martin manning wrote: ↑Wed Sep 02, 2026 2:24 am
18W Pa would be 60% of 6L6 30W Pa max, but about 5% of that is screen current so really it’s more like 55%. Still on the cool side. If there is no difference in the tone then that isn’t a problem.
I went a little higher than that, 42ma 21 watts (if my calculations are correct). My tendency is to run about 70%, removing the screen current that’s about 65%.
I think the clean is improved a bit, but the OD hasn’t changed.
I just grabbed some tubes randomly, I’ll go through and roll a few different tubes and see if that helps.
#1, I realized I put a 12AT7 in the PI, swapped that with a 12AX7, better punch on clean and dirty.
#2, I tacked a 1.5K across the 3k, first dropping resistor for 1.1k. That brought the plates to …V1a 192, V1b, 198, V2a 204, V2b 210
This seemed to tighten up the lows on the OD but still not the smooth OD I expect from a D
#3 swapped V2 from the Mesa branded 12AX7 to a Sovtek 12AX7LP. A bit better, especially in the upper register.
#4 adjusted the trimmer from 22k to 25k to ground. A bit more drive but still a muddy bottom end.
I’m wondering if the v2a coupling cap, currently .015uF is too bit. Maybe reducing that to .01uF might tighten up the lows. Also, perhaps a small bright cap on the OD level, or the 100k into the level, might be a help.
Well, I’m delayed, I came down with pneumonia so I’m sitting in the hospital.
But that gives me time to give this some more analysis.
My issues with the OD tone are 2 fold. 1j the low end is thin and raspy, not fat and smooth as I expected and 2) the highs are lacking.
I was thinking I was pushing too much on the lows at the OD1 coupler , but I’m beginning the think just the opposite, it needs more lows and less gain. To cap is now a .015, maybe a .02 like in gen 2. As for the highs I’ll experiment with bright caps.
What type of cap is on your mid switch? I can't tell if it is multilayer ceramic or traditional construction. The new production vishay caps sound nice and clear to me, while the blue TDK ones give a bit of pleasing distortion to the sound. YMMV
You may also play with your lead dress to get closer to how HAD ran the wires.
dbharris wrote: ↑Mon Sep 07, 2026 4:54 pm
What type of cap is on your mid switch? I can't tell if it is multilayer ceramic or traditional construction. The new production vishay caps sound nice and clear to me, while the blue TDK ones give a bit of pleasing distortion to the sound. YMMV
You may also play with your lead dress to get closer to how HAD ran the wires.
-Dan
Hmmm…it may be a single layer. I bought some before I knew the difference. I have some multi layer caps, I’ll have to check the values I have.
I’ve played a bit with lead dress earlier, I’ve been thinking that may be an issue.
I got back to work on the amp today and made a few changes.
1) I swapped the single layer cap on the mid switch for a multi layer vishay. It was a surprising improvement, but I felt it could be a tad brighter so I went from a 390pF to a 250pF and I’m very pleased with the result.
2) I felt the OD was muddy so I swapped the .015uF coupling cap on OD1 with. .01uF. It tightened up the lows nicely.
3) I added a 120pF bright cap on the OD gain. The jury’s out on that one but it did add a nice ping to the pick attack.
4) I adjusted the OD entrance trimmer to 22k to ground. When I first adjusted it I forgot to turn on the OD so I was off a bit. Again, a small improvement but that’s what this is all about.
5) I swapped the 6L6s with a test set of EL34s and rebiased. I’m not sure on that one, the OT impedance may be wrong for them.
I haven’t gotten to the lead dress yet, still recovering from pneumonia so about an hour is my limit.
1) I reworked the lead dress following HADs pictures.
2) I modified coupling cap values to see if I could remove some of the muddy lows.
3) played around with bright caps on the OD, Ratio and master
