Zoki Posted April 7, 2008 Share Posted April 7, 2008 Hey Gang, I wanna get this one last mod on my spare switch tonight in before packing this thing up and going.... (it's always one last mod ; ) So for the schematically chanllenged... Is the NFB circuit really as simple as running a lead and a resistor between pin 8 on the 12ax7 and the closest side to it of R9 with the switch between? Or am I misunderstanding the layout? Now wait a second - looking again....between pin 8 and the speaker out? Do I want to use the 16 ohm out it's a 16 ohm speaker and not the 8 or does it matter mmm, so where does the top of r9 come in? Help!!! Thanks Z Link to comment Share on other sites More sharing options...
layboomo Posted April 7, 2008 Share Posted April 7, 2008 You want a 28k to 47K resistor between top of r-9(junction of r-9 and c-3) and and one of the + output taps of the OT(try 8ohm first with a 28K)....I think 47K is a little too much NFB for my taste. Link to comment Share on other sites More sharing options...
Zoki Posted April 7, 2008 Author Share Posted April 7, 2008 hey thanks layboomo for the quick reply... so it's the farthest side of r9 from the tube and the 8 ohm speaker lead? don't know why i thought there was so much more to this one...... Link to comment Share on other sites More sharing options...
TWANG Posted April 7, 2008 Share Posted April 7, 2008 I have this running off the jumper.. C3 connect to one end of the switch.. the other end to the middle and the last end of the switch to the resistor. on off on switch gives C3 in C3 lifted NFB in. As I understand it, you can use the amp output connection to whatever you use most, but it will work on all three. I use an 8ohm speaker so mine's connected there. Link to comment Share on other sites More sharing options...
layboomo Posted April 7, 2008 Share Posted April 7, 2008 Just to clarify a little....lifting c-3 out of the circuit is a mild form of NFB. The resistor on the output tap is more NFB but c-3 still has to be out of the circuit(lifted) for it to work right. Thats why you see alot of guys put it on one switch the way Twang did it. Link to comment Share on other sites More sharing options...
TWANG Posted April 7, 2008 Share Posted April 7, 2008 yah I should have said C3 in C3 lifted C3lifted/NFB in. Some guys don't like the C3 lifted sound.. I do though. sometimes! I also found that 27K was what I settled on, but 33 and 47 weren't bad either. I almost went for 47 on head two. but I changed back and was too lazy to change again. TWANG Link to comment Share on other sites More sharing options...
Zoki Posted April 7, 2008 Author Share Posted April 7, 2008 Right on - thanks again guys - I'll try it tonight... Link to comment Share on other sites More sharing options...
bluefoxicy Posted April 7, 2008 Share Posted April 7, 2008 Why can't you use the power tube output rather than the OT output? Just curious. Link to comment Share on other sites More sharing options...
51xt33 Posted April 8, 2008 Share Posted April 8, 2008 OK, stupid question time here...:- ... I'm taking the board out of my combo tonight and will be installing the ToneQuest mods, or at least some of them. I'll be installing the JJ tubes and biasing tonight as well. I might want to do the NFB mod.... if only I knew what it was, how the tone is effected by it? What does this mod do???? Link to comment Share on other sites More sharing options...
bluefoxicy Posted April 8, 2008 Share Posted April 8, 2008 Negative feedback feeds some output signal back into the input, which reduces gain a little but when you crank the volume more the system stays stable longer. Overdrive happens later, so you have more clean volume. Link to comment Share on other sites More sharing options...
51xt33 Posted April 8, 2008 Share Posted April 8, 2008 Thanks bluefoxicy. I definitely will want this since I plan on using my PODxt through my valve Jr. I will eventually add a Loop d Jr, but for now, this will work. Link to comment Share on other sites More sharing options...
bluefoxicy Posted April 8, 2008 Share Posted April 8, 2008 http://en.wikipedia.org/wiki/Negative_feedback_amplifier may say it better. Link to comment Share on other sites More sharing options...
bluefoxicy Posted April 8, 2008 Share Posted April 8, 2008 I of course have a question: What's the Resonance circuit do? Link to comment Share on other sites More sharing options...
bluefoxicy Posted April 8, 2008 Share Posted April 8, 2008 While you're at it, here is a new diagram for NFB. Two SPDT switches, on-on, one engages NFB and the other sets resonance mode. NFB resistance variable between 0K and 71K when resonance mode off; when it's on I think the filter cap has an effect I don't quite grasp :) At any rate it's every NFB circuit suggested in the mod thread (fixed resistance NFB: don't touch the pot). Also added a bias pot in that diagram; stick it to the bottom of the chassis, add two taps where shown (drop two insulating gromits in the chassis and bolt two bolts down through them, connect them where tapped), and you can use a meter to measure your bias setting and adjust it by pot. Link to comment Share on other sites More sharing options...
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