Introduction

This was one of my first DHT preamps. I found a quartet of NOS CX-112a Cunningham (globe) back in Buenos Aires many years ago and built one of my first DHT preamps. I loved it. I played with it before I moved into the 26 and then started the long exploratory journey with DHTs. 

The CX-112a can be easily fit in an existing 01a preamp. Take a look at what Thomas Mayer recently blogged about this valve, worth reading it. 

Well, you can get more current drive than 01a (nearly double) but no thoriated tungsten filaments. Anyhow, the gain is slightly higher but is very easy to adapt to my gyrator-based circuit, that I couldn’t resist to take the quartet out of my valve stash and make them sing again after so many years.

The circuit

Again, an easy drop in without modifications. If you want to learn more about this circuit then I suggest you read the 01a blog posts. In summary, here it is:

I used my existing 01a, however I tried newer PCB boards with the IXTP08N100D top MOSFET as well as slight variation of the components as shown int the diagram. I like the fact I can drive it at 8mA, better Slew Rate and deeper bass as the BF862 transconductance is higher. 

Performance

This is an outstanding preamp. I have to admit that I was even impressed with the bass, closer to the VT-25. It has good dynamics but probably the level of detail not as close as the 01a. I think it’s really good and is worth listening for some time. I will leave it running as I enjoy it very much. 

The measured performance is great. Let’s have a look at the frequency response:

Flat response from 3Hz up to 700kHz. High-bandwidth, clear treble and strong bass. A real contender amongst the DHTs

Distortion can be as low as 0.005% for 1V output to 0.11% for 10V swing. The harmonics haven’t  got the best cadence when at full tilt, however, at normal levels they are great:

I don’t know how easy are the CX-112a to get these days, but certainly they are worth using. The sound of the preamp stage is great. Detailed and with strong bass and clear treble. I feel at this early stage is a good contender to the 01a as well as the VT-25.

i will play it for a while. Boards and preamp overall needs some break-in time. 

Cheers

Ale

Well, it was just a question of time to make an update to the VT-25 DHT preamp. I wanted to try the BSH111BK as lower FET as shown below:

The result is visible on the frequency response. It does have an improved FR and the bandwidth gets up to 800kHz on same testing conditions. If you increase the HT to 280-300V you can increase the anode current to 30mA which should be good as well. 

I love the sound of this stage driving the 4P1L PSE output stage. It has a deep bass as well as clear treble. Dynamics on the system are fantastic. 

The bass on my system is fantastic now. Stronger than with previous setup. That is what you get when you marry low ra (4P1L pair) with a high-quality OPT.

I have a pair of pre-soldered BSH111BK boards (which are painful to solder) so I will give them away on a first come first serve basis

Weather is really nice today in London, so I’m going out right now…

I’ve been working on repairing  a fantastic L3-3 valve tester. It arrived safely, albeit not working the anode regulation circuit. I replaced the pots for either 10T (for grid regulation) and the screen and anode pots (1M). The regulation of the anode isn’t working, so after some time of a frustrating troubleshooting, I decided to open a nice Ale and listen to Herbie Hancock on LP. Happy Friday!

Happy Easter to all! (whatever you celebrate, doesn’t matter, it’s always good to have some days off)

I have my preferred gyrator setup which includes a top (depletion) MOSFET IXTP08N100D, which has a unique high VGS(th) which helps improving the performance of the bottom FET, in my case the BSH111BK. The combination of both is superb and they do measure (and sound) superb. The frequency response is flat until 3.4Mhz (-3dB). Yes, a high bandwidth amplifier, so you need to be mindful of this when using high gm/gain valves. I read somewhere people complaining that gyrator “oscillate”. Well they don’t, however they create a high bandwidth amplifier which is therefore prone to oscillate if you don’t take the right measures. If you don’t know what you’re doing, it will oscillate for sure, you have been warned.

Ok, if you can’t get hold of (any) depletion MOSFET as the top device, there is an option, a la Gary Pimms.

