tictac wrote:a Blackface PI with a feedback loop and a 12AT7 would give you cleaner tone with tighter bass. You'll get no speaker damping with that open-loop power amp...
The Silverface PI with 330K grid resistors and 47K plateloads would also be a good choice; even cleaner and setup better for the 12AT7...
I'm sure your tired of making changes but that's what happens when you're building a prototype amp like you have here. Another reason lots of folks like making clones of amps... new designs can be a nightmare as you're finding out...
cheers,
TT
I don't mind making the changes. I want it to sound as good as it possibly can.
You folks have been a big help. I will try some of the changes after I get through this weekend. I am hoping this thing is going to be stable enough to use both nights. I will be happy when the "tweaking" is finished and I can lay it out proper and put it together to stay with new parts. It leaves quite a bit to be desired in the way of lead dress right now.
I am still a little hesitant to do anything to this sucker after the crazy day I had with the power recently. Makes me nervous just thinking about it right now............LOL. I'm gonna make it through this weekend then start again Monday morning when I am fresh.
billyz wrote:
I agree, 12at7 for the PI.
I think there is consensus on removing the 4th stage, too.
And rework your NFB loop.
After those are done I would revisit the Choke in place of the inductor/sag resistor.
Cool, thanks guys, I have a 12at7 lying around I will give it a try. NFB loop was making the distortion worse, when I removed it the signal cleaned up a ton. Maybe I had a wrong value on the resistor, will have to play with it.
I will see if the 4th stage is what is overdriving it by removing it.
Something is wrong if NFB made the distortion worse. look at a classic Fender ab763 circuit for proper implementation.
I tried a 12AT7 in the phase inverter, also dropped the plate R's on the PI to 47K. Somewhat of an improvement, but still distorts when past 1/2 volume. I also tried removing the last gain stage and going directly from the treble pot wiper to just before the .002 cap entering the PI. The distortion was overwhelming so I put the last stage back in and it cleaned back up. Do I need a pot or voltage divider behind the TS to keep a load on it?
I also removed the 100K pot on the pre amp out which increased both gain on the amp and voltage on the pre amp output AC.
Messed with the NFB for over an hour, tried from 10k to 150k with no love. Makes no sense at all, NFB just promotes the distortion, totally backwards. When I clip the DMM on directly in front of the PI to read signal strength it goes whacko and totally distorts and losses gain. Is this normal? Something the meter does? I am bringing the feedback back to the normal place. I thought I might be out of phase so I brought it in 180 out. Knew right away that was wrong. WTH am I missing here?