Today I managed to finally test the Lundahl LL2745/8mA specially designed by Lundahl for Thomas Mayer for a 26/01a preamp / line stage.

The circuit tested is here.

Initially did some tests with fixed bias, normal DC heater supply from my workbench and HT from a passive capacitor multiplier also available in my workbench. Circuit breadboarded has long cables and we shouldn’t expect good 50/100Hz noise levels as have many transformers and things around 🙂

Test 1: fixed bias

1. Vgk=-6,8V
2. Ia=6mA, Va= 119V
3. THD=0.033% @ Vo=1.4Vrms

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Test 2: fixed bias with Rod Coleman regulator

1. Vgk=-6,8V
2. Ia=6.5mA, Va= 119V
3. THD=0.031% @ Vo=1.4Vrms
4. Vf=1.4V (minor starvation)

Test 3: Filament bias with Rod Coleman regulator

1. Vgk=-6,3V
2. Ia=6.5mA, Va= 119V
3. THD=0.031% @ Vo=1.4Vrms
4. Rfilament=5Ω, Vf=1.4V (minor starvation)

As we shown earlier in other tests, filament starvation reduces THD slightly as expected. The OPT performs really well. Probably will look at starving a bit more filaments whilst doing a listening test

Will now proceed to rebuild the 26 DHT preamp with LL1660 with this circuit 🙂