Specifications

For example, with a Biaset voltage of 1.40 vdc (45 ma total current flow per tube), 1 kHz THD climbs nearly 55%. If you reduce it
further to 1.25 vdc (40 ma per tube) it climbs another 170%, for a total of 225%! So, reducing the current draw of each tube by
6% of its rated cathode current, when it is already operating at just 30% of rated cathode current and 62% of rated plate
dissipation, all to gain what, 225% more distortion? That is a very poor compromise to make, versus the very well thought out
operating conditions that Hafler set up for the tubes to operate at.
But what about using different tubes? The 6L6 in many of its various forms, manufacturers, and sources, is a popular alternate
for the ST-70. The pin out is the same, but requires slight alterations to the bias supply to bias the tubes at a reasonable value.
Assuming that appropriate bias supply changes were made then, using the popular 6N3C-E Russian derivatives will both
reduce power, and increase distortion, due to the less than ideal loading conditions offered for these tubes. Maximum power
drops to 24 watts RMS per channel at 1 kHz, with both channels driven, while THD rose to .5%. This represents a 36% loss of
power, coupled with a 61% increase in distortion as compared to sockets stuffed with EL34s under the same conditions.
These tubes had a minimum distortion operating point around 50 ma per tube as well. But at this level, these tubes are operating
at nearly 90% of rated plate dissipation capacity, and much more of rated cathode current capacity as well. Additionally, high
power 20 kHz sine waveforms take on somewhat of a triangle waveform at this frequency, with THD running nearly 6% at 24
watts output -- and this at the optimum 50 ma current setting. At 40 ma, distortion at this frequency is nearly 10%.
There are all manner of accolades available in abundance about the sound produced from using 6L6 class tubes, and reduced
quiescent current operation as well. I would never suggest what someone does or does not enjoy in their sonic quests, but I can
say that given the performance intentions that Hafler established for the ST-70, using 6L6 class tubes, and/or reduced
quiescent current levels in place the the designated tubes/setting, will cause it to fall far short of that mark.
Fig. 3 shows show a 20 KHz sine wave at the onset of clipping being squeezed into
somewhat of a triangle formation at elevated power levels when using 6N3C-E tubes.
Distortion of this waveform is ~5.5%. This is invariably due to the lower than optimum
loading conditions being offered to 6L6 class tubes, which are very much happier when
employing a 6600 ohm load, when operating in UL mode.
The Power Output Stage Pt.2: Configuration, A Short History
Speakers. Being basically current operated devices, tubes have a distinct
disadvantage in driving them, since tubes are basically voltage operated devices. This
is where SS designs have a distinct advantage since they too are basically current
operated devices, so the shoes and shoelaces match. For tubes to have a level playing
field, they need a transformer to convert their high voltage low current nature, into the
low voltage high current requirements of a speaker. But being a voltage oriented device, tubes have one other disadvantage
that a transformer cannot account for: Resistance -- referred to as Plate Resistance. It can be minimized by circuit design and
output stage configuration, but SS devices have such little internal resistance that they have a huge jump on tubes in this regard
when the load is a speaker.
The ultimate resistance of a given amplifier design then is called its "Internal Resistance", which is the primary element in
determining the Damping Factor of an amplifier. Quality vacuum tube amplifier designs will have an internal resistance of
around 1 ohm, while SS amplifiers will often display an internal resistance of .0xx ohm, many times lower than that of
conventional vacuum tube amplifiers. That is why SS amplifiers have much higher damping factors, compared to their vacuum
tube brethern. So why is this important?
Let's say an amplifier has an internal resistance of 1 ohm. Sonically, using an amplifier with a 1 ohm internal resistance is
effectively the same thing as using an amplifier with 0 internal resistance, but placing a 1 ohm resistor between the amplifier and
speaker. Besides the power loss this creates, there is also a loss in control of the speaker, and both increase as the resistance
increases. From a speaker control standpoint, any resistance (from any means) between the speaker voice coil and the
generator source driving it then, is effectively like placing a spring between the voice coil and the cone.
If an amplifier with 0 internal and external resistance commands the voice coil to abruptly stop -- as in the reproduction of a drum
kick for example -- it will, because the 0 impedance of the amplifier electrically shorts out the output from the voice coil, that
would normally be generated due to it's tendency to want to remain in motion after being excited. So the voice coil stops in our
scenario, but the cone, being connected to the voice coil by a slinky, doesn't. The result is muddy, low definition, BUT
ACCENTUATED bass response in many speakers -- because of the continued, uncontrolled motion of the speaker cone at low
frequencies. The same effect is produced by using a proper speaker (no slinky!), but with internal electrical resistance in, or
externally between the generator source, and the speaker. No wonder then that many find SS amplifiers superior in LF
performance: Not only is there typically more available power, but often, better quality of bass reproduction as well because of
the lower internal resistance they afford. So how does this all relate to the ST-70? We're getting there.
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Fig. 3 6N3C-E