The circuit can be tweaked slightly, as can be hacked the PCB (I can show you how if you’re intending to use this circuit)

Here is the design:

The main difference is that D4 provides a stable reference voltage (18V) which ones you subtract the VGS(th) of the top MOSFET (typically 2-5V) then will give you enough headroom to allow the bottom FET to operate under low output capacitance due to higher VDS. This is the common limitation of the cascoded pair of depletion devices. You can’t get more than 2-3V.  As the top device forms a “cascode” with the bottom, it also limits the maximum voltage possible to the drain of the bottom device. The protection zener of the bottom device can be removed to ensure maximum swing. This stage can do 20Vpp easily. C5 provides some filtering to the zener noise, which is very low. I can’t see an issue at the driving levels in place. 

The protection zener (D2) for the top device is needed unless the MOSFET comes with a pair of back to back as some do.

There are multiple options for the top MOSFET. I like the (nearly EOL) STP3NK60ZFP which is a FP TO-220 device, very handy for heatsinks and high voltage and comes with the bonus of the protection zeners. The best option is the AOT1N60 and also the easier to get hold off FQPF2N60C. 

So, the performance is great. You can get flat response up to 2.1 Mhz. Here is a snapshot with my buffer which limits to 1.5Mhz:

However, my prefered stage can do 3.4Mhz under same conditions!

New teflon sockets (UX-4 and UX-5) arrived from Jakeband. These are great quality and I’m looking forward to using them on my next projects. Stay tuned…

Well, the VT-25 preamp now is part of my main system. I modified the 01a preamp to turn it into VT-25. Now the mule is ready for further adventures. I can look into my list of other valves for exploration with the gyrator design

Watch this space, there is more to come soon…

I evolved my previous design here, thanks to the help of Rod Coleman and fruitful discussions with him.

There is an option to improve the design by bootstrapping the top MOSFET to avoid using a bias Zener and allow the bottom device to have a constant VDS. This can be achieved by double bootstrapping the FETs. Here it goes:

Similar design as before. Only difference is that R7 is used to create the bias of T3, and thanks to the bootstrap of C2, the bottom FET (T4) now operates freely regardless the swing. D1 is needed to protect T4. R7need to be adjusted considering the output voltage expected as well as the maximum VDS before D1 starts to conduct.

There is an stability challenge and it can be addressed as Rod Coleman clearly points it out, a “guard ring” :

The other pro trick is the guard ring: this will dramatically reduce problems of dc-drift, if the PCB surface gets contaminated, e.g. when soldered with some old or poor-quality solder. Or damp air, fumes etc. It’s a conductor (pcb trace) around the high-impedance network formed by the 10M resistors. A staggered-pinout version of the TO220 is needed to implement it, as the TO220 is the hotspot for leakage (B+ of drain to the 10MΩ-driven gate!).

If there is a leakage path, it leads only to the guard ring, which is only a few volts away from the intended bias – rather than if the leakage can reach ground or B+, which would drive the circuit crazy. Connect the guard to a low-Z source – the Output in this case.

 

Anyway – I hope it is useful in some way!

(Rod Coleman)

Not bad at all with 3MHz bandwidth. However, considering the circuit complexity, I much rather stick to the depletion version which performs much better in my view:

The VT-25 DHT Preamp is now part of my system as I do like it a lot. Therefore the Mule was vacant for a new experiment. It was the time for the 3B7 valve. This was introduced to me by my friend Paul, who has implemented it using the gyrator and is very happy with it. The 3B7 has a pair of triodes in the same bottle. I wired them in parallel as well as the filaments, which have same current specifications as the 01a. 

Most of the mid-mu DHTs tend to be microphonic. This one is an exception, although some singing comes out of them, its very minor and not impacting the stage. 

Here is my implementation:

I’m settled with the IXT08N100D and the BSH111BK. I have some daugther boards for the BSH111BK now. I also purchased the BSN20BK which has higher gm, haven’t tried them yet. 

 

Here is an interested picture of the frequency response:

 

 

ON both channels there is a slight dip coming closer to 10kHz. Less noticeable on one channel. I have some military JAN Sylvania boxed 3B7. Haven’t tested them all, but perhaps it’s my batch, I don’t know

Build

Well, a nice beer is needed to help out the build stage. What best than trying this cask aged Imperial Stout from Brewdog:

The build was fast as I said, that is the beauty of the Mule. I created a board to mount the Loctal sockets into the main  wood chassis, that will allow me to exchange different sockets in the future:

The sound?

