Input Stage Schematics
The input stage is exactly as shown in the design section. Both channels are identical.
And per the design section, the part values are chosen to get, approximately, 80V on the plates of the triodes.
Output Buffer Schematics
The output buffers follow the design exactly. The resistor values are chosen for a design center of about 22mA idle current in the output stage. The CRD is chosen to put 4mA into the input stage. The servo resistor are chosen as per the Buffer Design Discussion.
The two channels are shown to clarify that the buffer servo opamp is a dual opamp shared by both buffers.
Rail Splitter Schematic
Epsilon 12 with Lights
The CTH includes an simplified and modified version of the ε12 muting circuit and offset detector designed by Ti Kan (AMB). This version uses the CTH virtual ground as its reference point and has the Cavalli Audio ε12 light.
Note that this version uses BC550C instead of the original 2N3904. This was done because the amp already uses the BC550C devices. Using them here keeps builders from having to source yet another transistor type.
Warning: The original 2N3904 cannot be substituted directly because their pinouts are different from the BC550. They can be substituted if you rotate the devices 180d.
Notice that R1E and R2E are changed from 10kΩ to 18kΩ and that R4E is changed from 1kΩ to 1.5kΩ. The purpose of these changes is to make the CTH ε12 less sensitive and a bit slower. However, the SOHA II ε12 is still sensitive enough to prevent damage to headphones.
R10E is not in the normal ε12. In this amp the relay current would normally pass through Q3E and into VG. However, when the relay current is sunk into VG this simply means that the bottom half of the rail splitter has 10mA more current passing through it than the top half. This imbalances the splitter and dissipates useless power in the two bottom transistors. R10E is there to shunt the relay current away from VG. The coil resistance of the stock relay is about 1100Ωs. For our purposes a 1kΩ 1/4W resistor almost pefectly balances the rail splitter when the relay turns on. The excess power is burnt in the resistor and not the splitter.
If the modified ε12 in your CTH has problems, don't blame AMB since his original design has different component values.
The lighting circuit is below.
The Power Supply
The full CTH power supply includes the LV and HV supplies and the heater supply.
Some of the features of this supply are:
- The HV supply uses a 3X voltage multiplier to make about 100VDC from 24VAC
- The LV supply has a high ripple rejection pre-regulator filter
- The heater supply uses a switching regulator for high efficiency, lower power dissipation
- The heater supply is switchable from 6.3V to 12.6V and can handle 600mA tubes
WARNING - 100VDC is significant high voltage. Every precaution for handling HV should be exercised.