Introduction

Building a new version of the venerable 4P1L “Siberian” was very encouraging. This belated project finally came to life after some recent work on a new set of power supplies. So why 4P1L again? I always found the 4P1L sound to be unique. Great detail, overall tone and fantastic treble. What it makes it well suited for pre-amplifiers is not just its linearity (probably being the most linear valve out there) but the fact that it has a low anode resistance and current capability to ensure any challenging load can be handled effectively without any sound degradation. This can be heard particularly on the treble where the input capacitance of the amplifier is more evident and it is translated into treble loss. Other DHTs like 26, 01A, 30sp can only handle a few milliamperes of anode current and is not enough to charge and discharge the  parasitic capacitance at high frequencies. More importantly, the 4P1L has filaments which aren’t demanding. This is a unique feature amongst DHTs that is rare and very useful. Having low-current filaments that can be either configured at 325mA or 650mA, low grid voltages and high transconductance in a valve is very useful. This mean that filament bias can be easily implemented without burning unnecessary power by swinging many volts to perform the desired level of amplification.

However, nothing comes for free. The 4P1L is a dancing lady. You have to tame her singing (i.e. microphonic noise). This may discourage you and has done so to many people before. However, it can be done. Many ways have been discussed out there, but what worked well for me is the use of silent blocks and silicone rubber.  Also starving the filaments is a very effective way of reducing microphonic noise. I parallel the filaments to increase linearity and also to increase the filament current which helps keeping the filament resistor value low which doesn’t impact the output impedance much. Remember that any resistance in the cathode (e.g. the filament bias resistor) is reflected int the anode at (μ+1) times.

4P1Ls can be found in superbly NOS condition (probably not for long) and can be matched very easily. It’s surprising how close they are all to spec or at least amongst other ones in a specific batch.

Design

When I started planning my third 4P1L pre-amplifier, I thought about what I wanted to achieve that I felt could be improved from my previous designs. I came up with the following three things:

1. Reduce the output impedance: I don’t need further gain, just increased load drive
2. Increase current drive capability to handle capacitive loads. This is a must if you’re using step-up transformers as an input in your amplifier.
3. Reduce overall noise level (microphonic and residual hum). After building so many circuits using DHTs, this can be done effectively.

The first two objectives can be accomplished by the use of an LL1660 gapped at 40mA and in Alt Q mode or 4.5:1 step-down. This output transformer will help reducing the output impedance of the 4P1L whilst running the 4P1L at a very high current without compromising the frequency response given the relatively high inductance (i.e. 50H) for the gapped core it has. This feature married with the Ra of the 4P1L will ensure good low-frequency response. The transformer will also help to remove ground loop issues as it provides galvanic isolation between input and output.

The 4P1L likes it hot 🙂 You want to run the 4P1L higher than 20mA if you want to get the most out of this valve. Triode-wired can withstand 9W or even more at 250V maximum anode. As an output stage you can stress the 4P1L to get every juicy bit out of it, but is not what we want in a preamp. We want it to be quiet and very linear. I traced 4P1L in triode mode multiple times throughout the past 3 years and looked at the THD and sound impact of starved filaments in the search of the sweet spot (if there is really only one).

I aimed at running the 4P1L around 30-35mA as it was providing the lowest THD on my tests. A good operating point to start here was 163V and -8.7V of bias. I looked at filament bias given it sounds the best to my ears compared to battery or any sort of other fixed bias. I looked at an operating point which will lead me to a nice resistor value that could be formed out of high-quality wirewound resistors and 32mA seemed to be a great choice. The filament resistor then should be Rf=8.7V/(550mA+32mA)= 15Ω. This can be implemented with three 10Ω resistors or if you have a 5Ω at hand then better. The higher power handling capacity the better as it will help keeping the thermal noise down. The quality of this wirewound resistor is fundamental. I use NOS Dale ones. You should pay special attention to this point if you want to get the best sound of this preamplifier.

