Hello Tom!
On 24 Feb 98, Tom Scioscia wrote to Rich Lockyer:
 TS>        Really? Hmmm... Surprising about the low wattage... What if they
 TS> were to produce a tube with the voltage rating of an EL34 but the
 TS> efficiency (volt to watt) of a 6L6GC? That would be somewhere around 200
 TS> watts, huh?
It's not an efficiency thing.  Output wattage is purely a function of the 
plate voltage and the impedance of the tubes (which is very close on both 
tubes). Carvin amps can be "modded" to accept either 5881 or EL34 by simply 
flipping a switch on the chassis.  The EL34 and 5881 are perfectly happy to 
give you the same wattage on the same plate supply.
The EL34 is rated to handle upwards of 700 or 800 volts on the plate, but the 
circuit must be biased (and the drive level limited) so that the total 
dissipation of the output section does not exceed 25-30 watts per tube.
When you bump the plate voltage that high, you are BARELY driving the tube to 
get the rated dissipation, and at this point, the THD drops from the 50% 
level that we like down to hi-fi standards of below 1%.
It's two completely different applications for the same tube.  We want 
distortion, so we keep the plate voltage low enough so we can drive the tube 
into clipping without burning it up.  Stereo builders don't want distortion, 
so they carefully design the circuits so that they will never drive the tube 
into clipping.
Think back to my old theory post on clipping (with the simplified 10:1 gain 
output section):  At 10v drive, the output dumps 100v to ground.  At 20v 
drive, the output dumps 200v to ground.  At 50v drive, the tube would LIKE to 
dump 500v to ground, but only 400 or so is available, so the signal clips or 
flattops for the top (and bottom) 20% of the cycle.  The hi-fi designer knows 
that he can dump only 400v to ground without exceeding the tube's wattage 
rating, so he pushes the plate voltage to 700, and designs the preamp so that 
the output dumps no more than 400v with all controls on "10" with a typical 
input signal (guitar amp builders use 1v p-p at 1khz on the input jack).
A 6L6 equipped hi-fi amp may be rated at only 12 to 15 watts per tube, 
because the limited plate voltage would cause the amp to get into clipping 
well before max dissipation is reached.
The EL34 is by design more suited to hi-fi applications because it CAN be run 
closer to max dissipation without clipping.
C-ya! Rich
--- GoldED 2.40
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