All about electronic valves and hi-fi
I did some tests today and looked at minimising distortion of this 46 driver in filament bias and found that Va=230V (instead of 184V) to provide best performance:
Filament bias resistor array is now laid out horizontally to improve the dissipation of heat.
Here is the performance (0.05% at 17Vrms) at maximum drive input from my audio test set:
Finished the 4-65a filament regulators. Filament noise is only 1.5 mVrms at full load (3.5A):
After a lot of work today in designing many PCBs, I finally got a pair of mu-follower MOSFET gyrators for the 46 driver stage. The driver has to provide very low impedance to operate the 4-65a output valve in class A2. The gyrator in mu-follower configuration will enable the right bias point as the amplifier is DC coupled as well as maximum signal (and current in A2) with minimum distortion.
Many don’t like sand at all in their amplifiers. I have a lot of experience with gyrators and CCS loads in pre-amps and drivers as well. I have to say that with MOSFETs gyrators the sound is really nice. For an A2 driver, not many options are available and the gyrator is a great choice for this job.
I built two PCBs (one per channel) and the circuit is the classic depletion-mode MOSFET gyrator based on the high-voltage IXTP01N100D. I guess that a DN2540 should work as well here but I’ve been saving the IXYS for this occasion. The reference voltage for the anode bias point is provided by the CCS formed by M1 (LND150) which provides a higher impedance in AC improving the frequency response of the gyrator.
The 46 is operating in triode-mode and filament bias with a Rod Coleman filament regulator. R6 is approximately 1/gm and output voltage is set by P1 to achieve the 4-65a bias point as the amplifier has stacked power supplies given coupling is DC, so no capacitors in the path to the grid.
Next: some tests on these gyrators and the filament boards…
More pics after some nice time listening to the preamp
For those who are keen on the 26 preamp in filament bias and OT, here is my latest incarnation of this great pre-amp.
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!
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Finally, here it is.
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…
Today made significant progress on the new version of the 26 pre-amp. Only bits outstanding are the CCS modules part of the HT supply on board. Tested glow valves, output transformers and filament circuits…
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@DHTRob: I’m still thinking on what cartoon can draw for you. For the time being, here is one based on recent listening tests 🙂
After testing 4P1L with the 126C OPT, Paul Leclercq suggested a different operating point to see if distortion above 8Vrms on the OPT version was due to grid current. So firstly did a quick test with the CCS and found that -15V and 170V was a good operating point for 15mA. The 4P1L performs brilliantly with CCS and THD is only 0.05% when driven to 10Vrms.
Unfortunately distortion with 126C grows exponentially above 9Vrms. OPT is tested floating and not sure if is the Pete Millett interface a limitation with it.
Andy Evans suggested that secondary green should be negative so phase is 180. Aparently there is a thread meantioning the distortion introduced by the way I was using the OPT.
Anyway, problem sorted! Look at now how nicely the 126C can perform at 10Vrms:
