Why Tube Pre-SS amp over SS Pre- Tube amp?

>> 12-27-06: Rlawry
>> Tz7, I am an engineer but not an electrical engineer,
>> so maybe an EE type can weigh in here. My understanding
>> of MOSFET's is that their transfer function is more
>> like that of tubes than that of bipolar, HEXFRED, or
>> other transistor types.

the reason that MOSFETs can be made to sound more like tubes (compared to BJTs) is that the MOSFET current-voltage transfer function follows a square law. Thus, MOSFETs end up creating more even-order distortion (just like many tubes) than odd-order distortion (just like many tubes).
The BJT's current-voltage xfer function follows an exponential curve. Thus, BJTs end up creating more odd-order harmonic distortion that can sound strident.

>> They probably are also used in circuits with a lot less
>> global negative feedback than those of bipolars....

this is not a true statement at all! The type of device (MOS or BJT) has nothing to do w/ the amount of negative feedback used in a circuit. It has more to do with the amount of gain in the circuit & what parameter the designer is trying to control (output impedance, distortion, noise, etc).
Just FYI: vacuum tube circuits also use (global)negative feedback - some use a lot & some use very little & others something in between.
There are several tube amps that use zero global NFB & guess what........................
there are several s.s. amps that use zero global NFB. So, it is all design-based & not device-based.

The preamp signal is smaller,more delicate and fragile,so it benefits from the qualities of tubes.The amp signal is less fragile and so can more readily tolerate the disadvantages of transistors while benefitting from the better bass and oomph of transistors.

Adcom,
I've read and heard many technical explanations comparing solid state and tubed components but none like yours.
Thanks.

dc generated by some preamps can damage some ss amps, so ss pre and tube amps may be safer.

sonically, i prefer tubed pre and tubed amp.

however, if my back were to the wall and i could find a lush tube pre and felt confident that a ss amp would not be damaged, i would select that combination.

i would hope i could find a ss amp that didn't have the obvious signature of transistors.

Adcom sez:

The preamp signal is smaller,more delicate and fragile

No it isn't. Although yr thinking *should* be logical, it so happens that the line signal is usually higher than what goes into the amp. There's attenuation involved in the preamp, remember? And most amps are overdriven with the standard 2V output from a cdp, for example:).
Cheers