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:

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.

I’m not sure what the load capacitance is but I have the long leads to the SC interface and the oscilloscope lead connected in parallel so probably is somewhere around 150-200pF.

When I looked at the sweep response I found that the recommended Zobel resistor needed adjustment:
A minor dip around 10kHz was removed by replacing the 10KΩ resistor and adjusted the potentiometer to 3.23KΩ which showed the best response. The overall THD was impacted slightly and an average of 0.4-0.5% was achieved mostly across the entire audio band with an output level of 128Vpp/44.8Vrms. Not bad for this driver!

As expected from a 4P1L in triode-mode, the distortion is mainly given by H2 and the harmonic profile decays nicely. With my SC interface I can only get 128Vpp and distortion at 1kHz is 0.31%.

When driven with my external generator, I can get easily 200Vpp with distortion level increasing up to 0.5%.

As part of my research I looked at several triode-strapped pentodes which could deliver 0.3-0.4% @ 200Vpp when loaded with a CCS. Some interesting examples are C3g and 6e6P which proved to be the best driving at full swing:

Some H3 cancellation was taking place in the 6e6p and the harmonic content is well dominated by H2. D3a, 6J11p, 6J5p, 6J9p, 6j49p, 6j52p and others test really well, but not as good as the above with the exception of hand picked valves.

In summary, I just need to listen to this stage to feel how the treble performs in practice. From my tests it looks like this stage could perform well (not as great as a pentode / triode-strapped driver) but the sound of DHT in filament bias is unique. I love it, and if this sounds as good as my 46 in filament bias with gyrator load, then is worth the iron investment!

Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp) View all posts by Ale Moglia