Holy Crap!!! Smoke

[meanstreet]

The amp tech I use is excellent. I'm going to drop the amp off to him tomorrow. He'll find out what's wrong and most likely the cause. It usually takes a week or two to get an amp back from him...I'll know then if it was me or something inherent in the amp.

Is it Bob Konopka that you use?

 

[amiller]

The amp tech I use is excellent. I'm going to drop the amp off to him tomorrow. He'll find out what's wrong and most likely the cause. It usually takes a week or two to get an amp back from him...I'll know then if it was me or something inherent in the amp.

Is it Bob Konopka that you use?

Yep, that's the guy. I have to be careful and give him VERY specific instructions. He always wants to "improve" the amp. When he worked on my XTC he gave it a ton more headroom at the cost of "good" sag. I had to have Bogner un-engineer Bob's improvements.

 

[amiller]

...AS a far the cold bias, its a simple matter for your tech to change the "setting' resistor/trim pot value in the bias circuit to give a wider range of bias voltage. NOTE: that will most likely VOID THE FENDER WARRANTY !!...

Well, I'm wondering if higer bias values are a problem for the amp and maybe the cause of this mess in the first place.

 

[FUZZboat]

...AS a far the cold bias, its a simple matter for your tech to change the "setting' resistor/trim pot value in the bias circuit to give a wider range of bias voltage. NOTE: that will most likely VOID THE FENDER WARRANTY !!...

Well, I'm wondering if higher bias values are a problem for the amp and maybe the cause of this mess in the first place.

in a class AB amp (like the 5150 III)

The only problem would be the secondary winding on the power transformer - it wouldn't be able to supply the extra current.

if that were the case You probably would smell the power transformer cooking cause it'd get very hot !!

reason being, the only thing the bias control does is control how much current the power tubes draw from the secondary winding by adjusting the DC Bias voltage on the power tube grids.

less neg DC Bias voltage = less current drawn by the power tubes
more neg DC Bias voltage = more current draw by the power tubes

Since you said you saw smoke immediately and burnt parts , most likely screen resistor(s).

whenever an amp starts to smoke it a resistor !!

thats what resisitors do when they get HOT - they SMOKE big time !