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Last Edited:
31-Jan-2002

Type 220-A Monobloc
with "para-phase splitter"


(Click drawing for detailed view)

Almost without exception, H.H. Scott power amp designs used Class "A" push-pull circuits. The lone exception was the very rare, 211-A (6B4) Triode amplifier dating from 1948.

Virtually all  Scott power amp designs from 1947 to 1961 use a  "floating para-phase" splitter featuring a dynamic self-balancing inverter stage which automatically balances the output of the amplifier "beyond normal limits of audible detection."  This is accomplished by using the feedback developed in the plate-to-plate voltage divider from which the second grid of the phase inverter obtains its signal. An AC Balance adjustment potentiometer is provided to compensate for component variations, almost as overkill.

Scott's first amp, the Type 220-A Monobloc (see above), is an example of the "para-phase splitter" design. It uses a 6SN7 for the phase-splitter and a pair of 6L6 push-pull outputs. 

Around 1960-1961, when Scott-Kits were introduced, HHS started incorporating a "Split-load" phase inverter design that relied on more carefully selected and matched components. Apparently the change was made primarily for enhanced reliability and ease-of-maintenance.

The 208 Basic Stereo Power Amp is typical of the HHS 1st-generation "Split-loaders,"  with 6U8 phase inverters and 7591 push-pull outputs. This design was used on the power amp stages of the 200, 208, 222C, 296, 299C, and all LK-series kit amps; and the 340(A) receiver. Initially, Scott engineers took the "ease-of-maintenance" theme so far -- as to eliminate the bias adjustment potentiometer.

Note: 2nd-generation "split-loader" types: 200B/LK-30, 222D/LK-48-B, 233, 299D/LK-72B, and 340B/345/380 receivers, incorporate a bias adjustment potentiometer on each output channel, which is not used in the Type 208 shown below.

Type 208 Basic Stereo Power Amp
with 2nd Generation
"split-load" inverter


(Click drawing for detailed view)