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Original Message

RE: Quickie upgrades & modifications

Posted by Paul Joppa on October 11, 2009 at 11:52:31:
A choke behaves like a current source (except at low frequencies), so as a plate load it makes the load line more horizontal and reduces distortion.

In the Quickie as preamp, the stock plate load resistor is 4K, equal to the tube's plate resistance. So it reduces the effective source impedance to 2K, good for driving cables. At the same time, it reduces the voltage gain and the peak available output voltage. Replacing it with a choke will double the gain (another 6dB) and double the output impedance (can drive half the cable length).

In the quickie as headphone amp, the output transformer and headphone is an additional load, further reducing the gain. So more output power is available at lower distortion if the 4K resistor is replaced with a choke.

In simplified SE amplifier theory, a plate load equal to the plate resistance (4K) provides the most output power per volt of input signal, assuming the input voltage is the limiting factor. If the plate supply voltage is the limiting factor, then a load of two times the plate resistance (8K) gives the most power.

(If plate power dissipation is the limiting factor as with a power amp, then the best efficiency is obtained with a high plate voltage, low current, and high load resistance. For example the SEX amp runs a low impedance of 3 to 4 times the plate resistance.)

For the example discussed here, the choke's DC resistance is the same as the stock resistor (3.7K vs. 4K) so it won't affect the operating point. A lower resistance choke (as usually used in power amps) will allow the DC plate voltage to be higher with the same power supply, increasing the available output power (unless the input signal voltage is the limiting factor). The choke will also store energy, allowing the plate to rise above the power supply voltage on positive peaks; a resistor or a current source will not do this.