Tube amps - what 3 things…

Hidaleeho.  Having computer troubles, so just seeing the status of this thread.  Good exchange of info...good.  Ralph, I'm sure that I'm wrong because I'm even more sure that you're right.

Got some 8 Henry chokes on the power supplies of the amps at chez xeno, and wow, they're lurkers.  Carry on...

Cathode bias is not a good thing in input and driver stages, not just in the output stage.

I tried one solution in my DIY SET 300B amp. There is 6sn7 input tube and 6F6G in triode mode driver tube with coupling capacitors between stages. The solution is very big, around 100000uF cathode capacitance in the input and in the driver tube. It gave a huge improvement in everything: bass, soundstage, separation, high resolution.
It is a pity the same solution can't be used in the output tube cathode, because of high current start-up issues. The Maximum capacitor I can put in my 300B cathode is 5000uF. It improved sound but it can't exchange fixed bias in terms of SQ. 

Cathode bias is not a good thing in input and driver stages, not just in the output stage.

I tried one solution in my DIY SET 300B amp. There is 6sn7 input tube and 6F6G in triode mode driver tube with coupling capacitors between stages. The solution is very big, around 100000uF cathode capacitance in the input and in the driver tube. It gave a huge improvement in everything: bass, soundstage, separation, high resolution.
It is a pity the same solution can't be used in the output tube cathode, because of high current start-up issues. The Maximum capacitor I can put in my 300B cathode is 5000uF. It improved sound but it can't exchange fixed bias in terms of SQ. 

@alexberger I've yet to see a voltage amplifier or driver circuit that isn't cathode biased. When a cathode bypass capacitor is used, its still cathode bias. Its not cathode bias when you have a manual bias adjustment to make, or the amplifier is employing some form of autobias (both are forms of 'fixed bias' since the bias adjustment is made via the grid).

I've read controversy around the sonic differences between cathode bias and fixed bias but IME its really about design and how well the circuit is executed. Both types work just fine.

@atmasphere 

+1

Until they aren't. Tube amplifiers with output transformers, regardless of how good they are wound are not as transparent as their OTL or direct drive counterparts (given a level playing field). I've have all three design types and it's pretty clear what an output transformer adds to the mix. Getting rid of the capacitor at the output is also a benefit.

i agree with this expressed by @clio09 

witness ralph's otl amps, or my personal fave... the linear tube audio zotl40 ref+ and its remarkable clarity and palpability

no doubt output transformers do something to the sound heard out of an amp... whether that something is positive or negative is highly situational