Lower Threshold
[quote author="CJ"]I had the same problem with mine at first. Strange, but after a little burn in, it went away.
You can reduce the osc threshold by reducing the valure of the neg feedback resistor , R11, which is 68 K stock ( blue-grey-orange-gold).
Try a 56 K (green-blue-orange-gold) in there and see what happens.
Or, better yet, wire a 1 meg pot across the 68 k and turn it down til the osc stops.
Then pull the pot, measure it's resistance.
Now, you can either wire the same resistance across the 68 K in parallel, or calculate the parallel resistance and buy a new resistor at Radio Shack.
Just for the heck of it, open the door when the thing oscilates, and move it around. If the osc stops or changes, you have a door grounding problem.
Make sure there is no paint under your ground connections.
cj[/quote]
Gain Mod
[quote author="CJ"]While I had the LA2 out of the rack, I fooled around with the line amp. I tried all the mods I had for the 6BH7. The Jensen mod, a mod by Jhalibe, etc. I found that the Jensen mod did not make that much difference. The Jhalibe mod helped to soften distortion at onset, but unfortunately eliminateing the negative feedback made it so the amp had a lot more gain, which meant you had to turn the gain knob down even further to avoid distortion. Eliminating the negative feedback allowed the amp to gently round off the tops of the triangle wave, instead of lopping it off like a sword. But once you got past a certain point, you got the flat top haircut on the wave.
No matter what you do to the 6BH7, if those first two stages mess up the wave, there is nothing you can do down the line to fix it.
I did find something that really helped besides changing the first tube to a 12AY7. This involved changing the 220 k plates resistors to 100k. Gain was reduced as this mod affecected two gain stages, not just one, so the effect gets multiplied. Also, distortion became more symetrical. With the 220 k's, the top half gets lopped off way before the bottom half. This was due to the reduction of plate current from the 220 k resistors causing the bias to be too low, which caused the tube to saturate. With the 100 k's, the bias went up, which put the tube on a better place on the load line.
220 k's are great for those early Fender circuits where you want maximum boogie woogie, especially carbon comp 2 nd orders, but with something in the mic path, 100 k 1 watt metal resistors are the way to go, at least according to my ears. This is where you want to install them. Replace R9 and R13 with 100 k 1 watt. Now I could turn the gain knob up past 1 without clipping.
[/quote]
I hope you don't mind CJ, but I thought I would spare other noobs from having to search for these. Phew.....
[quote author="CJ"]I had the same problem with mine at first. Strange, but after a little burn in, it went away.
You can reduce the osc threshold by reducing the valure of the neg feedback resistor , R11, which is 68 K stock ( blue-grey-orange-gold).
Try a 56 K (green-blue-orange-gold) in there and see what happens.
Or, better yet, wire a 1 meg pot across the 68 k and turn it down til the osc stops.
Then pull the pot, measure it's resistance.
Now, you can either wire the same resistance across the 68 K in parallel, or calculate the parallel resistance and buy a new resistor at Radio Shack.
Just for the heck of it, open the door when the thing oscilates, and move it around. If the osc stops or changes, you have a door grounding problem.
Make sure there is no paint under your ground connections.
cj[/quote]
Gain Mod
[quote author="CJ"]While I had the LA2 out of the rack, I fooled around with the line amp. I tried all the mods I had for the 6BH7. The Jensen mod, a mod by Jhalibe, etc. I found that the Jensen mod did not make that much difference. The Jhalibe mod helped to soften distortion at onset, but unfortunately eliminateing the negative feedback made it so the amp had a lot more gain, which meant you had to turn the gain knob down even further to avoid distortion. Eliminating the negative feedback allowed the amp to gently round off the tops of the triangle wave, instead of lopping it off like a sword. But once you got past a certain point, you got the flat top haircut on the wave.
No matter what you do to the 6BH7, if those first two stages mess up the wave, there is nothing you can do down the line to fix it.
I did find something that really helped besides changing the first tube to a 12AY7. This involved changing the 220 k plates resistors to 100k. Gain was reduced as this mod affecected two gain stages, not just one, so the effect gets multiplied. Also, distortion became more symetrical. With the 220 k's, the top half gets lopped off way before the bottom half. This was due to the reduction of plate current from the 220 k resistors causing the bias to be too low, which caused the tube to saturate. With the 100 k's, the bias went up, which put the tube on a better place on the load line.
220 k's are great for those early Fender circuits where you want maximum boogie woogie, especially carbon comp 2 nd orders, but with something in the mic path, 100 k 1 watt metal resistors are the way to go, at least according to my ears. This is where you want to install them. Replace R9 and R13 with 100 k 1 watt. Now I could turn the gain knob up past 1 without clipping.
I hope you don't mind CJ, but I thought I would spare other noobs from having to search for these. Phew.....
