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.
EDIT
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:
This is a great unknown russian DHT. Just like his brother 4P1L :). An output beam tetrode with direct heated cathode filament. This 18W valve is a great candidate for an output stage, with its standard filament voltage of 6.3V and octal socket, many ones out there will be really attracted by this valve as it can easily be adapted in many classic output stages.
I did some initial tests about a year ago and this valve proved to be very linear and capable DHT. Was planning to use it as a pre-amp valve with filament starvation as THD results were more than promising.
Paul Leclerq, Andy Evans and I now want to test this one as an output valve. With 18W+3.5W=21.5W of anode power capability, it’s a great DHT for output stages clearly.
So I went back to the workshop to trace the curves first.
Finally, here is a THD plot for the 6P21S showing how linear this valve can be. The following quiescent point was found to be very good for this valve:
Continuing with the design of the 4-65A SE amplifier based on M. Koster design, I’m in the process of tweaking the 46 driver to optimise the operating point and provide maximum distortion to drive the demanding 4-65a. Here is the circuit I’m currently working on.
The current 46 driver will be biased at around 25-30mA using filament bias, so Vgk will be around -16 to -17.5V using a 10Ω filament bias resistor array. This will set the 46 at around 185-210V which will give sufficient headroom (i.e. need about 200Vpp max) to drive the 4-65a.
So today I look at varying slightly both anode currents and Vgk to see impact on THD.
So minimum THD is around -17V and Ia=30mA. So if setting the Rod Coleman filament regulator to ensure that Vgk=-17V and the anode gyrator to set anode voltage to ensure Ia is close to bias current would provide the minimum distortion (which is 0.04% in this example). Pa is close to 7W, but looking at the datasheet we can see that maximum Pa is 10W (as the latter 45 version).
46 Super Silvertone THD
So next I need to build a prototype of this driver with filament regulator and gyrator load.
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.
Well, after nearly 12 months of playing relentlessly my 45 SE amplifier, had an unexpected failure in the power supply (passive regulator) which forced me to do maintenance to the amplifier. It was a great opportunity to remove the 6J5 driver and do a quick swap for the greatly respected 7193 (i.e. military version of the 2c22)
45SE Amplifier upgraded with the 7193 drivers
Bias point remains unchanged: 7mA and 260V for maximum swing and minimum distortion. Need to look at my notes, but I remember that I was something around 0.30% at 100Vpp driving the 45 (which is not an easy load for anyone). Driver configuration was not changed, so had the 7193 now loaded with same DN2540 single transistor gyrator and mu-follower output for lower impedance. The valve was biased at about 8V with an LED array.
7193 in action
As a test, played the fantastic Symphony No. 3 from Henryk Gorecki….
Everyone loves this thoriated-tungsten DHT valve. I’ve only used it in a preamp and was hooked with its sound. Really warm and nice. Downside is, it’s very pricey these days and also is quite demanding from a filament perspective. You can check the characteristics here.
For those who like testing their designs with LT SPICE, I produced a model which matches really well the traced curves. Would like anyone to use this one, to drop me a note with any feedback 🙂
From an audiophile’s perspective, this is not the right approach to determine the optimal operating point. However, minimum distortion is a good indication of a good starting point for further refinement with your ears.
I have used mostly the 26 DHT with filament bias in the following point:
Vg=-10V
Ia= 5.5mA
26 DHT THD as a function of Ia
We can see in the diagram above that distortion decreases with the increase of anode current (lower ra and higher gm) and between 6-7mA it’s at its minimum of 0.04-0.05% at full output swing/
As posted previously, is well known that starving the filaments is a good approach to reduce microphony of the valve and THD as well. At the expense of increasing Ra.
THD impact of filament starvation
From the picture above we can clearly see that a typical 26 running at 5.5mA and with filaments at the normal level (i.e. 1050mA) can achieve a reduction of distortion of about 0.02% by starving the filaments to 700mA (66%).
I still need to test how this level of starvation will sound on my preamp, but is quite promising…(at least in theory)
Having repaired the Pete Millett’s interface (hopefully) I tested two 26 DHT triodes I had at hand. One was an used Hytron ST valve and then the other test was an NOS White Whestinghouse ST valve, which is actually in pristine conditions.
Both valves were tested with the same operating point:
Vg=-10V
Ia=5.5mA
input signal adjusted to produce Vo=10Vrms (+22.22dBu)
26 Hytron ST valve THD @ +22.22dBu
26 NOS White Whestinghouse ST valve THD @ +22.22dBu
Well, I think I have re-vindicated the 26 DHT THD performance at a decent swing. Surprised to see the WW valve achieving 0.03% THD.
