Increasing Crunch Range

[arledgsc]

My 2 month old JTM45 is settling in nicely. I have tweaked the tone of the amp to my liking by removing the mixer bypass and changing the bright cap to 100pF. It sounds great with a usable normal channel and just a slight bump of highs in the bright channel. Very 1986-ish territory.

I need more crunch range though. I get a nice lower volume crunch but went I try to get more of this sound a higher volume it comes out as a splatty mid-range distorted honk for lack of better expression. Its a fine line between nice crunch and honkiness. Seems to me that I may have too much negative feedback (8 ohm tap, 27K ohm NFB resistor). I have extrenely tight lows so I know the feedback is working properly. I'm hitting some hard wall but would like to more gracefully dance in the zone. Lar/Mar PPIMV is all the way clockwise and preamp vol. at 5 is area I am describing.

I am running VA KT66s and Tung Sol 12AX7s. I am going to experiment with raising the NFB resistor to 47k to see what this does. But would appreciate your thoughts and tweaks. Thanks, Scott

[arledgsc]

As an update I installed a 100K NFB resistor to get the most extreme effect and the results are positive - so positive that I am going to leave this value in for a while. The range and transition zone of clean-to-crunchiness has widened considerably. There still is plenty of NFB to my ear but the sonic effects now are pushed into the background. The amp can now breathe it seems. The bass is not as round or extended as with 27k but not exaggerated which I find most beneficial.

Reducing NFB has the following effects on the amp. Increasing gain, narrowing frequency response which I find beneficial especially on the bass side, and reducing distortion. The distortion reducing effect is where I think I had the most issue with 27k NFB resistor. If the power amp runs out of headroom and can't respond to NFB signal then the sonic effects are negative to my ear.

I am so tickled with this mod I can't stand it. The first mod that made a huge sonic impact. Scott

[Jsinca]

I installed a 100k pot so i can vary the NFB.

[arledgsc]

I played the JTM45 the other day after a long absence and think 100K ohm attenuates NFB a little too much but not by much. I settled on 68K in the JTM50 and like that setting a lot for that amp. But adjustable NFB sounds like a great idea. Thanks, Scott