Specifications

As a result, any modification that does not include SOME reasonable level of feedback is almost certainly going to color the
sound produced, either by way of its internal resistance, or distortion produced. Therefore, designs that claim that feedback is
not needed due to the use of a UL or triode output stage should be held in question, with regards to accuracy produced. The
stock ST-70 displays an internal resistance of .72 ohms on the 8 ohm tap, effectively removing any coloration it might contribute
due to the effects of internal resistance.
Internal resistance is only one amplifier characteristic that can influence reproduced sound. Like testing for frequency
response, it is typically measured at low power levels. The power response produced over the audio bandwidth is another. But
just as important is the quality of power response produced in the face of a varying load impedance, like that of a speaker. The
configuration of the output stage can have a significant impact on this issue as well.
ALTERNATE OUTPUT STAGE CONFIGURATIONS
TRIODE OPERATION -- As mentioned, the internal resistance of a triode connected ST-70 output stage is about 7.8 ohms on
the 8 ohm tap. Since triodes have the least gain of all output stage configurations, the feedback loop is not as effective in
reducing this down to an acceptable level as when operating in UL. But that's OK, because triodes start with a lower internal
resistance to begin with. With a triode connected output stage, the nominal 20 db of global NFB employed in the ST-70 is
reduced to about 16 db, which is still enough to reduce the overall internal resistance of the amplifier down to about .74 ohm --
low enough to effectively remove it from introducing any coloration of sound.
Against this however, is the poorer HF response that triodes display, causing the stock frequency response at 20 and 30 kHz to
go from -.25/-.5 db, to -1.0/-2.25 db respectively. A 10 kHz square wave presentation clearly shows the effects on frequency
response, when the output stage is converted to triode operation.
One major benefit of triode mode however, is the power response curve produced with respect to variations in loading
impedance. Triode mode will produce a flat power response of about 17 watts RMS within the audio bandwidth. But importantly,
as the load impedance swings from 1/2 rated load, to 2X rated load, the available power becomes about 16 watts and 14 watts
respectively across the same frequency bandwidth, while still retaining quite reasonable distortion figures in the process (<.4%
at 1 kHz). So while the triode operation presents challenges in the frequency response department, its ability to handle varying
loads while still maintaining low distortion performance is excellent.
PENTODE OPERATION -- A pentode output stage has the highest internal resistance. So connected, the ST-70 output stage
has a whopping 105.7 ohms of internal resistance on the 8 ohm tap! But pentodes also have the most gain of all output stage
configurations, so the feedback loop becomes more effective in reducing this down to an acceptable level, than with any other
configuration. With a pentode connected output stage, feedback rises to 23+ db, which is still effective enough to reduce the
overall internal resistance of the amplifier down to .82 ohms on the 8 ohm tap, and enough to prevent it from introducing any
coloration of sound.
Pentode operation also causes a rise in HF response, showing +.10, +.25, and +.5 db at 10, 20, and 30 kHz respectively. A 10
kHz sqare wave presentation clearly shows the effects on frequency response, when the output stage is converted to pentode
operation.
At a constant impedance and normal loading, the power response in pentode mode is flat across the audio bandwidth at 35
watts RMS, although distortion is at least double that of either triode or UL operation. And while power and distortion are
acceptably maintained when the load drops to 1/2 of rated value, distortion deteriorates notably at 2X rated load, effectively
becoming a 20 watt amplifier under this condition, with distortion climbing yet another three times over normal loading of
pentode operation (to ~ 2.4% at 1 kHz). This is as much a product of pentode operation itself, as it is the A-470 output
transformer providing marginal loading conditions for pentode operation of EL34s.
So pentode operation produces elevated frequency response, and an acceptable internal resistance level when feedback is
applied, but has increased distortion with a rising load impedance.
ULTRA-LINEAR OPERATION -- UL operation displays a low internal resistance as mentioned earlier (.72 ohms on the 8 ohm
tap), preventing it from introducing any coloration of sound from this characteristic.
Since the amplifier was designed with this output stage configuration from the ground up, frequency response is as is stated in
the Measured Stock Performance, failing to meet published specifications, but still being down only .25 db at 20 kHz.
With a constant impedance and normal loading, a flat power response at 35 watts RMS across the full audio bandwidth is
produced, at the lowest distortion levels of all configurations.
When the load impedance varies by 1/2 or 2X rated load impedance with UL, the power response is very similar to that of
pentode operation (with maximum power at 2X rated load being ~23 watts). HOWEVER, because of the UL connection,
distortion does not climb above .4% at 1 kHz under either condition.
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