6CB5a, as many TV power pentodes, is a great valve for a single ended amplifier. Specially now that popular valves of similar anode dissipation are getting really expensive.
Thomas Mayer came up with a great design for this superb TV pentode that wired in triode mode is very linear and can sustain at least 25W of anode dissipation. If you want to check Thomas’ design, have a look at his website which is very interesting.
If you want to look at the specifications, you can download the datasheet here.
When testing this valve on the curve tracer it was evident the fantastic features and linearity at every point.
As a power valve, testing a 6CB5a for harmonic distortion showed THD=0.07% @ Vo=+22.22dBu (10Vrms) at the following operating point:
- Ia=40mA
- Vg=-50V
- Va=253V
Transconductance in triode is high: 6.4 mA/V @ Ia=40mA, Vg=14.6V, Va=100V. In summary:
- Gm = 6.4 mA/V
- μ = 4.5
- ra = 700Ω
Here is a set of triode-strapped curves that you may find very useful if you’re planning to use this valve:
Also you may want to try the following spice model. It can be improved, but I think is an accurate representation of the curves shown above. Please try it and let me know what you think about it…
