Negative Feedback wire...

TommyShameless · Post by **TommyShameless** » Fri Dec 21, 2007 7:46 am

I hope someone here can help! I put a purple wire on the 8ohm tap on the impedence selector switch, like the instructions say, but where does the other end go? The instructions thoughtfully leave this part out. I have scoured the forum the best I can, and about all I can figureout is that is has something to do with the presence pot. Anyone know?

This is the 100 watt kit, 1969 circuit.

Also, I've read that running it to the 4ohm tap will make it looser and it will disort quicker. Would that still be true even though I'll be running 2 16ohm cabs, and therefore the switch will bein the 8ohm position?

Tommy

novosibir · Post by **novosibir** » Fri Dec 21, 2007 9:48 am

The 'other' end goes to the FB resistor on the board, usually a 47K, somtimes a 100K in the 100W models. The FB resistor's other end is connected to the 10K tail resistor of the PI cathode, from where the cable to the Presence control goes off.

Larry

TommyShameless · Post by **TommyShameless** » Fri Dec 21, 2007 10:45 am

Hmmm, okay, I think I got it then.

Question...since I was shorted 3 100k resistors, could I take that 100k out of the Negative feedback circuit and put a 47k instead, eventhough this is a 100watt? It would freeup a 100k resistor to usein one of the other spots, but what effect would doing that have?

Tommy

novosibir · Post by **novosibir** » Fri Dec 21, 2007 11:04 am

You may put in a 47K, what makes the power stage tighter and a bit wider in frequency range. You also may put in the 47K in and connect the FB wire to the 4 ohms tap, what's quite the same, as a 68K FBR connected to the 8 ohms tap.

Larry

toner · Post by **toner** » Fri Dec 21, 2007 11:05 am

TommyShameless wrote:Question...since I was shorted 3 100k resistors, could I take that 100k out of the Negative feedback circuit and put a 47k instead, eventhough this is a 100watt? It would freeup a 100k resistor to usein one of the other spots, but what effect would doing that have?

47k NFB is fine and actually more "correct" for the '69 circuit. Lowering from 100k to 47k will give you more negative feedback which makes the amp smoother and cleaner with tighter low end.

The 100k would have less NFB which makes the amp a little "looser" with more power tube gain; more "raw" sounding.

You can get the 100k's you need at Radio Shack, etc. or just email George and he'll just send you some if they were missing from your kit.

SDM also answered your questions here:
http://forum.metroamp.com/viewtopic.php?t=15814

Grosh_Guitars · Post by **Grosh_Guitars** » Sun Mar 23, 2008 12:51 pm

Page 35 of the 100 watt instructions shows exactly where the feedback "purple wire" goes.