Tag: filament bias

4P1L – LL2746 driver test

After a recent discussion in the DYI Audio forum about the 4P1L drivers, I decided to do some quick tests on an idea I had around to use a step up transformer (1:4) – 4P1L and step up interstage transformer (1:2) to drive a 300B or similar using the 4P1L in filament bias.

First suspicion is on whether the 4P1L has the grunt to drive a capacitive load which would be a real challenge in a 1:2 step up as load capacitance is multiplied by 4 when impedance is reduced by a factor of N^2=4.

I built a test rig with the 4P1L in filament bias using a 15Ω wire-wound filament resistor and connected the filaments in parallel to obtain easily a nice bias voltage with 650mA of filament current. Also lower Rf will improve the low frequency response as helps keeping low the output impedance:

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4P1L rat nest

The valve was biased at Ia=30mA / Va=160V and grid bias is about -10.2V. A 10KΩ resistor was added as a primary Zobel as per recommendation of the datasheet. Then it was replaced by a 25kΩ potentiometer (P1) and the right value was found by looking at the frequency response.

Initial tests showed a very good response at 1kHz with only 0.24% THD @200Vpp output. The gain is approximately 16. The mu of the 4P1L with paralleled filaments is around 8 and lower than when used in series which is approximately 9-10.  Albeit the results were promising initially, the real test of this stage is by looking at high frequency response where the capacitance will makes it real pain.

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4P1L / 6C6C SE Amplifier Design

Pushing to the limits

Weight lifting

 

 

 

 

 

 

 

We’re constantly obsessed to get the most out of our lives. Not a product of the capitalist world we live in, but a fact of our human nature. Its evolution.

When it comes to sonic power, unfortunately we are not too distance from this thought. We want more Watts. Yes, pure power. My generation back in the 80s got misled by the audio product marketing and their unrealistic metrics (e.g. PMPO)  to fudge the real power of a solid state amplifier.

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4-65a SE Amp: 46 filament supply tested

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Finished the second channel and tested the filament supply. The filament array which is formed by two paralleled pairs of 20W 10Ω wirewound resistors gets hot as expected. The array temperature is about 110-126°C at an ambient temperature of 24°C. The anode of the 46 gets to 49°C after 20 min of use and the heatsink stays at 42°C whereas the regulator TO-220 transistors are about 45°C. There is about  30W dissipated on each array. Yes I know, a lot of power but the filament bias is hard to beat in terms of sound in my view.

4-65a SE Amp building process

Some drilling and mounting work done this morning on the 4-65a SE amp breadboard:

Filament bias resistor array
Closer view of the filament bias resistor array
Mounting pilars for the SSHV2 regulators
46 valve sockets
Closer view of the 46 valve sockets
46 sockets, filament bias array and filament and HV supply connectors

During the afternoon, I managed to wire a filament supply for one channel 46 driver. Tested and working ok, now can move to the next one:

Filament regulator with its heat-sink at the back, anode loads and the 46 DHT drivers at the centre.
46 DHT driver under test. Gyrator anode load can be seen at the front
46 DHT driver

46 DHT driver final tests

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Having built the 4P1L filament bias driver stage in a breadboard, I now have the sufficient voltage swing to drive the 46 to maximum sweep. In my 4-65a SE amp, a maximum of 200Vpp is required to drive the amp into class A2.

The following tests conditions were used:

  • 4P1L first stage:
    • DN2540 gyrator in mu follower output
    • 220nF/450V Capacitor coupled into 46 driver
    • Filament bias: 15 ohms, Vgk=-10V
    • Vsupply=355V and Va0=210V
    • Output set to about 30-32Vpp to drive 46 at 200Vpp
  • 46 driver stage:
    • IXYS 01N100 gyrator in mu follower output
    • Load impedance is 100K (Pete Millett’s interface)
    • Filament bias: 10 ohm / 100W Vgk=-17V
    • Vsupply=355V and Va0=204-208V
    • Output set to 200Vpp

I tested 28 valves. Just a few of my lot are NOS. The average THD was about 0.4-0.5% but a good selection of 8 valves (mainly Sylvania NOS) provided a consistent 0.18% THD:

4P1L into 46 driver test2Happy now with the initial tests and selection of 46 pairs for the amplifier, I can now continue with the build…