Ugly distortion - Solved!

[johnnyreece]

Yeah, I'm not sure, but I can't believe this sound is proper. I like it, without that noise. I do notice that kind of goes away if I turn it up louder, and the sound becomes more swirly. Not entirely bad, but I think there's something I'm missing, that, once I see it, will be so obvious I'll smack myself on the forehead and say, "Duh!"

[Littlewyan]

I had the sams issue which turned out to be blocking distortion, however it wasnt so bad that it was blatant blocking distortion but enough to make the distortion sound bad. It could be as you turn it up the PI starts distorting and covers it up or the PI is distorting first and the power valves distort later. A scope would make things easier but going in blind is more fun

Does the amp play clean when turned down btw?

[johnnyreece]

I had the sams issue which turned out to be blocking distortion, however it wasnt so bad that it was blatant blocking distortion but enough to make the distortion sound bad. It could be as you turn it up the PI starts distorting and covers it up or the PI is distorting first and the power valves distort later. A scope would make things easier but going in blind is more fun

Does the amp play clean when turned down btw?

More fun...

Yeah, sounds good at lower volumes (or if I tap the coil on my LP, it starts in later). I love the clean sound, too. I think this would be my go-to amp if I could get it all sorted out. I'll play with the PI more and see what I can come up with. Thanks for the thoughts, everyone!

[Phil_S]

I think you are very fortunate to have someone like Merlin drop in on your thread. He basically confirms what I found, that the 6SN7 doesn't like being pushed too hard by the PI. If you insist on sticking with the LTP, dialing back the gain of the PI and the grid stoppers have a very good chance of addressing what you are hearing.

Another thought is to put a 6SN7 in the PI hole and see if that cures the noise. It seems to me, this is an easy way to tell if the PI is pushing too hard.

[Littlewyan]

That was 'the' Merlin? Wow, you are fortunate!

[johnnyreece]

Not gonna lie; when I saw that he popped in, I was like, "Okay, play it cool, don't act weird." I totally wanted to geek out, though. I also didn't want to make it seem like I didn't appreciate the other advice I was given, but since someone else brought it up.

It was actually some of his posts I'd seen over at AX84 that made me hesitate trying it in the first place. I don't have any appropriate resistors handy for rebiasing the PI, but I'll figure out something. In the meantime, I'll try the 6SN7 in the PI and see if that does anything for me.

I really do appreciate all the suggestions!

[Littlewyan]

I think bumping up those Output Stage grid stoppers to 10K will probably make a difference. Could also reduce the initial grid stopper to 10K, if it doesn't oscillate then leave it in as it should reduce the noise a bit and make it a slightly more sensitive amp

[johnnyreece]

Gentlemen, we're on the right track; 6SN7 in the PI gives me no ugly. It's pretty clean all the way up, though. So, I'm going to rework the PI to hopefully give me a little less gain, and maybe I'll have a useable amp!

Edit: Success! Upped the 470 to 820 on the LTPI bias, and voila! Not quite as hairy as I'd like it to be, but maybe that's where a pedal should come into play. More results later!

[johnnyreece]

In case anyone was still curious, I've made the circuit changes recommended, and I'm very happy with how this sounds. I love the sound of the octal pre/PI, and will definitely incorporate them into more of my future builds. I took this to a jam this weekend, and got to really run her through the paces. Everyone was impressed with the way it sounded. I realize that is due to the great advice I was given by everyone here, so thank you very much! I may make some tweaks down the road, but I'm just happy it doesn't sound like a hornets' nest when I crank it now!