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Lightning In A Bottle: ECC88 Tube Preamp with shunt regulated supply

Wharfedale Evo 4.2

doors666

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I am about to order pcbs for this preamp and wanted to share the info with you guys and get your opinions. This is an ecc88 based preamp that I am making. I did test a more basic version of this on a breadboard and a home made pcb a long time ago but put it into cold storage as I got diverted by other projects.

This uses a variant of the BluesPowerMini shunt regulated power supply. I will be running it at about 135-140v using 2*68v zeners. I will use a 110v transformer for this. Heater is 6.3v AC, but you can use an external supply to feed it 6.3VDC if required. My transformer will have 6.3-0-6.3V output, so I can use it for 12.6V AC tubes also. PCB will support both 6v and 12v tubes, with some change in component values as this will have a different tube operating point. The regulator can also be used for higher voltages for other tubes. It has 18 mm caps with 5 and 7.5mm pitch for input and output caps.

The board supports both a CCS or a resistor for cathode load. It can be jumper controlled (JP3 and JP4) or you can use a switch to change between the two Or replace the jumper with wire for a single configuration. ECC88 is a very linear and clean tube and gives very low harmonic distortion, so the switch can be used to switch between "clean sound" and "slightly sweet sound". You can get a lot more warmth (even harmonics) with properly tuned cathode resistor load with other tubes though.

Pin 9, which is an internal shield should be grounded by JP3 jumper. This should not be done for 12v tubes and be left unconnected which use pin 9 as a center point for the heaters. For ecc88 only use, this can be replaced by wire.

The default configuration is with B+ at 135v, Plate current at about 10ma and grid at -2v, tube dissipated about a watt. Input impedance is very high and a 3.3nf input cap is enough to get almost a flat response right down to 20hz. Lower value caps are better behaved than higher value caps. I have some wima fkps in that value...:). Output cap ofcourse needs to be big, from 2.2uf to 4.7uf depending upon the input impedance of your amp. Lower the amp input impedance, higher the cap requirement.

It should be able to easily handle all the variants of ecc88 like e88cc, e188cc, 6922, 6n23p, 6n23pev etc. Other dual triodes with the standard 9 pin sockets can also be used with some tweaks in components and B+/heater values.

preamp schematic -



Board Layout -

 

linuxguru

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FWIW, I have an NOS BEL E88CC dual-triode in original carton, completely unused and in the same condition that it was when it was manufactured c.1975 or earlier - 40+ years. This is the high-reliability one with gold-plated pins and grey plates - exactly the same as those which were manufactured for export and rebranded as Mullard. NOS E88CC tubes with gold-plated pins from various manufacturers (Mullard, Siemens, Telefunken, Amperex, etc.) go for $50 to $250 on EBay, but I'll take reasonable offers from FMs who want to build this preamp and need the tube badly (of course, if I only get low-ball offers, I'll probably keep the tube for future personal use - it can't hurt for it to rest in its carton for another 10+ years ).
 
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