Robustiano – Bartola® Valves

Hacked a simple PCB to build the follower to drive the 4P1L as suggested by Rod. I had to play with the LND150 setting resistor (R4) to achieve the 2mA of idle current. I ended up biasing the 4P1L rather hot at about 11.5W which exceeds the specs. The Q2 VBE was not possible to measure as the Q2 would oscillate I guess when I place the tester lead on Q1 collector and the voltage seems to drop when I try to measure it. Should have added a ferrite bead:

When measuring distortion against frequency, I was keen to see that the follower provided some impact in reducing the HF distortion. For example at 20kHz, THD reduced from 0.96% to 0.59% @1W output power and from 7.84% to 3.52%, that is close to half the distortion I had before:

What is nice to see now is the effect of the follower providing sufficient source current to the 4P1L grid. Above 2.5W, the grid current kicks in and we can see how “Robustiano” can deliver 3W at less than 1% until starts to clip about 3.5W:

I found that if I were to reduce the Rf further and therefore increasing the collector current but obviously exceeding the 4P1L power dissipation too much as collector current was about 45-48mA, the distortion at 20kHz falls significantly. I suspect I should increase the collector current to enable better drive of Q2 due to its Cib (30pF). To keep the current feedback arrangement this could be done by reducing the negative emitter voltage source (V1). Should try this I guess…

Cheers

Ale

Robustiano (version 0.6)

It was now the turn to the BJT to show what it can do in this circuit. Here is my quick breadboard with components I had at hand, so there is a slight variation from the simulation:

The Q1 is obviously an MPSA42. C3 was substituted with a 10uF/25V electrolytic (yes you read well). Rf is a 250k carbon pot and R2 is built of a series of 1K pair in parallel plus 100 ohms in parallel, four resistors are wire-wound 1W. C1 is actually a 30uF/450V ASC Oil cap.

I haven’t measured the collector current but it seems to be around 1.3-18mA. Variance is due to tolerance of RE and R3. I need to measure the actual resistance of Rf but is somewhere between 180 and 200K.

As Rod suggested, the circuit is very stable and easy to dial the right feedback with the potentiometer. The 4P1L is biased about -9V and is slightly under the max Pa in this case.

Distortion is as predicted in the simulation (e.g. 0.1% for 1W and about 0.2 – 0.25% for 2W). Just above 2.2W distortion creeps up rapidly given grid current, which is not modelled properly by the 4P1L pentode model:

Looking at distortion versus frequency, it’s interesting to compare the BJT performance against the depletion FET. With lower Cob compared to the Coss of the FET, the BJT should be able to drive better the 4P1L. In fact, the BJT is more linear when swinging many volts compared to the depletion FET, so the proof is in the pudding:

The BJT is indeed more linear but if we compare the THD vs frequency of both drivers when providing 2W output power, we can see that the BJT is suffering as much (and even more given poorer slew rate) than the FET at frequencies above 12kHz. Also FET’s THD versus frequency is more linear up to 10kHz, whereas the BJT has a peak around 6kHz and a dip closer to 10-11kHz. Either way, the BJT outperforms the FET in overall THD up to 11kHz.

I need to listen to this circuit now…

Robustiano (Version 0.4)

Finally back home after a long trip and had the opportunity to put the DN2540 at test and try the topologies discussed for the “Schade” feedback 4P1L SE amplifier. So I re-build my test rig and tried the DN2540 and LND150 at various drain currents. It was clearly to see that in order to keep distortion to a minimum, the VDS needs to be greater than 60V to keep the output capacitance of the FET low. Here are the results of the frequency response at nearly maximum output power (Po=2W):

It is interesting to see that the LND150 which has Coss (max) of 3.5pF doesn’t perform much better than the DN2540 which has Coss (max) of about 30pF. Operating points are different for both FETs but the 4P1L is running about the same operating conditions. What is also interesting to verify with this test is that the higher the drain current, the more capability the FET has to drive the 4P1L input (and Coss) capacitance at higher frequencies as the slew rate of the FET is higher.

We can see an interesting improvement from my initial tests at 5mA when drain current was just about 1.5mA. The yellow trace (Id=5mA) shows the best performance of the DN2540. Surely higher drain current will perform better but at a cost as the drain current is part of the OT primary current.

So how do we keep the gain of the FET when increasing the drain current? The natural approach will be to reduce Rf, but this affects the FET gain and the feedback. The alternative is to increase the supply voltage respect to ground. The price we pay here is to increase the cathode resistor and burning the power on it. With -4V as the negative source supply voltage, I had to only reduce RF to 51.5K to set 5mA on the DN2540. The supply power was increased to 350V, the screen (Vg2k) to about 140V (240-98.6V) which is lower than the 150V used before. There is a tad of extra power to extract on the 4P1L but here is close to its maximum dissipation. The Rk is a pair of wirewound 4K7 in parallel.

I think it is now time to try the BJT driver. I suspect that it will need at least 5mA of collector current to get on with the task of the input capacitance of the 4P1L when anode to grid feedback is in place.

cheers

Ale

Robustiano Schade SE Amplifier

Some topologies as discussed in the DIYAudio thread:

Robustiano: 4P1L SE Schade Feedback tests Part I and II

Here are some further tests using automated measurements. Firstly I tried same configuration running the 4P1L at closely 38mA and 160V screen voltage. Second test, I dialled up the screen voltage to 171V and tweaked feedback to get current up to 40mA

Interesting to see that the performance is more triode-like and has a higher THD at lower output levels, however at higher levels the THD is lower. Mainly dominated by H3 and significant rise of H5 given grid current I suspect:

Here is the harmonic distribution of the first test (more pentode like)