Tag: 4P1L THD

4P1L Pentode Driver (Test 2)

Improving the driver with a gyrator load

After the early experiments with the 4P1L driver in pentode mode, I decided to look at improving it somehow given advice given. The gyrator load is not a good match for a pentode unless the reflected impedance is low enough to control the gain of the stage. Gary Pimm recommends:

“In the driver experiments the plate resistor was increased to a value larger than in traditional Pentode driver stages to get more gain.A CCS was placed in parallel with the plate resistor to add plate current to compensate for the high value plate resistor. This allows you to have independent controls of the gain and operating current. The resistor is chosen to set the gain and the CCS is used to set the Pentode operating current.
To maximize the circuit performance the resistance in the screen circuit is adjusted for minimum distortion. There are draw backs to this- The circuit has to be tweaked for each tube. As adjusting the screen voltage and resistance also effects the gain of the stage you have to compromise some to have the gain match between 2 channels. This is not a circuit where you can swap tubes around without “calibrating” the stage on the test bench.
Another interesting way of applying the circuit is to place the plate resistor in parallel with the Pentode and have the CCS supply all the current needed by the stage. This allows the Pentode driver stage to have PSRR similar to CCS loaded triode stages. It also makes the signal current loop very small including only the Pentode, cathode, and plate resistors. The noise and capacitor colorations of the power supply are quite effectively removed.”

So I opted for adding a resistor in parallel (RL) to adjust gain, minimise distortion and improve PSRR:

4P1L Pentode Driver TestThe load resistor is 68K. I optimised the operating point to reduce distortion at maximum swing (i.e. 200V peak to peak). The input impedance of the soundcard interface which is 100K didn’t produce a significant impact on the distortion when measuring from the anode output or in the mu output:

4P1L pentode filament bias RL=68K

Interesting to see that distortion is now nearly half of previous operating point and 0.27% for 200Vpp is very good.

The screen current is approximately 1.8mA at 81V bias.

 

 

4P1L Driver Tests

4P1L is a sublime DHT. As shown before it’s one of the most linear valves in triode mode. I built a breadboard in filament bias to test 4P1L as a driver using a MOSFET gyrator in mu-follower mode:

4P1L triode driver filament bias 1

20130104-205239.jpgMy test set can only drive the 4P1L output to 30Vrms and the distortion is only 0.027%!

I was intrigued by the performance of this driver in pentode so did a quick modification to provide a screen fixed voltage instead via a source follower and adjusted the bias voltage to minimise distortion. I found that a bias of about 120V was the best. This setup wasn’t the ideal one as in filament bias the frequency response is really poor as there is no cathode resistor bypass. The gain is about 200 with the gyrator used:

4P1L pentode driver filament bias 1A distortion of 0.58% @ 200V peak-to-peak is really good. The filament bias is forcing the pentode to operate with low anode current so I guess that with a lower bias point performance will improve. I will have to test this.

The measured THD was:

  • 0.125% @ Vo=100Vpp
  • 0.34% @ Vo=150Vpp
  • 0.58% @ Vo=200Vpp

Interesting to see the increase of H3 and H5 as a result of the pentode operation.

The breadboard for pentode can be improved for sure. I will look next at reducing the bias voltage as a first step. Interesting results which show that 4P1L is a great driver both in triode and in pentode modes.

 

4P1L PSE tests

Testing parallel DHTs

After listening to a great incarnation of the 4P1L PSE in filament bias output stage from Andy Evans, I decided to have a look at the impact of unmatched pairs of triodes from a distortion point of view. Main reason was that when listening to Andy’s amplifier I noticed a bit of an uncomfortable treble with some strings. Perhaps the increase of odd harmonics, but wanted at least to see what was all about.

4P1L are very easy to match. you can easily get a pair with equal mu. Just randomly I picked from my collection a pair of valves with a difference of 0.5 in mu.:

4P1L PSE unmatched pair

THD is about 0.03% mainly driven by H2. It happened that one 4P1L from the pair had 0.02% where the other had nearly 0.04% distortion. The difference between H3 and H2 is about  8dB.

Then looked at a more closely matched pair (0.03 mu difference). The distortion wasn’t surprisingly different:

4P1L PSE  matched pair

Again, nearly 0.03% and difference between H2 and H3 is down to 7.5dB.

Looking at the individual performance of the 4P1L, now biased at 30mA and similar anode voltage, we can see that despite having a lower THD, the difference between harmonics is just 5dB.  This is the THD of the other  4P1L from the pair:

4P1L PSE test rig

Well, how rthis compares to a 2a3/6C4C? The latter valves are two triodes physically connected in parallel inside the same envelope. So, no matching can be done:

The previous was a low distortion 6C4C I have. Distortion is higher than 4P1L PSE, but not that much. H3 – H2 difference is about 12dB.

My early thoughts:

  • 4P1L are very easy to match
  • 4P1L PSE performs really well. Distortion of the pair is lower than a 6C4C performing at same level.
  • H3 component is higher in PSE and this could be the reason why is more noticeable when listening to strings – as I proved in practice.