Heuvelman, Brakenhoff and de Haan EL84 Direct Coupled Amplifier
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A Board for the Kees Heuvelman, Kees Brakenhoff and Wim de Haan's Direct Coupled EL84 Design
The Besta_PAS_ta  mods for Dyna Marantz Scott, etc.
 
 
Heuvelman,  Brakenhoff and  De Haan, published a direct coupled amplifier design in the January 2001 issue of Electronics World.  "DC-Coupled Valve Power".  Heuvelman has the schematic of the four versions on his website.  In order to NOT trounce on anyone's intellectual property we just provide this LINK to their site.  You can purchase a copy of the Electronics World 2001 CD-ROM from the EW site.

I made a printed circuit board for the amplifier using Ultiboard8 -- to access the files use this link to the zipped files ==> EL84DC_Gerbers  .  To make a board for yourself obtain a copy of Pentalogix Gerber Viewer freeware.  Download the copper-bottom and print at 100%  using the landscape orientation. You will have to use your own creativity in order to place the drill holes,  I would suggest printing the drill file on a piece of Mylar transparency film, then use a small punch to locate the holes.

The picture below is a screenshot of the board in UB8.    Red is copper bottom, green is copper-top (which may be hardwired.)  At the suggestion of various DIYAUDIO members, the filaments are hardwired with twisted pair.  I am grateful to Dave, Milan and Guido for their helpful suggestions.
Below is a 3D view of the PCB -- note that there is space allotted for TO-220 cathode resistors.  Ohmite or Caddock non-inductive types on heatsinks.

wims_el84_3D.gif (20698 bytes)
The authors provided a regulated power supply for their amplifier.  It uses an 0A2 regulator tube to derive the -150 volts for the current source, and one-half of the ECC88 as a regulator for the + 206 supply.  As I employ both halves of the ECC88 in this stereo board,  you'll have to  figure this out for yourself !  Voltage furnished to the center tap is +390. 

For my amplifier a Hammond 1608 transformer (available from Mouser) was used.  The best transformer for this design may be the Amplimo VD8020 torroidal output transformer which the authors suggest in their article.

The 20KΩ potentiometers are used to set the current source for the ECC85's.  The 200Ω potentiometer is used to set the balance.  When the current is properly set the voltage on each ECC85 plate should be +74V.

The authors use a 50 μA meter connected  to the EL84 cathodes through a 47kΩ resistor.  When the meter reads 25 μA the output stage is "in balance".

The authors do note that the filaments should be allowed to warm up for 30 seconds prior to application of the high voltage.