Fender Ultimate Chorus issue

[Randall]

OKay, so now I see the INH line (inhibit?) is for muting. In my case, the green LED switches to the red LED with no plug inserted, is this normal?

The thumping at power off I can live with.

[nickb]

OKay, so now I see the INH line (inhibit?) is for muting. In my case, the green LED switches to the red LED with no plug inserted, is this normal?

The thumping at power off I can live with.

Yes that is normal.

[Randall]

Yes that is normal.

Can someone explain why?

[nickb]

Can someone explain why?

With no jack inserted, the IHN line gets pulled to -16V. That via CR55 pulls the -ve pin 2 input of U10 to about -15.3V The +ve input pin 3 of U10 ( or U2 if you thought it was in binary ) section that drives SELA and the LEDS sits about +0.7V. So, when inhibited, that section output is high around +14V meaning the red DRIVE led is on. Ba-da-bing.

[Randall]

Thank you so much nickb, this has been very helpful!

[Randall]

I think this amp is back to normal, and big thanks to those, especially nickb who helped me. I have let this amp run for 3 hrs in a 90 degree room (no AC) and wonder how hot these amps normally run. The chassis is running about 95- 98 degrees. Too hot? Or more specifically, what is the heat sensor circuit triggering at?

[52 Bill]

I think this amp is back to normal, and big thanks to those, especially nickb who helped me. I have let this amp run for 3 hrs in a 90 degree room (no AC) and wonder how hot these amps normally run. The chassis is running about 95- 98 degrees. Too hot? Or more specifically, what is the heat sensor circuit triggering at?

Most thermal cutoff are around 170-180 degrees F.

You ran the amp for 3 hours and the chassis got to 95 degrees, was it idling or were you running a signal through it?

[Randall]

I was idling in the drive channel with a cord plugged in to defeat the mute, and the chorus was on, so I could hear the swoosh. So it was amplifying a little bit. I put a little extra heat sink grease on it before I put it back together.

[nickb]

I was idling in the drive channel with a cord plugged in to defeat the mute, and the chorus was on, so I could hear the swoosh. So it was amplifying a little bit. I put a little extra heat sink grease on it before I put it back together.

There isn't an easy answer to your temperature question.

A perhaps not obvious characteristic of class AB/B power amps is they get hottest when running somewhere between full and zero power out, half is probably near enough worst case. To get a meaningful maximum temperature rise you need to play it under real conditions. If you were to set it up for the worst case output, I an almost guarantee it will either fail or trip the thermal cutoff. They are generally just not designed for that.

The maximum working temperature is set by the maximum junction temperature of the output transistors, usually around 120C. The power dissipated times the thermal resistance junction-to-ambient gives the temperature difference between the junction and ambient. Thus there is a constant temperature rise above ambient for a given power output. If the ambient is too high then the junction max will be exceeded. Fender didn't specify the maximum ambient operating temperature so we are in the dark there. We can find out if you measure the power in and the power out. You then know the power dissipated. Now measure the case temperature of the output transistors and use the published junction to case thermal resistance and known ambient to estimate the junction temperature.

If you have a concern then the answer is to lower the thermal resistance a lot, a bit of grease won't do much, you have to add a fan.


PS: After all that and based on the temp rise you stated as idle, it doesn't sound like you have a problem. The amp will work as designed, whatever that was.

[Randall]

yes, I can see this amp shutting off if it were used even at moderate volume if outside in the Florida sun. (as many other things here tend to do) But, in air conditioning, I'm not worried about it. Thank you again nickb for your excellent explanation.

[olddawg]

yes, I can see this amp shutting off if it were used even at moderate volume if outside in the Florida sun. (as many other things here tend to do) But, in air conditioning, I'm not worried about it. Thank you again nickb for your excellent explanation.

Have you considered installing a 120vac whisper fan? They are cheap and it's done all of the time.

[Randall]

No, I will leave that up to the customer. But I will suggest it.