The first tweak to the basic circuit so far outlined is to add a capacitor at the end of the voltage divider resistor string. This capacitor will shunt away the AC signal while preserving the DC voltage of the pentode's plate, which is needed to establish the correct DC output voltage of the Cathode Follower.
     The second tweak is to replace the voltage divider resistors with a single potentiometer. The potentiometer allows for some amazing adjustment to the pentode's mode of operation: at one extreme we have pure pentode; at the other, pure triode. In between these two extremes lies ultra-linear operation.
  What is the best potentiometer position? Best for what? Best for rock and roll or best for opera? Best for what amplifier? What speaker? What day? What wine? What mood? This control gives the user a choice over the sound quality that is not frequency dependant. For the longest time, I have wanted to build a preamp that sported several knobs that were labeled "Dynamics," "Darkness," and "Granularity"  and that controlled such circuit parameters as idle current, coupling capacitor composition (from paper to Teflon), feedback ratio (without altering the output level by reducing the input signal level correspondingly), and mode of operation (triode to pentode). Imagine
Stereo Review doing a review of this preamp: "But it measures the same no matter what the position of the knobs!"
   Since small triodes usually come two to the envelope, a Cathode Follower for buffering the pentode's plate from the load can be easily had. The pentode specified in the schematic is a favorite of mine, the 6AC7. It is a low noise, linear pentode in a metal envelope. It requires about 10 mA's of idle current to sound best. Avoid the Ken Rad versions of the tube as half of the samples I have tested have proven severely microphonic. Alternate tubes are the 6SJ7, EF86, 12BY7, EL84, etc. One sleeper pentode is the 6AU6, which many tube fans regard as just being an RF tube; it isn't.

Ultra-linear Cascode gain stage. The two 100k resistors return 50% of the output signal to the top triode's grid and provide a DC bias voltage for the top triode.

The Ultra-Linear Cascode
   This circuit approximates the ultra-linear pentode circuit by using two triodes. Like the pentode version it boasts both lower output impedance and distortion than otherwise, but differs in that the top triode's grid has a very high input impedance that is easy to drive with just a simple voltage divider.
    A further advantage this arrangement holds over the stock Cascode is an increased maximum output swing, as the top triode's plate swings down, so too its cathode moves down. This increase in output headroom would not be needed in a preamp, but in the first stage of an OTL power amplifier it would prove significantly better than a stock Cascode circuit.

OTL Power Amplifiers
  Ultra-linear mode can be added to an OTL amplifier in much the same way as it was added to the line stage amplifier by sampling the output and feeding this signal into a Cathode Follower to drive grid 2. Here some complexity intrudes into the design. How do we derive the correct ratios of output signal for top and bottom output tubes? The bottom tube works in much the same way as an output tube in a conventional push-pull ultra-linear power amplifier. Consequently, providing the bottom pentode's grid 2 with the right percentage of the plate's voltage swing is easy enough to conceptualize.


pg. 4

NEXT >   Copyright © 2000 GlassWare   All Rights Reserved