The Bijou is tightly designed around the operating points of the 6922 and ECC99 tubes. There is only one variety of ECC99 from JJ Tesla, but there are many varieties of 6922 including NOS 6DJ8s and 6922s as well as various current production tubes. Any of these should work.
It is also possible to roll other types of tubes as described below.
Correct operating points are critical to proper operation of the amp. To get good results with other tubes you must change the value of the first stage's cathode resistor. The first stage is designed to have about 100V at its plate. Since the first and second stages are direct coupled, this voltage is also important to proper biasing of the phase splitter. To use other tubes, adjust the cathode resistor (R2) as shown. See the Amplifier Schematic and the Boards sections for the location of R2.
Cathode Resistor (R2)
WARNING: You can direct sub other tubes without making these changes, but you will certainly get bad results. Please do not blame the ampifier or its design for this result.
WARNING: Do not use tubes which have a heater tap on pin 9. This includes all of the popular 12XXX series tubes. The Bijou amplifier board has pin 9, the internal shield in the 6922, grounded to reduce noise. Using tubes with a heater tap at pin 9 will damage the heater biasing circuit and, possibly, cause other problems.
Output Stage - 6N6Pi
The ECC99 is based on the Russian 6N6Pi. The tubes are similar except that the 6N6Pi has sligtly lower plate resistance (higher current for a given operating point) and ECC99 uses pins 4,5 and 9 for its heater while the 6N6Pi uses only 4 and 5. Nevertheless, these tubes are close enough in performance to substitute the 6N6Pi for the ECC99. The Bijou amplifier board is equipped with configurable heater connections for the ECC99 socket so that 6N6Pi can be used.
To use the 6N6Pi, see the diagram in the Wiring and Ground section.
Using the EZ81
Some of the Bijou builds that use 6N6P output tubes are drawing more current than was shown in the prototype process causing the input to regulator to drawn down below 250V and, thus, preventing the regulator from regulating to 250V. The easiest fix for this is to simply subsitute the EZ81/6CA4 rectifier for the EZ80. The EZ81 is pin for pin compatible. It draws 1A heater current instead of the 600mA drawn by the EZ80, but the Hammond 270DAX heater winding can support this additional current draw. A schematic is not really necessary, but here is one anyway:
Regal's mods require the EZ81 Power Supply.
Using Hammond 370DAX
The Hammond 370DAX or 370DAZ have the correct secondary voltages but with a universal primary for operation worldwide. To use the 370DAX follow the wiring diagram below for the transformer connections. Note that the Violet and Orange wires are not used.
Wire colors are according to the Hammond website. See the Hammond website for wiring the primary for your line (mains) voltage.
The original Bijou was designed to operate with 220V B+ using the Hammond 270AX transformer. To run with 220V, make the following changes.
- Amplifier - Change R5, R6, R7 to 33.2kΩ.
- Power Supply - Change R2 to 34.8kΩ.
Use these links to get schemtics for amp and PS:
Choke Power Supplies
Parts which change are shown in red. The 470u caps have been reduced to 47u. This keeps the ripple into the regulator to below 1mV. However, the regulator has a high ripple rejection (>100db at 120Hz). Thus, you can try making these values even smaller in order to use large film capacitors instead of electrolytics.
In this version the choke replaces both R7 and R8 and eliminates one filter cap. Because of this, the remaining filter cap can't be reduced in size.
Removing NFB for High Z Headphones
The Bijou is equipped with a variable NFB circuit. This helps the amplifier handle the range of headphone impedance from 32R to 300R. However, very little feedback is needed for high Z headphones. Some listeners may prefer no feedback at all.
If you are sure that you'll only be listening with high Z headphones, you can remove the feedback and improve the bandwidth of the amplifier by doing the followng:
- Eliminate the 250kΩ feedback pot
- Remove (or don't install) R12
- Change R1 from 10kΩ to 300Ω
There are no harmful effects from this change. And you can always install the components needed to include the variable NFB capability.
Regal, a member of both Headwize and Head-Fi forums modifed the input and phase splitter stages run at higher idle currents. The original Bijou was designed for a particular power budget. The current in both stages was kept to around 1mA. However, tubes generally sound better at higher currents. Regal's mod moves the operating point to about 5mA for both the input stage and the phase splitter.
“Broskie is quoted as saying that the 6DJ8 sounds better over 3 mA, Modjeski recommends 3-5 mA, Others on DIYAudio say 10 mA is good. So I came up with this biasing scheme for 5 mA, based on the fact that we’re almost all using a bigger transformer and better rectifier than the original design. With the new bias in place, I tested the voltage at the input to the phase splitter with brand new tubes and real old tube and got 105V (new) and 95V (old) , you can adjust to 100V by dialing B+ up or down.
The sound before was great but just a tad too smooth and polite. After the mod everything became fuller, punchier, and more detail lifelike. Big rich imagery is most important to me with an amp and the Bijou with the hotter input tubes really excel in this area. I do recommend 6DJ8 tubes vs 6922 tubes as they tend to deal with the low-ish (even at 5mA) bias better than their 6922 counterparts.
In comparison with my DV337 the Bijou is clearly superior with its lifelike imaging and conveyance of harmonics and overtones. Your milage may vary as my Bijou has been optimized for high ohm phones without using NFB and using far fewer electrolytics than typical. I really want to thank Alex for his time designing and supporting this amp.”
Parts changes are in red:
Here are a few cathode resistors for rolling the most popular tube in with Regal's mods.
Cathode Resistor (R2)
Regal's mods requires using ECC99s (not 6N6Ps) and the EZ81 Power Supply.
Amphead's Heater Mods
Amphead, a member of various forums, struggled with a build that had induced noise/buzz in the headphones. After many attemps with various changes the one that finally fixed the buzz is this one. It is to remove the heaters from the standard heater bias chain and ground the heater secondary of the transformer in the manner shown in this schematic:
This type of heater grounding is very common when the heater windings can be tied to ground. This balanced approach is better than tying just one side of the heater secondary to ground. 1/2W resistors are fine for this duty. They will only dissipate about 100mW.
If you are building from scratch and intend to use this mod, then leave out all of the components X'd out in red. Add the two 100Ω resistors tied to the star ground point.
If you have already built the PS you should remove R9 and C7.