Primaluna 300 integrated running 6L6 with high bias switch

I doubt the plate voltage changes. That switch most likely changes the voltage to the power tube grid. Much easier -- and much safer -- to do it that way.

Plate voltage doesn’t change. Bias current changes. If you know what the bias cureent is then current x 417= plate dissipation. 6l6 has a max plate dissipation of 30 watts. I like a little margin so I use 25. 25/417= 59mA which is what I would use for my maximum bias current. this is why I really don’t like auto bias. you can’t just set your bias at 55 and know it is right.

Jerry

edit to add that KT88, 6550 and KT120 all have a higher rated plate dissipation power (watts) so high may be too much bias current for the 6L6, resulting is reduced life and possible red plating.

I really like the primaluna presentation. The detail and warmth are great, but the bass is a little soft.  It’s not lack of bass, but the bass is a little mushy compared to my Audio Research VSI 60.  
 

The VSI 60 has hard hitting solid bass with a colder overall presentation.  I’m afraid to run this amp very often.  It tends to go out and take the tubes with it every couple of years.  I’ve had it maybe 10 years, and always rotate my amps so it’s not running all the time.  In that time, I’m on my 3rd set of KT120s, plus it has to be repaired every time.  That’s a lifetime of power tubes in the primaluna.  

I’d like to keep the primaluna’s mids and highs, but have firmer bass.  
 

Randy you might like KT150, they provide the bass plus great transparency and spatial qualities, tone is super. In my PL I found 120s too sterile and KT77 pretty good but not enough mid bass. I occasionally swap the 150s with 7581a, which have surprisingly good bass.

I haven’t tried them yet, but I’m sure I eventually will.  I tend to rotate the power tubes on a regular basis.  One of my favorite combos are JJ 6CA7 in the outside slots and Siemens EL34 on the inside.  Just the stock tubes inside with the 6CA7 on the outside are pretty awesome.  Really helps with the bass.

+1 on the KT-150s. I'm a bit of a basshead and the 150s in my Evo 300 int gives me all the bass I want. Highs are also fantastic - mids are excellent but not that tubey sounding midrange from the EL-34s - not that I remember those that much as I only had them for a few mos before going to the 150s. I did get some of that tubey midrange sound back though - and unexpectedly from my rolling my Modwright 9.0x from Amperex to Telefunkens ($$$).