Fender Harvard 5F10

[Bruce / Mission Amps]

Wow, that's darn near Mexico brother....what shock from VT!!
I flew down to Carlsbad, NM two summers ago, a couple hundred miles east of you.... hot as all heck.
That is about the furthest I've gone in 4-5 years.

[gtownblues]

Bruce, how would you go about removing the negative feedback from the 5F10 circuit?

[Jazz P Bass]

There is a 56K resistor that takes the output transformer signal & injects it into the cathode of 1/2 of the 12AX7.
http://music-electronics-forum.com/a...5f10_schem.pdf
That is the NFB resistor.
If you do not want NFB, remove the connection.

[Bruce / Mission Amps]

There is a 56K resistor that takes the output transformer signal & injects it into the cathode of 1/2 of the 12AX7.
http://music-electronics-forum.com/a...5f10_schem.pdf
That is the NFB resistor.
If you do not want NFB, remove the connection.

Or get a 50K to 100K pot to put in series with the 56K resistor, wire it as a variable resistor and mount it somewhere so you can increase or decrease the amount of NFB.
I don't think those amps had two speaker jacks so.... you have to be selective on where to and how to mount the pot so as not to wreck the vintage value.

[ngteleblues]

My Harvard copy is my favorite guitar amp.
http://i422.photobucket.com/albums/p...s/Harvard1.jpg

[mort]

I frequently find that these old Harvard amps exhibit an ultrasonic oscillation right at about 75% to 90% full output power.... must be the wiring layout... if you remove the negative feedback, it goes away.

I have a Harvard circuit that I built up a while back that does exactly this but I haven't worried too much about it because it's just a little shop amp that I don't need to crank. I know this is an old thread but can anyone comment on how to avoid this problem without cutting the nfb? I wouldn't mind cutting it but would also like to know if there is a real fix. Thanks