in every single amp I have built.

Gil

Green wire input. Blue wire out, Black w ground, red w from B+5.(307 Vdc)
Drain 10k, source 3k3, the one missing is the 8k2 to ground. How would you suggest to be the best combination of resistors here to get the tight dirty sound, with just a little of breakup.

Frank

[ayan]

If so maybe I should try 3k7 then?

Let me offer a simpified approach to solve your problem. I agree that the FET is most likely misbiased. Leave the power supply alone, and leave the drain 10K resistor the way it is The source resistor is commonly 3.9K or so, bypassed with a 4.7uF cap, leave the cap alone... You can do one of two things: connect a 10K pot there instead of the fxed resistor (using the wiper and one of the other inputs) and turn it from 0 to 10K to find the sweet spot. Otherwise, use 6.8K resistor and you will probably be fine... that value has worked in every single amp I have built.

Gil

Green wire input. Blue wire out, Black w ground, red w from B+5.(307 Vdc)
Drain 10k, source 3k3, the one missing is the 8k2 to ground. How would you suggest to be the best combination of resistors here to get the tight dirty sound, with just a little of breakup.

Frank

Frankie, based on your picture, temporarily pull out the 3.3K resistor off the board. Connect a 10K pot in its place, using the wiper and one of the edge lugs only. Then, starting with the pot's resistance at 10K (you can use any value you'd like, 10K makes the job easy), start turning the pot until YOU hear the sound YOU like. When you find the sweetspot, measure the resistance of the pot. Then, grab a resistor with the closest commonly found value to your reading and you're all set. As I said before, I ended up with 6.8K... and my method was to only vary that and leave everything else alone.

Cheers,

Gil