Now that I have a very quiet preamp indeed, I’m a happy bunny. Changed today the pair of White Westinghouse 26 ST NOS that I’ve been using for over a month with a pair of RCA CX-226 and have to say that I enjoyed a far more sweeter and rounded tone out of this preamp.
Bass is still deep and treble is clear with the sweetness of the globe valve fingerprinted in the tone as you would expect. I will run this setup for a month and report results…
You can still see the clips used to provide earth return at the output to avoid the ground loop that I unnecessary created. God knows how may tweaks I did to improve the HT rail ripple before I realised that it was a simple ground loop.
Glow tubes fed by a CCS are not just very quiet, they are really cool
A nice pair of CX226 in the new preamp. They are in very good shape, very low distortion measured in the workshop time ago and no hum induced, so great for this preamp.
Sound is definitely warmer, without loosing any of the bass and bright treble from this preamp. A great preamp so far, happy like a bunny!
Improved sound compared to LL1660 version. This OT (LL2745/8mA)is better suited for the 26 in this configuration. Sacrificed output impedance a tad to get more gain. Filaments starved at 760mA. Distortion is as low as 0.02% @ 3Vrms output.
First listening session with Mingus, exceeded my expectations. Rounder bass and more space. Like this sound!
Will do some proper measurements but for now will enjoy and listen to it…
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
Vgk=-6,8V
Ia=6mA, Va= 119V
THD=0.033% @ Vo=1.4Vrms
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Test 2: fixed bias with Rod Coleman regulator
Vgk=-6,8V
Ia=6.5mA, Va= 119V
THD=0.031% @ Vo=1.4Vrms
Vf=1.4V (minor starvation)
Test 3: Filament bias with Rod Coleman regulator
Vgk=-6,3V
Ia=6.5mA, Va= 119V
THD=0.031% @ Vo=1.4Vrms
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 🙂
Before breadboarding a pre-amp with these nice OT provided by Thomas Mayer, I decided to simulate some options in LT Spice to see what results I got.
CX301a DHT pre-amp
First one is my preferred CX301a thoriated-tungsten DHT. I wired the LL2745 in 5.6:1 step-down configuration. This should provide a low output impedance which is what we want in this configuration where we need to drive the cables to the amplifier with sufficient capability:
CX301a preamp based on LL2745
Looks very promising. Of course gain will be much lower than a gyrator-based pre-amp, the 01a anode load is optimised providing very low distortion: 0.015% based on my SPICE model with curves taken from real CX301a. Gain is low at 3.1dB, but we don’t want loads of gain in this pre-amp. Sound is what we are after…
26 DHT pre-amp
Now it’s time for the revered 26. I used Dmitry’s model based on the RCA manual curves. I’d like to simulate this again using a model based on starved filament curves with a real 26.
Originally planned to bias the 26 in a different operating point based on feedback from Andy Evans, however after playing a bit with the OP I found that a more linear point was around Ia=5mA and Va=114V @ Vgk=-6.85V
26 preamp based on LL2745
26 looks more interesting in principle as the output impedance with this model is lower than the CX301a thanks to having a lower Ra (7KΩ against 11KΩ) so LF response will be slightly better in a side by side comparison.
Will be trying these two with filament bias and Rod Coleman’s filament regulators. The HT will be provided through a Salas HV shunt regulator.
New Lundahl LL2745 OT pair arrived today in the post. Very excited, Thomas Mayer kindly send me this pair for trial. These are specifically designed for the low current and high anode resistance DHT used in pre-amps (e.g. 26, 01a, CX112, etc.). Wiring and primary details are similar to the LL1689. Hope we can get a final datasheet in September 🙂
These are gapped at 8mA. So the primary inductance is about 200H in theory (160-180H in practice according to Thomas). The labels are incorrect. This is 2×2.8:4×1, providing the option of wiring it as 5.6:1 if both primaries windings are in series and all secondary windings are in parallel.
Will rebuild the 26 or 01a preamps and do some test.
I would like to build a quick breadboard with the 01a to do a comparison against the gyrator loaded preamp I’m using at the moment.
You can still see the clips used to provide earth return at the output to avoid the ground loop that I unnecessary created. God knows how may tweaks I did to improve the HT rail ripple before I realised that it was a simple ground loop.