The Mule saga continues and it was time to modify the 3B7 preamp and to test the 2P29L valve. This was quite easy as they both have loctal sockets. I had to modify the Rod Coleman filament regulator to set the filament current down to 120mA. Then a bit of wiring work, and in less than an hour a new preamp was ready. Job done, this is why I built the Mule:

The circuit is quite close to my original design time ago. I modified the filament resistor to use an existing Russian NOS wirewound part I had in stock and suit this preamp quite well. Added grid and screen stoppers as well:

The rest stays the same. The gyrator board is based around the BSH111BK and the top MOSFET is IXTP08N100D (with its own heatsink). 

I have a series of daughter PCB boards to adapt the BSH111BK to the gyrator PCB. I will be offering these shortly as pre soldered boards with the gyrator.

The 2P29L is run hot at is nearly maximum Pa. Well, I’ve got plenty of these so I won’t worry about it. It sounds better at 20mA in my opinion. If you can lower the HT a bit may help with power dissipation of the top MOSFET. I’d go for 200V to provide enough headroom.

Frequency response is really good, mind you there is no 100K load (I forgot to add it on my test) as in normal operation. When loaded it will reduce the response. However, this is really good. Gain about 18dB is as expected and similar to other DHTs I use. Enough to feed the output stage (4P1L PSE). 

Distortion is about 0.045% for 5.5Vrms and 0.12% for 11Vrms output.

How does it sound?

 Well, another great DHT in my opinion. Compared to the recent version of the 3B7, the 3B7 lacks of some warmth and or strong body sound I think. Hard to explain, I liked it but not as much as the other DHTs I have been using. This one has a powerful bass and clear treble. No coloration, great sound overall.

I wasn’t wrong when I said this was a good DHT to explore. Microphonic noise is minimal, surprising. No need of special arrangements. 

High gain stage with DHT

Some time ago a colleague  (Shawn Fox) contacted me to find out whether I could test some rare high-mu DHTs. I didn’t have them in my stash, so he offered to send them across for testing. He was quite keen to find out the performance with a gyrator load due to the particular characteristics of the DHT in question. The valve in question is the CX-340. There isn’t much information about this valve am afraid and coincidentally, Thomas Mayer (Vinyl Savor) wrote not long ago a review of this valve.

Tracing the curves, the first step

The high anode resistance as well as the low anode current in which this valve operates makes it a real challenge to implement successfully. Hence, this is why the gyrator load plus an output follower stage comes into play as the best companion for this valve. Before we look into the circuit itself, I submitted the 40 valve to the mercy of my tracer:

A very linear triode indeed. The specs between the pair of valves varied quite a bit, this was expected anyway.  Here is the best sample out of the pair:

With a mu of about 30-32 and a transconductance of 200 μmhos,  the anode resistance is about 135-150kΩ. A real challenge for either a resistor load or a choke/transformer. 

A Spice cx340 spice model was easily developed:

The circuit

If you want to use a valve of this type in a RIAA stage, then a circuit like the below is the best option as the gain stage:

I already shown the SiC MOSFET follower stage on this post. The gyrator is well known, so won’t cover it at all.  The filament bias introduces degeneration which helps with improving the linearity of the valve (not needed) and increases the overall impedance presented by the valve. As the stage is a mu-follower, the output impedance is only determined by the J1 FET. This is ideal with a valve of this type. However, due to the low anode current, the output impedance is higher and we also need a follower to avoid slew rate issues and also increased distortion when the stage is loaded. This stage can be easily the 2nd RIAA stage as well as the output as it provides about 30dB of gain.

The DC-coupling of the valve to the follower is ideal. You need the high impedance of the SiC (or MOSFET) gate to improve linearity and increase the bandwidth of the stage, in particular with low current and high-anode resistance like this valve. In my practical implementation, I used a SiC MOSFET, here is the famous AOT1N60, which works just fine as well.

The penalty of increased anode resistance is that we need to increase C1. If you want to avoid this, you can opt for 2 or 3 SiC Schottky diodes (like the C3D02060) to reduce the dynamic impedance presented in the cathode (filament). 

