Tube amps - what 3 things…

@atmasphere

Great synopsis 👍

Checking bias monthly is OK, but knob dicking the adjustment pots will shorten their life. Tweaking for the last fractional mA is pointless. Great amps were biased with moving coil voltmeters for decades before digimania took over.

The bias is going to shift slightly with line voltage, time on and ambient temperature variations. Home users would do better to check bias at the end of the listening session with the amp and components are fully warmed.

Tube amps tend to be more noisy than solid state

Not if they are properly designed and installed.

The world of tube sound is vast and very variable--much more so than the sound of good solid state.

Not true. Tube amplifiers are current devices. Many hook up tube amps to roller coaster, Mt Everest peaky impedance speakers. Not the best match. Solid state is  just as variable. A lot of it comes off a production line and is improperly set up. 

Tube gear has a magic that solid state doesn't. If you're a bottlehead, you're hooked for life.

Tube rolling is a fools errand. Tubes are not identical, not even matched sets. Rolling various tubes in and out without any idea of the tube condition and how far it varies from design center is silly. VERY few tube rollers measure and balance different sets and what they hear are level and distortion changes. See ieLogical Rolling

Autobias is nice and generally works very well. However a catastrophic tube failure can take out the AB unless there is other monitoring to shut down the amp. Today, I wouldn't have an amp without it. Most AB circuits allow you to select a preferred bias. It requires a trimmer and a voltmeter and a healthy appreciation for sticking your hands in and around several hundred volts DC. MESSING ABOUT IN A TUBE CAN KILL YOU IF YOU ARE CARELESS.

Nobody has mentioned the driver stage. Some amps have CCS for phase splitter AC balance. Others have fixed resistors that are calculated with an ideal tube, but are seldom exact. Others have a pot for adjusting AC balance. This is the most flexible BUT requires test gear for optimum - read lowest distortion - results.

The ability to switch between modes (triode, ultralinear, pentode) is nice.

A fish with wheels and wings would be nice too.

Get a great sounding amp, not an overly complex spagheti bowl.

+1 on P2P wiring with silicone or other high temperature, NOT PVC, insulation. Tube amps require service and good designs can be made great with component upgrades and cct tweaks. One of my favorite amps was a Citation II reworked in the mid 80s. IMO, it ate my AR studio amps for lunch.

Ralph, stated another way, if the grid voltage remains constant as the transconductance changes, the current in a bipolar [CT] transformer will be unequal.

Seems like a greater evil... unless one is OTL 😉

Only if you keep digging...

I am not a EE...

Please don't post myths. I can assure that autobiased amps don't suck the headroom out of amps. As long as the bias time constant is long enough, the speakers will be toast long before the autobias affects the gain... assuming you're not trying to drive 80dB speakers with a 20w amp.

Well designed autobias is precise. The tubes idle @ the setpoint very shortly after turn on and after several hours of robust use. It's probably an order of magnitude more precise that a pot and resistor in fixed bias designs.

The biggest advantage with a quality CT OPT is both halves idle identically, thus reducing distortion and noise.

@atmasphere 

+1

I compared sound with 100uF, 10000uF and 100000uF. It was a big difference in SQ between all 3 cases.

Were they:

• the same technology
• the same size
• formed and capacitance verified

Any of the above could change the SQ... and what reference other than different?

@alexberger 

The ESR would be quite different with parallel caps... and topology counts