Valve Final Amp (1)
35 watts from a no-frills design
This valve power amplifier is a push-pull design using two EL34s (or their 6CA7 US equivalents). It has been kept fairly simple to avoid problems with DIY construction. The output power is well above 35 watts, with low distortion and a wide frequency range. This amplifier provides excellent sound reproduction when used with a pair of reasonably efficient, good-quality loudspeakers — and it shows that even a simple design with quite conventional specifications can sometimes make you tremble with excitement when listening to certain musical passages. This final amplifier is based on a Philips design dating from the late 1950s, with a few modifications suggested by Claus Byrith. These modifications consist of a separate supply for the negative grid voltage for the EL34s, an AC balance adjustment for the output stage, an EF86 pentode wired as a triode in the preamplifier stage and a reduction in the amount of overall negative feedback (20 dB). Two documents on this subject have been published on the Internet. They describe the design in detail, and they are certainly worth reading if you are interested in this topic (see ‘References’).
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CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor Members can exclusively download these files for free as part of their Membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.
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