Testing the stage

A new rat’s nest was built for this, and opened the way to other experiments which I hope to publish shortly.  Here is how it looks:

Ignore the bottom Gyrator PCB for now, it is a converted screen regulator for pentode tests. 

So how does it measure?

Really well in my view! You can get 3Hz-35kHz flat response at 30dB. Not bad at all for an DHT. I didn’t measure the miller capacitance, that is something to bear in mind for the RIAA stage driving this valve. 

The distortion as well as the harmonic profile is very nice. The CX-340 provides the expected decay of harmonics of a DHT. I measured distortion to be 0.022% @ 1Vrms output. 

What are the disadvantages? Well, microphonic noise as expected. You get some ringing at 200Hz as well as 2kHz. Probably with proper dampening and care this can be managed. I’m not sure of how it will really perform in a RIAA Stage.  Alternatively, the EML30A should be considered for a similar job.  Pricey but a great valve indeed. There has been an evolution here and Emission Labs has taken care of tackling many of the challenges of a high-mu DHT valve. 

Next time, I looked at alternative Russian directly-heated pentodes to play somehow a similar role and will share my findings.

The 2K2M is an unknown directly heated pentode from Russia with has variable mu. I have several pieces of these, and was wondering how useful they could be. This is part of my on-going saga of the “Mule“.

There is not much information around this valve. Here is the data sheet and here is in Russian:

It is also interesting to read this “confidential” report from the CIA about this valve. 

The filaments have low requirements: 2V and 60mA. The maximum anode voltage is 120V and transconductance is about 1mA/V. Anode current is about 2mA. The internal resistance in pentode mode is about 1MΩ .

The first test I conducted was in pentode mode, albeit being vari-mu, you should expect a high distortion, H2 in particular:

The circuit is simple. A gyrator load with a load resistor (RA) as explained on this post. The screen regulator is actually a gyrator PCB modified with the addition of a current limiter transistor. The output follower is added given the high anode resistance and limited current drive capability. This was already covered in the previous post here. 

So, a simple test was made with this valve in pentode:

The minor dip at 1.5kHz wasn’t noticeable in other sample valves. Either way, you can see a nice 56kHz bandwidth for 32dB gain. The gain is set by RA, 68kΩ in our case and the transconductance. 

Let’s see the distortion profile:

As expected with a vari-mu valve, the H2 is high. We get 0.33% for 1V, which ramps up to 0.65% for 2Vrms. If you would like to use it as second stage in a RIAA amp, then probably it will add too much H2 distortion in my opinion. 

Now, for a guitar FX stage, this may be an interesting valve in pentode mode.

Well, what about triode?

We always need to experiment and see if we can get better performance out of them. Let’s see.

Wired as triode, the gain is about 16-17. Very nice for a DHT:

A lovely 180kHz bandwidth for nearly 24.5dB. Not bad at all!

Well, let’s see the distortion in triode mode then:

Which is very low 0.0081% for 1Vrms output, so for 10V is still below 0.05% which is outstanding:

the harmonic profile is what we want. Hey, I think we have another candidate for the Mule. I should wire it up and listen to it, hopefully soon.

Some years ago my friend Paolo brought up this lovely valve which looking at the curves seemed to be a great candidate for a driver which required large voltage swing. This indeed is needed for 300B, 211/GM70, 813 SE Amps. I build it and tested with the 300B, which is great. 

The immediate challenge (and probably the reason why it hasn’t been used extensively in the past) is that has a high anode resistance as well as very low current capability. These 2 things are of course a killer for driving these demanding output valves. However, with a gyrator load and a follower stage, the reality is different

The circuit is dead simple. The triode is biased with SiC Schottky to minimise the impact on the reflected anode impedance due to their low dynamic resistance. A pair is sufficient to set the bias to provide large voltage swing. The triode is very linear so no need to add degeneration in the cathode by replacing these SiC diodes with a resistor instead. You can do that if you want. 

The output load is minimised by adding the SiC follower which presents a light load to the delicate 6SF5 whilst providing the current to handle any grid of either a 300B or any transmitting valve. In order to add the grid bias to the output stage, this one is DC-coupled to the SiC FET and instead we add a 100nF capacitor between the gyrator mu output and the gate stopper (R10). Here we insert a 1-4M7 resistor with the bias voltage. The T5 and R12 instead of returning to ground they do return to the negative voltage allowing the swing needed. That will vary depending of the output valve. 