The 4P1L wired in triode will have a μ=10 and an Ra=1.6kΩ. Transconductance is high at about 6mA/V. Simulation showed that the preamplifier should provide a very low distortion about 0.02%.  Will see how this performs in reality.

The final circuit design is here:
Nothing new for a classic DHT pre-amp. The devil is in the detail, though. The HT supply is regulated by a Salas SSHV shunt regulator which allows adjustment of the operating point but also is very quiet and proved to be reliable when delivering large currents as well. I used it on my 46 DHT driver with great results. I added a 10μF ANSAR Kelvin capacitor to improve input filtering and HF response. The regulator is set to 210V and 75mA. A 10Ω sensing resistor is added on each anode to help measuring the anode current.

Simulations show that the maximum capacitive load that can be handled by this pre-amplifier is in the region of 18,000pF – 20,000pF before the -3dB point is below 20kHz. This handling capacity should be more than what most of you need in your systems. However this is good enough to drive my step-up input transformer (1:8) and a Zobel network which included a 220k + 390pF.

With the LL1660 wired in 4.5:1 the overall gain is about 6.7dB and output impedance around 425Ω. I ended up wiring the 4P1L with G3 and G2 tied up to the anode via a 300Ω resistor. I usually connected G3 to the cathode filament end which end up with a lower anode current. I therefore ended up with a 34mA anode current.  Not a major difference, but couldn’t be bother to rewire the G3 as I’m fine with a bit more current anyway.

P1 is a stepped attenuator in my case and you can get rid of it if you have a TVC or other volume control. In that case R3 should be changed to something like 100kΩ. I don’t use grid stopper, instead I use a ferrite bead located close to the grid pin. Never had an oscillation problem with the 4P1L so far.

The most critical part of this pre-amplifier is the filament supply. You can’t underinvest here. I designed and built a choke-input filament supply which delivers 16-17V at full load to the Rod Coleman filament regulators. If you haven’t tried proper DC regulation in DHTs, I suggest you read this article. The difference in sound is overwhelming. The slow rise in filament current helps in extending the valve’s filament life.

First tests and listening impressions

It took me half-day to upgrade the 26 preamp and convert it to 4P1L. I had to replace the transformer block (now with cover), the filament regulators, the SSHV2 with a bigger heat-sink, the 4P1L loctal socket board and the filament resistors.

The initial tests on my workbench where really good.  The key highlight measures are:

1. Bandwidth (-3dB points): 10Hz – +100kHz
2. Noise level: -87dBV (45μV)
3. THD: 0.014% @ 1Vrms or 0.0188% @ 2Vrms output

Here are some quick screenshots of my preliminary tests. Load in this case was the Pete Millett’s interface (100kΩ + 250pF or more):

Sweeping the input to see how much the stage can handle reveals a nice performance. I have to optimise the automated sweep measurement at LF, but either way you can see that up to 8Vrms the THD stays below 0.04% which is really good:

What is important to highlight is the fact that since this is a DHP in reality and is triode-wired, the H2 and H3 levels are not strictly speaking performing as a pure triode. You get about same levels for large voltage swings. For example at 2Vrms I measured the H2@ -62dB whilst the H3 at -64.8dB.

Sound-wise, I’m surprised with this preamplifier  It’s great.  The microphonic noise is very low, only you can hear it when you tap the valve or when it heats up you will get some metal expansion dings here and there. Once warm (it can get to 105°C measured) it provides the most delicate DHT sound. The dynamics and tone are superb. I can now get some high-treble detail that I lost with my 26 preamplifier. I simply love it. I’m burning in a matched pair of Svetlana NOS 1973 production which my friend Vyacheslav sent me from Ukraine. These are fantastic.

I’m still looking forward to listen this pre-amp for many hours before making further observations. I will also take further measurements when taken back to the workshop.

Now, I will enjoy the 4P1L stage for some time…

Ale