5687 preamp done...

and it's about time... since the holidays are well over and work has taken on the 4th gear. :P

an idea for a preamp that started off as a transformer volume controller, to a dht preamp, and finally to a noval indirectly heated preamp with output transformer.

and it turned out that i built the 5687 preamp with LED biasing and tamura A4714 output transformer. all these is with parts that are on hand... mostly...

even the chassis is made from aluminium that are lying around for some forgotten project... just so happens that the width of the aluminum plates are 10mm longer than the width of the A4714, making it a suitable candidate for a tower chassis... and so it is. for an easy chassis build, i got 2 MDF to be the front and back of the preamp. the pesky thing about 3/4" thick MDF is that it is a hassle to make a hole for any connector thru it...

the 5687 was wired as double sections to lower the plate impedance so that it will better leverage the 5k output transformer. heaters are in series, fed by 50000uF of elna capacitors and 2 paralleled LM317 set up for 830mA current regulation each. this gave around 11vdc. a little low as i used a common 1R5 coffin resistor for the LM317. maybe i should series a 1R and a R47 to achieve higher output current... but well... i have not known anyone who used tubes till they die of cathode poisoning... plus i used raytheon 5687wb and not the super expensive bendix 6900... so what the heck...

rca plugs for the input selectors are actually stripped from my neutrik rca from the m10 project. the output plugs were from canera rcas. volume pot is a 100k blue alps. signal wires are silver upocc wires, copper hook up are upocc copper wires, umbilical are multistrand 14ga upocc copper. all wires from ahfartaudio. in fact, my drd used all upocc wires internally. silver was used for signal path.

power supply portion was built on a hammond chassis meant for another forgotten project, using some handsome irons from nick chua. it is rectified by the rca 83 mercury vapor full wave rectifier, two 50H chokes and many 220uF philips capacitors.

future mods would be to add the constant current source that i implemently wrongly... which led to a loud bang upon power up, with bits of the transistor flying out. hahaha... ;)

here's a flickr set of the preamp.