New Output Transformer for the 01a / 26 pre-amps

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.

45 SE Amplifier upgrade

Replacing driver for 7193 valve

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.

As a test, played the fantastic Symphony No. 3 from Henryk Gorecki….

DHT sound

Unique tone from a DHT valve. Haven’t experienced it?, worth trying…

4-65a glowing in the dark

These were taken at my lab in the attic during the summer evening twilight.

4-65a HT +300V supply

First breadboard of the 4-65a SE amplifier. Will start this long project with tuning the HT supplies. Will need a +600V, +300V and +100-150V supplies to be stacked.

Here is a simple breadboard using metal rails over an old wooden frame I had around. Oil caps will be glued temporarily and damper tubes will be fit later:

A snapshot of first test….

4П1Л/4P1L THD sweet spot

Playing around today with the 4P1L chap found a very good operating point where distortion is minimised at maximum anode power disipation:

Va=250V
Vg = -21V
Pa=9W (7.5W anode + 1.5W screen)
Ia=35mA
THD measured at Vo=+22.22dBu (10Vrms)

Interesting to see this valve swinging beautifully at just 0.027% THD….

4П1Л/4P1L triode curves

4P1L triode curves

4П1Л (or 4P1L) is probably one of my favourite valves. It was an unknown device to me until was suggested by some friends in the forum. Many discarded it as being a howling beast in pre-amp stages :). I found that albeit it can be microphonic, this can be controlled to a certain extent, but in my opinion this is a great valve in most of the roles: pre-amp, driver or output stage. Preferably is such a linear valve that can easily match 2A3 and 300B characteristics (when arranged in parallel) at a fraction of their cost. You can get a view of this beauty in the datasheet here.

Looking at the specifications, the key points to highlight are:

Filament Voltage: 2.1/4.2 volts @ 650mA/325mA respectively
Recommended anode voltage: 200V with 150V on the screen
Maximum operating voltage 250V anode or screen.
Maximum cathode current: 50mA
Anode dissipation: 7.5W
Screen dissipation: 1.5W

I became aware of this valve when Anatoly (a.k.a. Wavebourn) recommended the 4П1Л directly heated pentode which was used in military transmitters. It is very popular now among Russian audiophiles. Apparently is the Russian equivalent of the WWII era German Wehrmacht RL2 / 4P6 RF oscillator / transmitting amplifier tube. It’s a brilliant valve when triode strapped, better than 2A3 / 300B in terms of linearity. See my post around THD here and will see why 4P1L is at the top of the chart with less than 0.03% THD @ +22.22dBu!

As Anatoly suggested, they are very nice for class A in triode, and give up to 2.5W per valve when driven with up to +12V on control grid. It is easy to parallel them, since they are consistent and very linear: paralleling linear valves you are loosing power on mismatch, i.e. the valve with higher transconductance will draw more, no distortion raise caused by mismatch. 2A3, for example, has a pair of paralleled triodes inside. You can parallel ten of 4P1L matching them (it’s easy), to get 100W dissipation and 25W output.

Many found a sweet spot around Va=235V, Ia=40 mA, Vg=-18V providing Pout= 2.5W on a 5K OT, triode connected. Capacitance between anode and first grid for 4P1L is 0.1 pF. Capacitance between screen grid and control grid is about 1 pF. It has a 10 pF Miller capacitance which is not high value, and for 20 KHz it is slight less than 1MΩ impedance. Any driver with 10 mA idle current will make it happy.

I tested this valve a lot as a DHT preamplifier with great results. Starving filaments and suspending the socket with cord can reduce significantly its microphony to very low levels. I could listen to it perfectly fine whilst my friend Tony still have some issues with a 30sp DHT stage bolt to the aluminium top cover 🙂

I loved the sound of the 4P1L pre-amp. I will build a 4P1L SE in the future, is on my list…

4P1L Siberian Preamp — 4P1L Siberian DHT pre-amp

Recently, a friend from the diyaudio forum asked me for the 4P1L curves which I posted previously. Here is a new trace of the curves under the following testing conditions:

G2 and G3 tied to anode
Filaments are in parallel, so If=650mA powered by DC supply.

You can create your model or use the curves to produce your load lines, etc.

Hope this helps

26 THD analysis

As I’m proud of my 26 DHT pre-amp and also looking to use this valve as the first stage of my 4-65a SE amplifier shortly, I looked at how linear this valve is.

Unfortunately I don’t have an extensive set of valves of same brands, so albeit I have about 40 valves if this type there is a big mix of different brands and many of them are used ones. Having said that, I think this may be the case of many of you out there, so I think that probably the results of this test may be relevant to you (if you are still reading this post).

