February 2006
Grommes 360 Mono
Amplifier: Measurements
All amplifier measurements are performed
independently by BHK Labs. Please click to learn
more about how we test amplifiers there. All measurement data and graphical
information displayed below are the property of SoundStage! and Schneider
Publishing Inc. Reproduction in any format is not permitted.
 Measurements were made with 120V AC line voltage and one
channel driven (this is a mono amplifier). Data shown for the unbalanced input unless
otherwise noted.
 Input impedance
 Unbalanced: 9.8k ohms.
 Balanced: 14.1k ohms.
 Gain
 Pentode: 49x, 33.8dB.
 Ultralinear: 30.5x, 29.7dB.
 Output noise, 8ohm load, unbalanced input, 1kohm input
termination
 Pentode: wideband 1.3mV, 66.7dBW; A weighted 0.26mV,
80.7dBW.
 Ultralinear: wideband 0.81mV, 70.9dBW; A weighted 0.12mV,
87.4dBW.
 Output noise, 8ohm load, balanced input, 600ohm input
termination
 Pentode: wideband 1.0mV, 69.0dBW; A weighted 0.21mV,
82.6dBW.
 Ultralinear: wideband 0.82mV, 70.7dBW; A weighted 0.17mV,
84.4dBW.
 AC line current draw at idle: 1.45A.
 Output impedance at 50Hz
 Pentode: 17.3 ohms.
 Ultralinear: 8.5 ohms.
 This amplifier does not invert polarity.
Power output with 1kHz test signal
 8ohm load at 1% THD (P): 14W
 8ohm load at 1% THD (UL): 12W
 8ohm load at 10% THD (P): 77W
 8ohm load at 10% THD (UL): 75W
 4ohm load at 1% THD (P): 6.3W
 4ohm load at 1% THD (UL): 7.5W
 4ohm load at 10% THD (P): 88W
 4ohm load at 10% THD (UL): 75W
General
The Grommes 360 is a mediumpower, mono, pushpull tube
power amplifier utilizing one pair of KT88 output tubes. Not usual in this day and age is
the use of two tube highvoltage rectifiers and a 6L6 tube used presumably as a voltage
regulator for either outputtube screen grid voltage or frontendtube 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 tubeamp 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 opencircuit, 8ohm, and 4ohm 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 8ohm loads in either pentode or
ultralinear modes although, as usual, distortion is higher for the 4ohm loading.
Total harmonic distortion plus noise as a function of
frequency at several different power levels is plotted in Chart 3 for both outputstage
modes. Amount of rise in distortion at low frequencies is quite pronounced, but it is not
atypical for many tube power amps. Highfrequencydistortion 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 pentodemode
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 nonharmonicrelated
components present in both modes.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Pentode
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Cyan line = NHT dummyspeaker load
Ultralinear
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Cyan line = NHT dummyspeaker load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
Pentode
(line up at 1W to determine lines)
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line: 8ohm THD+N
Ultralinear
(line up at 20W to determine lines)
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line: 8ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
Pentode
8ohm output loading
Cyan line: 60W
Blue line: 30W
Magenta line: 10W
Red line: 1W
Ultralinear
8ohm output loading
Cyan line: 60W
Blue line: 30W
Magenta line: 10W
Red line: 1W
Chart 4  Damping Factor
as a Function of Frequency 
Damping factor = output impedance divided into 8
Magenta line: Ultralinear
Red line: Pentode
Chart 5  Distortion and
Noise Spectrum 
Ultralinear
1kHz signal at 10W into an 8ohm load