I hope this is clear enough, the above circuit represents the test rig, not the output stage.

With a mu of 100, be prepared to have a source which can handle a 250pF capacitance due to the Miller effect on this triode. Not impossible, but worth considering when building this stage.

Here is a sample 6SF5GT I used:

The circuit behaves as expected:The

The distortion is very good. At 52Vrms (circa 150Vpp) you get 0.35%. very nice indeed.

Here is the frequency response test, which is the cherry of the cake!

With 40dB of gain you can get a bandwidth of nearly 50kHz. This has sufficient gain to drive any demanding valve to full output without any preamp when driven from a CD player or DAC. 

Very simple circuit which performs great. I will have to test it the GM70/813 next.

Some 3 years ago I built the famous “Starlight” discrete DAC and CD transport, the DIY version from Sonic Illusions.  This is, in my opinion, one of the best DACs I’ve every listened to.

One of the disadvantages of the DAC output is the high (10kΩ) resistance and AC coupling requirement (unless you are prepared to balance DC, etc.) The traditional output configuration is an output capacitor feeding a step-down transformer to provide lower output impedance. The DAC can do only 1Vrms maximum due to the TTL output logic. With a 4:1 output I had to date, I wasn’t able to drive properly my VT25/10Y preamp into the 4P1L PSE amplifier. 

I decided to try a 1:1 output transformer but I had a clear challenge. The big primary inductance (Lp) needed demands a careful design and construction of the transformer to minimise the leakage capacitance and inductance which will impact the HF response. As I’m driving a long cable into the DHT preamp, this presents a demanding load of 47kΩ||400-600pF.  

I contacted DvB transformers to order a custom transformer to fit my needs:

With this configuration I could get nearly 2dB of loss and a good response above 20kHz. I asked Dorin for a minimum Lp of 200H, and Dorin sent me a first prototype of 307H.  

DvB used for this transformer the highest grade mumetal-permalloy available aiming for lowest THD under 200Hz/2Vrms and finest magnet wire produced by Elektrisola. Computerised non-bifilar hybrid winding technique, involving high interleaving rate and organic materials for isolation, in order to achieve the lowest leakage impedances I needed for this transformer.

So I submitted the transformer to some tests before replacing them on the DAC:

The initial results were very encouraging. Without the capacitance load this transformer can do 150kHz but when loaded properly, the response is like this:

The LF pole is about 2Hz due to the high inductance of the primary. The HF rolls off about 26.5kHz. Ideally should aim closer to 30kHz, which is something we will look doing by reducing the primary inductance and therefore reducing the leakage impedances from both primary and secondary windings. 

How does it sound?

I have these transformers running for a couple of weeks now. The sound of the DAC is very clear and detailed. I have now the gain needed in my system, so I can drive effectively the amp. These are a pair of amazing handcrafted transformers. I will review further prototypes as DvB have them available.

Time ago I wrote about this sterling Russian valve. It’s extremely linear in triode mode, sounds superb and isn’t microphonic. My friend Paul LeClerq has used it as first stage of his guitar amplifier and is delighted. A real dormer one. I hope it doesn’t disappear when valve hoarders go out and grab every big lot of valve that exists. Anyhow, I have more than I need myself, so I’m not worried.

The triode driver

Well, the easiest implementation is the triode driver. In triode mode this valve has a μ of about 50. So you should expect a maximum gain of 33-36dB. Very useful in phono stages (2nd stage), or as a driver for high demanding output stage (i.e. transmitting valves). This lady can swing 150Vpp at only 0.13%! The penalty of course is the Miller capacitance.

Here are two snapshots of their frequency response using the gyrator load:

The above test is with a BF862 FET. Let me clarify that I have added a 1μF as C1 in the board as I was intending to do tests in pentode mode (as you will read later below).  If you use instead the new Rev07 PCB which can fit BSH111BK FETs instead, then you will get a bit more bandwidth as the transconductance of this FET is higher than the BF862 at same anode current:

 This is fantastic. However in triode mode it can’t be used in a phono stage. I haven’t measured the Miller capacitance of this valve, but I’d expect the resulting input capacitance to be higher than 200pF. 

