24a Tetrode Driver THD

Having traced curves for the 24a tetrode, now is time to look at the THD performance to drive either a 45 or a 6C4C output stage as requested in the previous post.

To avoid the area of negative resistance the output signal must always be above 100V. So a quick test was made to test output performance at 100 Vpp:

• Anode voltage (Va) = 180V
• CCS Load (DN2540 cascoded MOSFET) sourced by 300V filtered HT line
• Screen bias (Vg2k) = 90V from a source follower
• Grid bias (Vg1k) = -1.7V

So anode current was set to around 5mA. Noticed that the harmonic profile does change. If current goes slightly above 5.05mA then the H2 increased and become more triode-like distortion. When reduced below 5mA, the odd harmonics increased with more pentode-like distortion.

Transition is too quick given gain of this stage and not easy to fiddle around and set the minimum distortion point.  Distortion varies from 0.60% down to a 0.50% minimum if correctly tuned:

I need to increase anode voltage to 200V and conduct further tests to find probably a better operating point. However, not bad at all for this driver so far!

 Edit (10th November 2012)

Testing a tetrode/pentode with fixed bias in my testing box is proving hard to stabilise. Where is input noise or purely the fixed bias can make the valve to drift in some minutes.

Here is my last take on Vg1k=-3V for an output voltage of 10Vrms:

The above 0.17% isn’t that great compared to some tetrode/pentodes triode-strapped as drivers such as 6P21S, 6e5p and 4P1L which distortion is below 0.05%.

Looking at the performance at full driving capability (i.e.. 100Vpp / 35.3Vrms):

You can see that performs slightly worse than biased at Vg1k=-1.7V. However the anode voltage drifted a bit from previous take. With a large gain, the fixed bias trimming of Vg1k becomes challenging in my tester.

My early thoughts are:

• THD measurement of high-mu valves such as tetrodes/pentodes needs some sort of negative feedback to ensure the valve quiescent point doesn’t drift. Some sort of cathode bias needs to be implemented.
• 24a is a good driver, but from a distortion perspective there are some better ones in terms of linearity.
• THD down to 0.57% at full 100Vpp  can be achieved if bias is properly set. However in practice it is expected this to be around 0.60-0.70%
• I would be keen to build a driver stage with a 24a and test how this sounds!