Pentode driver with feedback (Part I)

It was about time to get my hands on this driver experiment. I’ve been trying to find the time for a while and could only make it due to the obliged COVID-19 isolation upon return from holidays.

The idea is simple. I wanted to use a pentode driver to swing large volts (e.g. 200Vpp) whilst retaining the triode-like characteristic from harmonic perspective and low distortion. A nice challenge and fun to work on.

Have to say that the parallel/parallel feedback (also referred to as “Schade feedback” by some in audio) when applied locally in the output stage, does sound very nice and is a very nice way of implementing high-gm pentodes used for vertical service in TV. They can produce very low distortion and sound amazing when implemented correctly. I’m not covering this now, as it has been dwelled on for some time by many good people out there.

The circuit topology is as follows:

Instead of driving the grid, we fix the grid and we drive the cathode. This has a benefit of avoiding signal loss when the PMOS (Q1) is driving the feedback resistor array (R1 and R2). However, the screen to cathode voltage isn’t constant so distortion may increase. The benefit also is to avoid loading the anode and have flexibility on the values of R1 and R2. This can be done also by placing a NMOS transistor between the divider and the grid. This is good technique when operating in A2 and grid current is needed. C1 and also potentially another Capacitor between R2 and ground may be needed to equalise the HF response depending on the anode load and the output valve, etc.

It’s a rather complex arrangement but does work. The hybrid mu-follower is formed with the pentode and the active load (aka gyrator) which provides the maximum output swing at very low distortion.

The gain of the stage is derived by the voltage divider R1 and R2. Ideally if the valve had infinite gain, it would approximate to R1/R2. However, gain is limited on the pentode so we need to apply the formula but measuring (or estimating) the open loop gain of V1.

Challenge here is introducing some level of DC feedback to avoid drift of the valve.

The added complexity is a negative supply (in most of the cases) to feed the PMOS driving the cathode. There is a way of avoiding this at the expense of extra complexity, and this is what I did to test the stage:

The added complexity is a voltage follower and DC shifter (Q1) for the feedback divider with some element of DC feedback from the anode. In the above diagram, R1 and R2 will derive a DC voltage which will be followed by Q1 and feed the end of the feedback divider (R5 and R6). This way the negative supply is avoided, and some level of DC feedback is introduced.

 There is alternatively a way of introducing some level of DC feedback via the screen as well. The above circuit, shows Q2 providing the screen voltage. In this case, the screen voltage is obtained from the mu-output via the divider R9 and R10.

D3a Pentode Driver

So, I couldn’t wait longer and with the limited time available I put together a rat’s nest testing jig with the following implementation of the D3a pentode as driver:

A few additions. Firstly, I derived the DC feedback from the mu-output. It varies more than the anode with changes in anode current as there is a mu-resistor in series, although is rather small in the grand scheme of things.

I did a few simulations in Spice and found that reducing the screen voltage to about 70V and anode to 200V provided the best performance. There is a first board which Has a simple voltage reference (T1) which takes the input from the Screen Supply. This voltage reference provides the DC reference to Q2 and sets the cathode DC voltage. The signal input comes via C5. The feedback divider (R1 and R3) is DC-shifted by Q1 which takes the reference from the mu-follower output via R2-P1-R6. P1 is the only one I ended up using to do the DC adjustment.

The measured open-loop gain (A0) is about 280 given the low anode and screen currents, transconductance is rather low. With R1=68KΩ and R3=470Ω, I measured closed-loop gain of 98. This is perfect for a  one-stage  driver in a 300B SE Amp for example.

The D3a operates at a very low current, about 5-6mA.

The above measurement shows a good performance of 0.53% for 180Vpp and a 100KΩ.

You can see that H3 dominates from the low feedback ratio provided so there is more of a pentode-like behaviour. Yet, for a pentode stage is good performance for 40dB of gain.

The circuit drifts a bit and takes some time to stabilise. I left rather too-long  time constants (e.g. R4/C4) which should be halved or more. That will work better.

If we apply more feedback, as expected the gain is reduced. Here is an example of the same circuit with gain of 27 (29dB):

You can see that distortion is now reduced due to greater feedback to 0.22% for same output level of 185Vpp. This is very good performance, albeit the harmonic profile still has the H3 domination as before.

Interesting experiment, learned a lot from it as usual. There are ways of improving this stage to get better stability for sure.

Will I build it? Not sure, this is a much more complex circuit. Despite having a great performance, it still needs a lot of sand around it to work. It does excel in avoiding the Miller problem of the high-gain triode stages, so the source will drive without a problem the PMOS.

A similar topology is very very useful for output stages and that is where you can obtain a big improvement in implementing the traditional “plate to grid” feedback in a clever and effective way.