The Pentode Driver

Ok, now you want to use a pentode driver in a RIAA stage. Here’s the testing rig I’ve used for this valve:

Actually C1 is 1μF as I mentioned before. I conducted a series of tests with different gain setups by simply changing RA1:

The 68KΩ resistor provides a gain of gm⋅RA1 or about 52.3dB for a degenerated gm of about 6.2mA/V. Without the SiC Mosfet follower output buffer you get a bandwidth up to 87kHz. Very good indeeed. If you want to extend this a bit further, you can add the follower to get up to 100kHz.

There is a dip about 2kHz which I can’t explain why. Perhaps is the valve itself, as you can notice some bump on the triode mode test. I should re-run the tests with other samples to diagnose where this is coming from. 

You can bump up the gain by increasing the value of Ra1.  Here is the response of a 100KΩ resistor:

You can get 54.5dB but HF pole is impacted slightly down to 68kHz without the buffer. 

Now, let’s see the performance of a 60dB stage. You will need to increase Ra1 to 270KΩ: 

You can see that at this gain level the response is limited further down to 38kHz. The dip is a bit more evident with about 1dB difference. Yet, this stage is fantastic at 60dB gain with still enough bandwidth to perform well in a RIAA stage. The input capacitance of this valve is less than 10pF (close to 8pF) in pentode mode. So it can be used with MC cartridges.

If you add the buffer you can increase the bandwidth to 45kHz.

The distortion in pentode mode is high. However, for lower signal levels is good enough. As a reference see the output of 2Vrms:

The disctinct H3 signature of a pentode is present. At lower levels like 500mV in a fist stage of the phono preamp we should expect THD to be around 0.15% maximum. I think it would do a good job as first stage. 

Back in 2012 I did some experiments with this Russian valve.  The 2Ж27Л / 2Z27L is a fantastic valve. I discovered that if I run it hot, it’s extremely linear and non-microphonic. Bingo! With its highish μ of about 16, it’s ideal as preamp stage or driver. 

Here is my implementation for an DHT preamp (as usual with the gyrator load):

This is very similar to the 2P29L which is described here. You only need to change a resistor on the Rod Coleman regulators and you’re done. It took me 15min to do the upgrade, I wish a mod in a preamp could be done every time like this. The loctal pinout is the same. The anode voltage is reduced to about 116-120V to push the valve to the maximum cathode current which is about 4mA. 

The result is great, check out the distortion profile:

Distortion is mainly H2 and below 0.03% for a 10Vrms output. This is very nice.

When you tap hard the valve you get some microphonic noise at about 2.5kHz. Surprisingly, when I built it on my Mule preamp, no noise is picked up! It’s extremely quite for a 24dB DHT stage! Nice discovery.

The frequency response is good, see below:

Flat from 5Hz to 170kHz (-3dB points). Expect worse response when fully loaded. The only disadvantage of this valve is the 4mA maximum current. If you need, you will have to add a FET follower at the output to drive more demanding capacitive loads. 

How does it sound?

I will have to get back to this a couple of weeks later when I manage to spend some proper time to listen to the stage. The initial impressions are (after listening Freddie Hubbard’s Open Sesame record for some time:

1. Treble is very clear and sharp
2. Tone is very detailed and clear – I like this valve
3. Bass not as powerful as a VT-25/10Y, well what can you expect with only 4mA through the BSH111BK FET

What a great valve…

Will report back in some weeks.

The return of the Siberian

After trying out so many DHTs and pre-amplifiers, I decided to wire up my 4P1L preamplifier Gen3 and fit the gyrator board to drive my 4P1L PSE Amplifier.  

I have a pair of 4P1L/4П1Л dated 1968 which are properly burnt in. I’ve used them lately in my previous preamp incarnation with great results. 

The circuit doesn’t need explanation, I think I’ve covered this repeatedly for a long time.  I will only point out the differences:

The main change was fitting a pair of Russian wirewound 27Ω resistors in parallel to get closer to the 15Ω used in this position. I found these Russian wirewound resistors to sound extremely well as filament bias resistors. I tend to be skeptical about the “sound” of some components in circuits, however, they do make a big impact in the cathode of a filament bias arrangement. 

