Since no one else responded so far, I, a rank newbie will take a stab at it. ???

My SE84ZS is really a Select packaged in the new chassis, I think.  There is an optional switch and a pair of 1.5K resistors that are in parallel with the 47K plate resistors.  If my math is correct, flipping the switch adds the two resitors in parallel and drops the resitance to about 1.45K.  (Is that right?  1/Rt = 1/R1 + 1/R2)

That's a significant change.  On my amp, flipping that switch increases the brightness a lot.  The amplifyier sounds more dynamic to me.  On some recordings it is benificial.  On many it's just to bright and harsh.

I suspect that a 2K difference will not be very much of a change.  I think it will be slightly brighter but not an enormous amount.

Any one else out there to verify my assumptions?

KD
San Antonio, TX

[quote author=chrisby  link=1170982794/0#2 date=1171046932]IINM, the switched optional resistors are on the cathode of the input tube - for adjustment of bias levels.  

Now, it's my turn to go out on a limb - changing the plate load resistor by a factor of approx 10% might not make as discernible a difference as toggling the bias . [/quote]

I´ll second venerable Yoda..Uh I mean Chrisby 10 % only half of what a gold band on the resistor (5%) is and I think the topology runs in the 50% range to make a noticeable difference.  A lot of resistance values are chosen on the back of tradition. like 47kohm, 100kohm, 2.2 k ohms ect it´s not the make all or break all of design's

Well, your avatar say's it all..

OK, here's the story. After months of on-off sputtering, the left channel of my SE84B died. It wasn't a tube, I checked that straight away, and I couldn't see any broken solder joints or damaged components when I opened the chassis. Sending back the amp across the Atlantic to Decware was a bit of a problem for me.

Eventually I took the amp to a competent audio electronics person, closer to where I live. Power stage is OK, he told me, let's check the input stage. Eureka! The left 47k plate resistor didn't work...yet it looked absolutely fine. He couldn't find a matched pair of 47k's in his box of resistors (surplus from the aeronautics industry) but 42k would do. Would I be able to hear the difference? Barely, he said, perhaps a small improvement in dynamics.

Which I do hear, along with an improvement in clarity. My reference in the comparison, though, is an amp that had been running for a while with a damaged half. Even when it wasn't sputtering, I think the bad resistor had been playing up with the overall sound in some subtle way.