Power output with 1kHz test signal
- 8-ohm load at 1% THD (P): 14W
- 8-ohm load at 1% THD (UL): 12W
- 8-ohm load at 10% THD (P): 77W
- 8-ohm load at 10% THD (UL): 75W
- 4-ohm load at 1% THD (P): 6.3W
- 4-ohm load at 1% THD (UL): 7.5W
- 4-ohm load at 10% THD (P): 88W
- 4-ohm load at 10% THD (UL): 75W
General
The Grommes 360 is a medium-power, mono, push-pull tube
power amplifier utilizing one pair of KT88 output tubes. Not usual in this day and age is
the use of two tube high-voltage rectifiers and a 6L6 tube used presumably as a voltage
regulator for either output-tube screen grid voltage or front-end-tube supply voltage.
Distortion behavior of the amp was essentially the same for
balanced or unbalanced inputs. One thing that is a bit puzzling is the low input
impedance. This parameter, for tube circuits, can easily be much higher than this and is
typically 50k ohms or higher for most tube-amp designs. The low input impedance of this
amp could penalize the performance of some otherwise very good tube preamps used to drive
it.
Chart 1 shows the frequency response of the amp with
varying loads for pentode and ultralinear modes. The output impedance, as judged by the
closeness of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading in the
pentode mode, is unusually high and would cause major aberrations in the frequency
response of many loudspeakers. For instance, with the NHT dummy speaker load, the
variation is some +/-5dB. In ultralinear mode, things are a bit better, but still the
output impedance, in my opinion, is unacceptably high. All of this technical logic aside,
it may well be that this amp with some speakers may be complementary to each other and
sound very good.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for a 1kHz and SMPTE IM test signals and amplifier output load for
both pentode and ultralinear modes. This design, with its single output connection for
speaker loads, is about equally good for either 4- or 8-ohm loads in either pentode or
ultralinear modes although, as usual, distortion is higher for the 4-ohm loading.
Total harmonic distortion plus noise as a function of
frequency at several different power levels is plotted in Chart 3 for both output-stage
modes. Amount of rise in distortion at low frequencies is quite pronounced, but it is not
atypical for many tube power amps. High-frequency-distortion rise is moderate and
reasonably good.
Damping factor vs. frequency is shown in Chart 4. Here, we
can see the unusually low damping factor in pentode mode and the approaching acceptable
and typical value for some tube amps in ultralinear mode.
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal is plotted in Chart 5 for the ultralinear mode. The pentode-mode
signal spectrum was very similar, but had more hum components. The principal signal
harmonics are second and third with the remaining harmonics about 20dB below the level of
the second and third harmonics. However, there are some spurious non-harmonic-related
components present in both modes.
![[SoundStage!]](../titles/sslogo3.gif)
Home Audio 
