F5 Bias Problem

[MJPAudio]

Hi there,

Finished building the F5 amp. Everything looks fine but the bias does not want to go higher then .6amp. P2 is set to max and output set to 0vdc. Used Peter Daniels board and components from Tech-Diy.

Martinus

 

[MJPAudio]

Its on both channels. So something has to be wrong 😕

 

[6L6]

If it's on both, that's actually a good thing.

What is the voltage across R12 and R11?

Is it all warmed up and toasty?

You said Tech-DIY, but to be sure, these are not Toshiba outputs?

 

[Andersonix]

Is your PS large enough? Bottleneck somewhere?

 

[MJPAudio]

Voltage across R11-R12 is 0.3v. Devices are IRFP9240 IRFP240.

Rail voltage 25volt+ and 25volt-

 

[MJPAudio]

Nothing toasted 😉 not yet

 

[Nelson Pass]

You can increase the value of the 2.2K resistor which is
in parallel with the pot to get some more bias. Try 2.7K
or 3.3K

😎

 

[MJPAudio]

Hi Nelson,

Yeah i was thinking the same thing. But the circuit should work like this right with the original value's?

 

[cjkpkg]

silly question but have you validated the ohm value of R11 and 12?

You should be able to easily get 800-1000mV(not that you would run there unless you had some MONSTER sinks) across with the stock values and 25V rails.

Also maybe doublecheck the values and orientation of the trimpots themselves - the adjustment screws should be to the outside of the boards.

 

[nigelwright7557]

You can increase the value of the 2.2K resistor which is
in parallel with the pot to get some more bias. Try 2.7K
or 3.3K

😎

The IRFP240/9240 each need 3v5 to switch on.

 

[MJPAudio]

Changed the resistors over the pots to 3.3k and now it works like a charm!

Pictures tomorrow!
Thanks

 

[Nelson Pass]

Little higher Vgs figure for your Mosfets.

😎

 

[ENSen]

I have a similar problem.

I can not get Vgs over 750mV on any of the mosfets. When I start turning P1 and P2 Vgs starts increasing and goes up to 750mV pretty quickly where it stops. It does not matter how much I turn the pots after that, Vgs does not change.

The voltage over R122 and R12 is 0V all the time.

I have exactly the same problem on both channels.

I use the Fairchild MOSFETs.

Any suggestions on what it could be?

 

[Nelson Pass]

Same solution.

😎

 

[ENSen]

Ok, I will change the resistors.

I did not think my problem was exactly the same since my max Vgs of 750mV is very far from the~3-3,5V needed for the fets to switch on.

 

[Melon Head]

Have you installed the correct pot?
Or have you been supplied the correct pot? Did you measure it before installing it?

 

[Melon Head]

Just one more possibility. What is the Idss value of the Jfets you used?

 

[ENSen]

Have you installed the correct pot?
Or have you been supplied the correct pot? Did you measure it before installing it?

I usually measure all components before installing them but I am not 100% since I do not have any memory of measuring the pots. I have some unplanned hours this afternoon, I will spend them checking all components, starting with the pots.

 

[ENSen]

Just one more possibility. What is the Idss value of the Jfets you used?

They are all ~6,5mA.

 

[ENSen]

I measured the resistance from the positive rail to the gate of the mosfet when turning the pot. It goes from 0 to 1.2 kohm nice and smooth. It is the same on the positive and negative sides. I think 1.2kohm is the value to be expected with a 5k pot?

These measurements where done with the PSU disconnected. First I measured the resistance with the PSU still connected, then I got ~140ohm max between the rails and the gates of the mosfets. Could that be the problem or is it just that the capacitors in the PSU are still charged and thus interfering with my resistance measurement?