F5 mosfet thermistors on fire

[Hikari1]

I hope I'll be happy with the IRF's because I couldn't get any 2SK1530-Y/2sJ201-Y. So it will have to be the IRF's.

New Jfets are on the way. I hope I don't hate the IRF's.

 

[tinitus]

 

[vdi_nenna]

I hope I don't hate the IRF's

You won't hate them. Just a slightly different flavor.
IRFs are more critical or sharper.

 

[Hikari1]

I thought the Fairchilds were suppose to be sharper and brighter w/less 2nd harmonic distortion in the midband. I've heard the IRF's are darker and more "tubey".

Regardless, I hope I like them. But I have a feeling Toshibas will be going in someday. I just can't seem to leave this amp alone.

 

[vdi_nenna]

Compared to the Toshibas, the IRF are brighter. I don't know the Fairchilds.
You know who would know the sound for sure is TechDIY.com.

I have some Toshibas that I got with a parts kit. I don't think I'm using them. I don't remember the exact model numbers either.

 

[Hikari1]

I think I know why the thermistors caught fire.

The tip or probe or the thermistors I was using is CONDUCTIVE. Didn't realize that. I put them right on the metal washer holding the mosfets. ZAP and fire.

 

[Hikari1]

Got everything put back together. New thermistors, New IRF mosfets, new Jfets, new ztx 450/550.

Damn thing won't bias.

P2 does nothing. I can get the bias up to .5V w/P1 but the offset remains around 700mv. And P2 wont bring down the offset and has now effect on bias.

Nothing smoked or hot. PS is spot on 24V.

What now? No resistors are open. Both channels exhibit this behavior.

Fried P2 pots? is thing is driving me crazy...

 

[AudioSan]

massure resistance across the parallell resistor of P2. if it change resistance with P2 adjustment, then its fine. if not....

 

[Hikari1]

The R4 resistor in series w/P2 does not change resistance as the pot is turned.

In fact it just slowly climbs in value regardless of whether the pot is turned or not. Both channels.

Replace the pot and R4 just to be safe?

 

[Hikari1]

All fixed. The P2 trimpots were fried (thanks Audiosan)

Lessons learned:

Make sure the tips (sensor) of your thermistors are not conductive. And if they are, make sure they contact the plastic body of the mosfet only. Don't let them touch the bolt/washer.

Since it took me a month to fix the amp my memory is not so good but the change from Fairchild to IRF mosfets doesn't seem to be that big of deal. I could be imagining things but the Fairchilds seem more detailed and extended and the IRF's seem to have more midrange body. Not a back-to-back test though, there was a month between listening to them.