Power Tube Idle Currents - Briefly Swapping 33mA TAD 6L6WGC with Mesa STR 420 37mA Without Rebiasing

[Greenback25]

I've been playing with my 50W Fender blackface clone head and noticed that the amp decreased in power output all of a sudden and the tone got a little bit fuzzy. I swapped the preamp tubes but they don't seem to be the problem, so I assume it has to be the powertubes. I want to test the amp with another set of 6L6 tubes but they don't have matching idle current values.

I have only a couple spare Mesa STR 420 color code blue tubes (which I don't want to use in this amp, I do it just for troubleshooting purposes) that I could directly swap into the amp that's currently biased for TAD 6L6WGC Blackbase tubes with PC (idle current) values of 33 mA.

TAD says that their PC value measurements of a Mesa Boogie STR 420 color code blue are 37 mA.

If everything remains untouched, including the bias setting, and I drop in these STR 420 tubes, how much more over- or underbiased will the amp be assuming that the 6L6WGC are set to their optimal 70% dissipation value?

Even though the 6L6WGC is still a 6L6, it's not a big bottle 6L6, so maybe there's problems here as well.

Could I use this setup just to play the amp fairly cranked for a minute and check if the behavior is still fuzzy in the top end or would I possibly ruin the output transformer and melt the tubes?

[xtian]

I gather you don't know how to check bias. Time to learn?

6L6GC is a "big bottle," or most robust version of 6L6, unlike the old 6L6 in metal enclosures. And, even if a pair of tubes is measured at 37mA on someone's test equipment, this does NOT mean they will draw the same current in your rig. Target current is based on plate voltage also, so you need to measure your plate voltage during biasing. Finally, 70% of max dissipation is no longer a desirable target, IMO. I get just-as-good tone, and longer tube life biasing at ~50%.