I was using a Snark SA-1, which is labeled for 400 mA @ 9.6V. The Jamman Express XT requires 150 mA or less, so presumably that's not it, but who knows. Checking power is a good point at any rate. I've gotten rid of it already though, and picked up a Boss RC-1, which works much better for me...
Also just found a knob that looks like it might have the sort of tactile feedback necessary to tell you when you're at the sweet-spot, without a detent: https://jescarmusic.com/products/sever-pot-knob-bk-dome_i_knurled-spiral
I've been thinking about (and experimenting, to an extent) with...
Do you have any references for that you can toss my way? I haven't been able to find much in the way of a technical explanation of this beyond "my tech told me so." I would think biasing the 6550's like 6550's isn't going to cause the amp to put out more than 50W, because other things will limit...
Just found out about this neat gadget, though it unfortunately only comes in strat-style knobs: https://www.thatklickknob.com/
It's a knob with an on-the-fly settable detent.
Is that considered hot? 6550's are 35W tubes, 42W for some like the Sovtek 6550WE's, and 39 mA is only 18.64W at that plate voltage. So somewhere between 44% and 53% dissipation.
I use the .8 mm core version of the stuff I recommended above, for just about everything. You'll have to feed more than you're used to, but there's a bit more control due to that. Haven't tried 1.6mm core so can't comment on specific differences. I'm usually cooking at 390C.
They have a table here for the "44" flux solders: https://www.kester.com/products/product/44-flux-cored-wire
Looks like the closest is their 1.5mm diameter solder:
On digikey: https://www.digikey.com/en/products/detail/kester-solder/24-6337-0061/31179
They also have one with less flux, 2.2%...