So in summary, this is the sample set I tested:

39 valves
12 brands
ST (34 valves) and Globe (5 valves)
Type: 26, 226 and 326
State: used (80%) and NOS (20%)

All DUT were tested with the same test set and operating point:

Operating point: Ia=5.5mA and Vg=-10V
Anode load: Cascoded DN2540 CCS set to Ia
Test signal: 1kHz, Adjusted to obtain Vo=10Vrms (+22.22dBu)
THD analysis: Audiotester via Pete Millett’s interface:
- 32768 sample FFT (2.96Hz resolution)
- 256 averaged windows
- Van Hann window
- THD for H2+H3+H4

So in summary the results showed that you should expect a 26 to have around 0.08% (Std Dev = 0.00047). Here is an histogram showing a summary of the tests done:

Looking at brands, the following average THD ranking was produced:

26 THD by Manufacturer/Brand

Interesting to find National Union leading the chart. I found RCA and Sylvania to be my preferred ones in terms of sound. THD shown above are average of sample sets of 4 valves or more. Although I tested 12 brands in total, some of them were just a pair so they are not a representative subset.

Finally, here is a sample THD of a very nice 26 valve:

I wish I could have a bigger collection of 26 to improve the accuracy of this statistical analysis. Either way you can get a view of what you should expect from this great valve…

26 DHT THD (continued)

Looking for the optimal operating point

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 operating point — 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.

26 DHT operating point vs filament current — 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)

CX301a DHT pre-amplifier

CX301a DHT preamp

Here is my latest incarnation of the DHT pre-amplifier:

Many claim that the 26 is the best sounding DHT valve for a pre-amp. I will agree to a certain extent, however I personally found the thoriated-tungsten filament sound a bit more rewarding to my ears. A more clear and defined treble in my opinion.

Since I plugged in my CX301a incarnation of my breadboarded preamp, I just left it there as I loved its sound. Certainly there are things to be improved to enhance the dampening of microphony, albeit I have to confess it hasn’t been a problem to me. Have heard some valves to howl, and this is not one of those. Clearly suspending the valve socket or adding the rubber dampers to the valve holding plate or socket will help massively.

Filament bias is a must in my DHT designs. Since discovered it, can’t avoid not removing most capacitors that I can from the signal path. In this case the filament resistor R9 will increase anode resistance by R9 times (μ+1). This will also impact the stage gain, but here all this is not a problem. You may find this is way too much gain in your system. Rod Coleman’s filament DC regulators are crucial to provide a hum-free stage. Attempting AC or other DC regulator is likely to bring frustration to your design. Believe me, I’ve been there before…

Now turning our attention to the anode load I will not open a debate here (or a can of worms!). You can make your choice of using a superior quality output transformer (and by superior means a lot of money!) or you can look at various options. A choke is a great idea, but special care needs to be taken to ensure choke is not picking up any hum from the remaining parts of the circuit – specially the supply transformers, etc. I have experimented for some time various types of CCS or gyrators as sandy loads for the valves with excellent results. If you are one of those that feels that sand is a sacrilege, then I suggest you stop reading this post now.

Gyrators are superb. They can simulate the AC response of an inductor of 300H (but without storing energy as a real inductor) or above very easily at 1/100 of its cost. You can easily adjust the valve operating point ensuring this is maintained despite the ageing impact of the valve or the eventual replacement of it. The anode voltage will be fixed by the gyrator, the current not. Cascoded MOSFET gyrators provide better supply ripple rejection and isolation. Using Q3 as a CCS instead of a high resistance potentiometer to set the anode voltage is better as it helps providing a better frequency response as impedance on this node is increased. A higher value of R10 will help reducing the size of the gyrator capacitor and the smaller the better it will sound in my experience.

M1 and M2 can be your depletion FET of choice. M1 should be a 250V rated one at least. Depending where you live, you will be inclined for using BSP129, LND150 or DN2540.

Previously I mentioned in some other posts that the mu-follower setup of the gyrator here provides a better output impedance and improves the performance of this valve significantly given its high anode resistance compared to other more suitable DHTs for this purpose such as 4P1L, 46 or 71a.

I’m not going to cover the HT supply here, but using a shunt regulator such as Salas, is one of the best choices here.

With Russian PIO capacitors you will get a fantastic result here, no need to start burning serious money on the capacitors until you are happy with the end to end build and you can then start looking at how to improve the sound of it by replacing some bits with better (or preferred) quality components

With an operating point of Ia=3mA you can get THD=0.08% at Vo=10Vpp. This will be subject of the quality of your CX301a. Some older globe 01a’s have a great sound, but they are not that linear. Hard to pick and chose your precious ladies here without testing them for linearity.