Referring
to Fig. 1, the harmonic content in SRPPs can be “moulded” by making the
cathode resistors R2 and R3 (normally identical) asymmetric or varying R4.
In this context I will act on R4 in order to leave unchanged the preset DC bias
of the first stage. Full Data Sheets for 300B Vacuum Tube (Western Electric)
show for the following dc bias:
Vplate=350Vcc,
Iplate= 60mA, Vgrid=- 74Vcc,
theresults summarized in Table 1.
Fig.
1
Tab. 1
Power
2nd
3rd
7Wrms
30dB
Below
Fundamental
44dB
Below
Fundamental
with
a load impedance of 4K and a sinusoidal driving signal of 70Vp. In [1]you can find a good Pspice Vacuum Tube Modelsrepository with an excellent model for 300B.Infact with the same previous bias condition you obtain by simulation the
results of Table 2corrisponding
to a THD of 3.84%.:
Tab. 2
Power
2nd
3rd
7.2Wrms
28.5dB
Below
Fundamental
47.4dB
Below
Fundamental
By injecting
70Vp on the 300B’s grid and varyingR4you can easily observe the action of HCT, Fig.2. A minimum in the THD
equal to 2.26% exists for R4= 11k; Fig. 3 shows the spectrum in this condition.
Fig.
2
Fig.
3
The same condition doesn’t establish also a minimum forthe THD into the driver, on the contrary the SRPP is “persuaded” to
work outthe best performing
condition in order to improve the global result: a key aspect of the Harmonic
Cancellation Techinques. If you decide to build a stereo version of this circuit,
a 45Watts H.V. power supplies is approximatively required.
