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6922 / E88CC tube pre-amp




 

After finished my simple 12AX7 tube preamp, I decided to try other medium mu tubes, including the 12AU7 and also this 6922.

 

The design is almost the same as the 12AX7 preamp, it is a single gain stage triode tube with cathode follower. However there is an important difference, a 1k "stopper" resistor is added at each grid input. This 1k resistor is required to avoid parasitic oscillation. Without it, the 6922 would oscillate easily.

 

Please note that you should NOT use 6DJ8 in this design, as the heater-cathode maximum voltage limitation is different with 6922. Besides 6922, the E88CC or E188CC tubes can be used.

 

Schematics :

 

Gain stage loadline :

Cathode follower loadline :

When design the loadline for the cathode follower, the 6922's maximum cathode to heater voltage limit of 150V has to be taken into consideration. The cathode voltage is chosen at 90V so that the 150V limit is not exceeded. If the cathode voltage exceed 150V, the heater is needed to be elevated. However, if the heater is elevated then its ground is not grounded, the heater supply wire can pick up noise or EMI more easily. My measurement showed that when the heater is elevated the amp's noise floor is increased by more than 10dB. So I would avoid heater elevation if possible.

 

For 90V cathode that means the tube has to drop 160V. I also want the tube to have a high output current at 9mA, so the operation point (the red dot) is chosen at 9mA and 160V. The load line can then be drawn by connecting the red dot and B+ at the voltage axis.

 

I am not going into the details that how the resistor values are calculated, please reference to my 12AX7 preamp page if you want more information.

 

The gain for this amp can be calculated by this formula :

Gain = (Tube Gain x Plate load resistor)/(plate load resistor + plate resistance + ((tube gain +1) * cathode resistor)).

 

With 6922, the plate resistance = 3k, tube gain = 33. Using these value in the above formula, the calculated gain is 22.

 

The output impedance of the 1st gain stage is equal to the plate load resistor paralleled with the plate resistance, so in my case it is about 2.7k, which is a bit high. So a cathode follower is needed to reduce the output impedance. With the cathode follower, the output impedance would roughly equal to 1/(tube's transconductance).

 

For 6922, its transconductance is about 12000 micromhos. So the cathode follower's output impedance would be at about 85 ohms, which is a good low figure.

 

It is very important that the regulated power supply can provide a clean B+ and heater voltage supply. For this design, any ripple in the B+ line will appear on the output signal. Noise or ripple in the heater supply will also be coupled to the output.

With the use of the copper component stands, I can change components easily. This is especially useful for trying out different loadlines.

DACT switched volume control, it has very smooth switching action

DACT 4 poles 5 positions input selector. Both signal and ground are switched.

 

The 6922/E88CC has low distortion, high output current, and has high transconductance which make a cathode follower with low output impedance of 85 ohms. All these properties make the 6922 a good choice as a preamp tube.

 

In fact, among the 3 tube preamps I created, this 6922 preamp sounds the best, it has the most resolution details and having excellent dynamics. I like it so much that I searched for the best 6922/E88CC tubes to use in it.

 

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