WAD PreII conversion to Cathode Follower stage

[FAQ Team]

WAD PreII conversion to Cathode Follower stage

The PreII was designed to provide a 5x gain for low output sources such as the Leak Troughline tuners. However, the majority of equipment has sufficient gain to make a preamplifier with added gain unnecessary. In such cases a preamplifier/buffer stage with no gain or unity gain would be more suitable. There have also been reports of hum issues with PreII; the following conversion should be totally hum free.

A cathode follower is simple and provides .90 to .97 gain (slightly less than unity). It also possesses high input impedance and low output impedance. This means it can be driven from low or high impedance sources and, in turn, can drive low or high impedance loads. Depending on the valve it may also drive high capacitance cables.

A cathode follower is so called because the Vg-k (BIAS) remains constant despite the cathode voltage changing under active working conditions. The cathode literally follows the input signal, maintaining correct BIAS.

As standard the WAD PSU II delivers 100V HT to PreII and will need to be modified to provide twice this for a cathode follower circuit. Also, an elevated heater supply will be needed as the cathode will sit above ground at approx. 200V. Luckily WAD have already provided us with such a heater supply for the PSUII.

It is possible to use a different valve choice for this conversation such as the excellent 6SN7. This is a very linear low distortion valve with a great sound but requires an Octal base and ideally needs more HT than PSUII can deliver. The heater current draw is also greater but keen experimenters might like to consider this valve with appropriate modifications to the power supply.

Also worth considering is the 6072. It is a really sweet sounding valve and would be a good choice but does draw slightly more heater current so the circuit would need to be optimized for it, along the lines of HT= 240V, Rk=1.2K, BIAS=1.65V, Ia=1.35mA and RL=100K

The following conversion stays with the 6922/ECC88 B9A base small signal double triode valve as supplied with the PSUII kit, making conversion simpler as the PCB is retained from PreII. You may have to adapt some things to make all the components fit.

Circuit board changes

* Remove all components from the PCB except for the valve socket and R9/10. Solder a shorting link between the GND pcb track and the GND end of R8/R7. The o/p transformers also need to be removed.

* Solder an 820R 1W & 33K 2W resistor in series into the R12/R11 position. Stand these two resistors off the board by 5mm, you need to build a resistor network off the leads. The 820R resistor connects to the Cathode of the valve, it connects to ground via the 33K resistor. See circuit diagram.

* Solder a 1M 1W resistor from the signal input end of R9/10 to the connecting junction of the 820R and 33K resistor, this same junction will be used to take the output from

* The HT Anode and Heater connections are as before, the GND and signal connections from the PCB are as before.

* Solder to the input PCB pins one end of a .047uF (47nF) quality polypropylene capacitor, the signal input is to the other end of the capacitor, it can be sat on the PCB and either blue tacked in place or Hot Glued. This capacitor is necessary to isolate the Volume control from any DC leakage, do not omit it!

* Using the original screened cable, connect up the input and ground signal connections.

* The o/p from the 820R/33K resistor junction needs to be taken to a 1uF o/p capacitor, this is a DC blocking cap. Quality film&foil polypropylene capacitors can be large so tie-wrap the two o/p capacitors to the chassis where the o/p transformers normally sit and take one end of the cap directly to the o/p socket.

* Solder directly to the o/p sockets 100K 1W shunt resistors (signal to ground) to form a high pass filter with the o/p cap, this gives 1.6HZ low F cutoff. And a 3.2HZ cutoff if the input impedance of the next stage (power amp) is also 100K. If the impedance of the next stage is lower than this then increase this resister to 1M ohm

The Bias is set at approx. 2.6V. There should be around 95-100v across the valve.

The circuit is below.

PSUII changes

* PSUII needs to be modified to deliver 200V, if you built it correctly R4 should stand off the PCB by 5mm. Cut it off leaving the legs sticking up, also cut out ZD1 near the body of the Zener so there is some leg left to solder to.

* Replace R4 with an 18K 3W (minimum) resistor soldered to the legs. Replace ZD1 with a 200V version (RS 447-2055). Bypass ZD1 with a .1uF 400v cap. You should be able to do all of this without taking the PCB out.

* If you run PhonoII you may find it’s HT voltage is now too high, replace R6 with an 18K 3W resistor to bring the voltage nearer to 250V.


That’s it! Hope it works for you.

Neal Gibbons