Tube Preamplifier 5687 Led Biased

Posted on Feb 4, 2014

The RCA-83 rectifier makes use of 5VAC for the filaments. The heaters for the 5687 tubes are provided from a full bridge of MUR860 diodes, followed by 5 x 10, 000uF Elna caps. 830mA of current regulation is supplied using a LM317 regulator plus a 1R5 resistor between ADJ plus VOUT pins. Votlage to the tubes is about 11. 5V, so I am actually running

Tube Preamplifier 5687 Led Biased
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starved filaments. The power supply makes use of one RCA-83 mercury rectifier. An Omron timer is used to delay the B+ in order to permit the rectifier to warm up. The last 2 RC sections (3k6, 220uF, 3k9, 10uF) are separate for each channel, but in future I plan to remove that plus use a CCS for each channel instead. The 10uF capacitor is polystyrene. Yes, those are 50H chokes! The preamplifier makes use of the double section of a 5687 tube for each channel, with each section using it`s own grid stopper (220R) plus it`s own pair of green LED for a 4V bias. The double section is used so as to lower the effective tube impedance, leading to better compatibility with the Tamura A4717 5k output transformers. The 5687 tubes are operated at 115V, -4V plus 25mA, which is one of the more linear regions for a 5687. Initially, I attempted AC for the filaments plus the result was far much hum, so DC is used for the heaters. The schematic of the preamplifier section is shown in Figure 01.

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