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An Amplifier Using New 6CZ5's
AUDIO, JULY, 1958, VOL. 42, No. 7 (Successor to RADIO, Est. 1917)
Most experimenters have sufficient equipment in the "surplus" department to construct this simple amplifier which gives good performance in a small package
The 6CZ5, a new RCA miniature power tube, holds much promise for the art of audio power amplification. This tube is not to be confused with the 6AQ5, or English 6BQ5, both of with it resembles in construction and purpose. It is not interchangeable with them.
It has the same filament, plate and screen, and load characteristics as the 6V6, of which the 6AQ5 is the miniature type, and costs about the same. However, the other characteristics are different and provide a considerable improvement over tha latter types. The negative bias on the signal grid and the transconductance are about 15 per cent greater and the power output about 20 per cent greater for a plate voltage of 250. In push-pull operation the 6CZ5 resembles the 6L6 in that it generates a low percentage of odd harmonics and can be operated as a pentode with a plate voltage of 350. Under this latter circumstance and with 280 volts on the screen, a bias of -23.5 volts on the signal grid, and a plate-to-plate load of 7500 ohms, two 6CZ5's are rated by the manufacturer to deliver 21.5 watts of audio power and with only 1 per cent of harmonic distortion.
This adds up to lower supply requirements, lower distortion, lower all around cost, and higher power output. To try a pair of 6CZ5's the writer built an amplifier from parts in the junk box including a husky output transformer which was manufactured about 20 years ago. To avoid expense in obtaining good voltage regulation, the writer used a large bleeder and worked the amplifier half way between the recommended pentode and the lower-plate voltage tetrode operation. With 325 volts from plate to cathode, the results proved better than expected. The plate voltage did not vary from minimum to maximum power output, and total variation of the screen voltage was only 3.5 per cent.