In hifi applications, you see primarily preamp (input) tubes and power
(output) tubes. This makes sense ... tubes for each half of the equation, so to
speak. There are tubes for other applications: rectifiers; ham radios; guitar
amps. Prior to 1970, tubes were used in most televisions. A tube, when you
get down to it, is an amplification device for a circuit. People like tubes for
the sound they produce in a component. In your Prologue One, the 12AX7
and 12AU7 are your preamp tubes and the EL34 are your power output tubes.
Tubes produce a very specific type of sound. Better tube components have
been described as having a warm, full, and analog type of sound. I have been
knowingly around tubes long enough (40+ years) to have heard both good
and lousy sounding tube equipment.
Solid state/transistors have been described as thin and harsh sounding ...
though the better solid state components sound just fine and can have a tube
like sound to them. It's like most things, the better equipment sounds
good ... the lousy stuff sounds lousy. The solid state vs tube debate can
sound like the CD versus Vinyl debate. In the end it is a matter of preference.
By the way if you ask my dad, who fixed radios and stereos in the 1940's -
70's for a living, which he preferred ... he would say tubes. To fix something,
you just had to replace the tube (which was cheap and easy) as opposed to
fixing or replacing a printed board (which was expensive and time
Most tubes produced today come from the Soviet; Eastern Europe; or China.
Your 12 series tubes cost about $10 each to replace. Your EL 34's cost about
$20 to replace. You should buy them in matched sets (tubes that have been
bench tested and have been measued to operate within a given range for a
number of functions). Then again, you can just replace the burned out
tube ... that's what my dad would have done.
NOS tubes cost a lot more. NOS stands for new old stock ... the original
tubes made in the 50's through 70's from such brands as RCA; GE; Mullard
(original British); Raytheon ... that some dealers maintain an inventory for. As
remaining supplies are used up, prices will go up, as well.
The tubes that come with the Prologue One are Chinese made tubes and are
pretty good ones. If you give Kevin Deal at Upscale Audio
a call, he may recommend some Svetlana new production EL 34's and some Electro-Harmonix new production for your 12AU7's and 12AX7's. Kevin was in on the design of the Prima Lunas and he know his products as well as anyone else does.
This just scratches the tube surface, but should be enough to get you going.