Fender PA135 Feedback loop question

[TheMadMax]

Hi there,
I'm new to this forum, but not to electronics ;-) I have an old Fender PA135 head that I use as a Bass head, sounds fantastic. Some previous owner tampered with it however and built in a lousy little micro-flip-switch as an impedance selector for the speakers. Doing this he changed the feedback loop.
In my change back I came across this funny issue: I thought the PA135 ist genereally the same as the Bassman or Twin 135W amp, but it has this one different resistor value in the feedback path (red circle: PA135 has 220R, all others have 820R). Can that be a typo in the schemo? Mine had a new 390R 5W there - obviously not stock.

Second question:
The feedback path has the 220R and a 100R (green circle) in series to ground, makes 320R to ground. Now I wanted to decouple the shorting jacks on the speakers so I can use one tap (YEL/GR) for 4 Ohm and one tap (GR) for 8 Ohm output. I would then add a 470R 5W over the 8 Ohm putput creating a "sort of" protection against no-load-condition. But this would now be in parallel with the 320R I already have to ground... As the 100R (green circle) is only a half-watt, that thing would probably immediatley frazzle in a no-load-situation, wouldn't it?

Just curious why Fender chose shorting jacks and not better output protection (a blown speaker presents no load which shorting jacks won't prevent...).

I'd be very thankful for any help regarding these noob-questions

 

[agedhorse]

The series resistor value just changed the amount of global feedback. Since the shunt resistance is always 100 ohms in the voltage divider the 820 ohms series resistance decreases global feedback. This has an impact on feel and also affects the gain of the output stage. Any of those resistances may be correct, though you don’t want to go below 220 ohms because the circuit can become unstable.

The shorting jack was a simple and effective solution, that’s why it was used.

 

[TheMadMax]

Thanks agedhorse for the reply!
That was roughly as I had figured out, I was just curious as it's almost the only difference in otherwise identical setups... So it wouldn't be advisable to add an additional R (i.e. 470 or something) across the output?

 

[agedhorse]

The existing shorting jack has proven reliable and effective, I would leave it alone.

 

[TheMadMax]

The PA135 was built for PA operation with two cabinets (4 x 8 inch columns each side), so it only had the 4ohm secondary of the OT connected - with two shorting jacks, one per "side" (although it's not stereo, of course). Any inserted plug would open the short. But that would always leave the 8ohm secondary open, which I wanted to use on the second jack. I'm curious, because my brain tells me that when the 4ohm-jack is shorted we've still got the remaining turns from the (shorted) 4ohm (=ground) to the 8ohm tap open and seeing no load, if not terminated by a resistor. But I don't see the 8ohm secondary connected at all in the schem... Yes, the OT number is correct.

 

[agedhorse]

With the jack shorted, there will be no signal at any of the secondary outputs.