The gyrator has my preferred combination: IXTP08N100D and BSH111BK. I have now an upgraded PCB Rev07 which fits the BSH111BK and similar FET and I will offer them shortly. 

The latter benefits from the 30mA idle current. The result is lower output impedance whilst providing a great frequency response overall.

M3 needs a proper heatsink, it does get hot with about 2W of dissipation. 

How does it perform?

Well, this valve has the reputation of amazing performance and low distortion. The gyrator setup provides the best out of this valve in my view. You can get a flat response as well as great bandwidht from 10Hz up to 3MHz loaded with 100kΩ:

The distortion is very low and is lower than 0.05% below 10Vrms. Dominant H2 with a lovely harmonic profile characteristic of this valve. 

How does it sounds?

i’ve been listening and using this valve extensively since 2011. I have to say that it sounds amazing. I never get tired of its sounds. Before I listened to a 4P1L-4P1L system and found a slight edge on the sound (probably due to its H3 component) which I don’t hear on my system. The drive, clarity and tone is amazing. It can drive the 4P1L PSE perfectly well and you get a strong and clear bass. Very powerful. My +600 hours 4P1L are very quiet in this setup, no microphonic noise. I don’t have even dampers in the 4P1L sockets!

Anyway, if you need 19dB (x9) gain in your system or you need a driver for your  SE amp, then this is the valve to go. I Still can be found cheaply and is a great contender to the thoriated tungsten filament DHTs like 01a and VT-25. 

Build this one and enjoy!

The gyrator PCB has been updated to fit now a wider variety of lower enhancement MOSFETs with low capacitance and high transconductance. The best examples are the BSH111BK and BSN20BK which are great options for currents above 25mA:

The board offers now all the flexibility needed in terms of different TO-92 and SOT-23 package pin-outs to use whatever FET you want.

I’ve been posting about the use of source followers in the circuits with some interesting results from testing. Some interesting posts to read, If you haven’t read them so far:

1. Slew Rate, Slew Rate (Part 2), Slew Rate (Part 3) and Slew Rate (Part 4).
2. 6SF5 driver for 300B/GM70/813 SE Amps
3. DHT Phono Stage Test. 

After several tests over a variety of circuits, I finalised the prototype for a Source Follower PCB. The circuit is incredibly useful. Some examples of uses cases are:

• Amplifier output stage grid drive
• Screen drive amplifiers
• Screen voltage stabiliser for pentode stages
• HT voltage stabiliser for preamps
• Buffer stage for high-mu/high-anode resistance stages – either triodes or pentodes (e.g. Phono)

Some key aspects of the board are:

• The PCB has been designed to accommodate all sorts of power MOSFETs (both TO-220 and TO-247), in particular the high transconductance and low Crss ones which perform the best in this role.
• The tail CCS is simple and leverage the option of using same MOSFETs.   
• The board takes into account the use of any bipolar supply up to 450V diferential. You only need to change a resistor depending on the supply voltage levels and make sure there is a sufficiently big heatsink on the MOSFETs.
• There is a current limiter circuit built in to protect screen or grid from excessive current. This is also very useful when the board is used as a voltage stabiliser for a preamp. You can limit the peak current and avoid destroying the MOSFETs when capacitors are charged or if accidentally the output is shorted. This circuit can be bypassed easily with a jumper.

Here is one of the boards submitted to the usual abuse during testing:

This is a very useful PCB in my view which can be used extensively in preamps, line stages and amplifiers.

If there is sufficient interest, I will run a batch of PCBs for the DIY audio community:

I previously implemented a preamp with the UV-201a. These are very old globe valves, somehow fragile and hard to get in good shape. Despite all this, it’s a superb valve. I have managed to acquire a decent set of them to pair the best valves to use in my preamp.

Recently I developed a prototype PCB for the source follower circuit. The source follower is ideal to place at the output of this preamp due to its low driving current. My 4P1L PSE amplifier will be pleased with more current to pump the Miller capacitance effectively. 

