5687 LED BIASED TUBE PREAMP
Vincent Wong 
Page 1: Introduction, 5687 Preamplifier, Power Supply
Page 2: Enclosure Construction
INTRODUCTION
This is a 5687 preamp project that I recently completed. This idea for this preamplifier started off as a transformer volume controller, evolved to a DHT preamp, and finally to a indirectly heated preamp with an output transformer. This 5687 preamp uses LED biasing and Tamura A4714 output transformers.
5867 PREAMPLIFIER
The preamplifier uses the double section of a 5687 tube for each channel, with each section using it's own grid stopper (220R) and it's own pair of green LED for a 4V bias. The double section is used so as to lower the effective tube impedance, resulting in better compatibility with the Tamura A4717 5k output transformers. The 5687 tubes are operated at 115V, -4V and 25mA, which is one of the more linear regions for a 5687. Initially, I tried AC for the filaments and the result was far too much hum, so DC is used for the heaters. The schematic of the preamplifier section is shown in Figure 01.
Figure 01 - 5687 LED Biased Tube Preamp Schematic
Photograph 01 - 5687 Tube Pre-Amplifier
POWER SUPPLY
The power supply uses one RCA-83 mercury rectifier. An Omron timer is used to delay the B+ in order to allow 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 and use a CCS for each channel instead. The 10uF capacitor is polystyrene. Yes, those are 50H chokes!
The RCA-83 rectifier uses 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 and a 1R5 resistor between ADJ and VOUT pins. Votlage to the tubes is about 11.5V, so I am actually running starved filaments.
The schematic for the power supply is shown in Figure 02.
Figure 02 - 5687 Preamp Power Supply Schematic
Photograph 02 - 5687 Preamplifier Power Supply
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Enclosure Construction |