Here is the updated diagram for this preamp:

No major changes on the preamp itself.  Just some component adjustment due to availability and preference.  I’ve posted several times around this design using the gyrator PCB, you can read the blog if you want. The top MOSFET is IXTP08N100D and the lower jFET is BF862 which are my favourite combination for lower current (i.e. <20mA). The filament bias is formed by a pair of NOS Russian wirewound  51Ω resistors in parallel. Rod Coleman regulators V7 set to 200mA. 

The addition of the source follower is very simple. The follower is DC-coupled to the SiC MOSFET (C3M0280090D). The back to back Zener diodes D4 and D5 protects the gate of the SiC. You can use your MOSFET of choice, ideally with low Crss and high gm.  The SiC follower has a tail CCS formed by M5 and T1. The CCS current is set by R13. In this case is 10mA, however it’d be better to dial it up to 20mA. Penalty is the power dissipated across M4 and M5. Both need their heatsinks. With 10mA, you just need a clip-on heatsink for M5. 

If you want, you can use any other FET of your choice. The PCB I designed includes both TO-247 or TO-220 pads for the top device and also you have back to back protection zener pads for the lower MOSFET in case it doesn’t have them built in on the same package.

The output coupling capacitor C2 is now connected instead at the output of the source follower. 

Finally, C4 may be needed for HF decoupling and avoid oscillations which I experience with my variable HT bench supply. 

The ground lift circuit was inherited from previous build and has a back to back Schottky power diodes which aren’t in the diagram. Not mandatory but useful to have if you have ground loops. In my case, I don’t need it as layout and grounding has been thought through carefully.

Prototype build

Here is another ugly breadboard for you:

Measurements

As usual, I conducted the measurements to check the preamp before the listening tests just to spot out any issues:

On my first test I found a poor FR due to a leakage cap in one Rod Coleman reg I damaged during careless testing. Gladly I replaced it and got the preamp working perfectly well. You can see the good response of this stage from 12Hz to about 400kHz loaded at 100KΩ.  Now looking at the harmonic profile we can see:

The distortion is really low. More importantly, the even harmonic profile is nice with suppression of H3. At 10Vrms output THD is below 0.02%. Nice, but this doesn’t say much more until we listen to this stage.

Listening

Well, the most valuable part. After playing several weeks with the same configuration (4P1L into 4P1L), it was pleasing to get back the  01a. My amp has only one stage of 4P1L PSE and despite being manageable well with a 01a at 3mA in terms of slew rate, I can notice a nice improvement in the overall detail at the treble side. I already praised and enjoy Tony’s 01a preamp which has a simpler follower stage (not this one). It’s revealing to see that the addition of a well implemented source follower stage has no noticeable detriment in the sound. In fact, the 01a works in better conditions which allows it to perform at its best when loaded with a more demanding capacitive load (due to Miller effect).

I will continue to listen to this stage for a while to give firmer impressions. 

Some time ago, I did some initial experiments with the 4P1L (4П1Л) with the screen performing as anode instead. Some DIYers claim the improved sound of the mesh type anodes. Kees Brakenhoff kindly sent me some PL519 to test in screen mode. He has done multiple builds with this mode of operation with great results. Unfortunately I’ve not had the chance yet to build such an amp.

What I could do instead though, was to mod very quickly my 4П1Л preamp to screen mode. It was a very easy and fast modification. I kept the same heating wiring and just adjusted the screen (anode) current down to 10mA:

The 10mA operating point keeps the screen dissipation within its limit. I’d probably use a BF862 here instead of the BSH111BK, but I reused the existing PCB gyrator board for simplicity.

Some measurements

Before listening to the preamp, I conducted a series of basic tests on the preamp:

Due to its lower quiescent current, the transconductance of the jFET is lower and so it’s it bootstrapping resulting in a slightly worse HF response compared to the 4P1L Siberian preamp. Still is outstanding. You also get a slightly better gain of about 1dB close to 20dB, instead of 19dB. 

Distortion profile is very good:

It has the characteristic harmonic footprint of the 4P1L with a predominant H2 and cascaded H3 and H4. A very low distortion indeed.

How does it sound?

I played it for about a couple of hours yesterday. It sounds as good as the 4P1L I think. Unfortunately I modded my 4P1L preamp for this so can’t do a side by side comparison.