CX371a / 71a DHT Preamp

More than 4 years ago I ran a lovely 71a preamp which sounded amazing. I used it for some time and enjoy its sound up until I continued with my exploration around DHT preamps. Recently I was asked about how to implement this lovely valve again.

The CX371a / 71a valve is a great candidate for a line stage with its low mu and anode resistance. In my experience you have to run it above 20mA and over 100V to get the best out of this valve:

The implementation of this preamp is dead simple and a few components are needed on top of the gyrator PCB:

I haven’t starved the filaments as I found this valve not to be microphonic. If you have an 01a preamp you can modify it slightly. The interesting thing is that you can run it with just 180V. Even 150V should work and you need 25mA on each channel. A J310 or BF862 lower JFET device will work fine and you will need a heatsink for the top device (e.g. DN2540). Filament resistor is anything close to 50Ω. I used some 51Ω Russian NOS wire wound resistors, but any combination will be fine.

Enjoy

Ale

Russian PSE in Steroids (01a into 4P1L) – Part IV

More power

Our previous west meets east circuit can be improve further. In fact, a compromise made with the filament bias design is that coupling between driver (FET follower) and the output stage wasn’t DC. We want DC coupling to get best performance, to ensure we can drive well the output stage and provide sufficient grid current even when not operating in A2. This can be done with filament bias, however, since we are already introducing a negative supply, I’d prefer removing the filament bias and go for proper grid bias to get best performance of output stage in terms of maximum power and linearity.

The below circuit can be easily implemented with just few modifications from previous version:

What has changed here? Not much, the coupling cap C2 is now between the gyrator and the FET follower. The gate bias resistor R6 provides high impedance to the gyrator load to ensure maximum performance of the 01a driver (minimum distortion given size of load). Not as good as previous version, but good enough. The R6 is connected to a potentiometer which sets the bias voltage. The bias voltage is derived from V2, the -50V negative supply. You can see that this circuit will put more stress into the M1 FET as now there is an additional 25V of drop across it so power burned on this device increases.

The output of the follower is directly coupled (DC) to the output stage. The filament bias resistors are removed and we use the Coleman regulators directly on the filaments of the 4P1L.

This amplifier responds better to the grid current of the output stage once the output power goes over 3.5W. At 4.5W the distortion is just above 3% (3.2%) with a 3Vpp input signal. A tad more and you can get to the 5W and a bit more into A2 operation.

Russian PSE in Steroids (01a into 4P1L) – Part III

From Russia with Love

East-West Divide, Copyright by Justmeans

The interesting combination to explore from our previous designs is to mix some western valves like 01a into the Russian parade.

The result would be quite interesting, as the sound of the 01a has proven to be amazing. Therefore 01a driving 4P1L is possible as the 4P1L doesn’t need a lot of drive. Instead of using 4P1L as a driver, we can opt for the 01a which has a similar gain. What is interesting is that the voltage swing required by 4P1L wouldn’t force the 01a outside the zone in which is highly linear, hence, with some modifications, it can work as a great driver here.

The circuit

Instead of starving the filaments of the 01a, given the voltage swing requirements for a driver, we ought to drive it at full tilt. In the circuit above, the 01a hasn’t got the stones to drive the 4P1L pair, therefore we have added a cathode follower as explained here. The M1 follower will then drive easily the output stage.

Russian PSE in Steroids (6E5P into 4P1L) – Part II

Well, it was obvious I couldn’t leave last post as it was. There is an option to change the driver for a different valve. You can use a C3m (low gain in triode mode which is ideal here), a C3g, E180F/E280F. 6S45P or my loved 6E5P (or 6E6P) as the driver. Not longer a 100% DHT, but a nice option for sure. The 6E5P is extremely linear, good driver, with a nice gain (μ=30) in triode – perhaps more than enough for a 4P1L stage and would help in avoiding additional filament supplies.

The 6E5P has curves not dissimilar to the 4P1L as no further distortion cancelation can be seen. Here is the updated schematic if you’re interested in playing with:

Again, the gyrator PCB can be easily used to simplify the build of this amp. The 6E5P is not driven hard, but at a nice current of 20mA which makes the driver operate in a linear region (and with good sound) with just a pair of red LEDs. The nearly 30dB of gain will make this amp to be very sensitive. The 5W can be easily achieved with 1Vpp, so you will need to have an attenuator, no preamp needed clearly. The 6E5P will drive an 300B nicely here which needs the voltage gain, not like the 4P1L.

As you can see, there are plenty of option to try on this 4P1L PSE amplifier.

Russian PSE in steroids (4P1L into 4P1L)

Introduction: DHT madness

I’m not going to dwell on DHT sound. I’d rather say that if you’re looking for a stellar DHT candidate, the 4P1L beats them all. It’s dirty cheap, reliable and sounds amazing. You can go any route you like, it’s your own decision of course. However if you’re looking for a 100% DHT amp to build, here is an interesting example for your consideration.

I’ve tried 4P1L in many topologies. The advantage of its low filament requirements is that you can implement it in filament bias and simplify the circuit significantly.

The gyrator driver using the PCB I designed recently, can be used to avoid iron and have an excellent first stage and make this Russian Amp in steroids: 4P1L driving 4P1L.

One minor caveat around 4P1L in excess. I have found (as well as many others) that if you use too many 4P1L stages (e.g. 4P1L line stage driving a 4P1L-4P1L amp) then it will sound a bit harsh in the treble. i suspect this may be explained due to the H3 component level when triode-strapped. I’d rather limit the number of 4P1L stages to two. You’ve been warned.

The other great thing about the 4P1L is that is quite consistent between samples and easy to match pairs. Also in PSE mode you can drive it to full tilt with only 20Vrms and achieve up to 5W in class A1 with a pair of valves.

Continue reading “Russian PSE in steroids (4P1L into 4P1L)”

Gyrator PCB boards arrived!

Great pleasure this morning to receive the first batch of the gyrator PCB. After extensive testing we refined the layout and options for the PCB and now we have the final product ready for shipping! Unfortunately I will not be able to do some testing and shipping before end of February and I received a large number of requests already. I’m sure this batch will fly very quickly so please confirm your requests.

I will post soon the specifications and some circuit examples for this flexible gyrator board which can be used for DHT preamps (e.g. 4P1L, 01a, 26), amplifier drivers, A2 drivers, LTP drivers, parafeed output stages and more!

So check the For Sale section soon for more information.

Some days ago we discussed in DIYAudio using this board for a 2J27L preamp like this one:

The output FET follower is needed to for valves which have low current and high anode resistance which will struggle to drive large capacitances. The FET follower of your choice can be used instead.

A minimal circuit which sounds fantastic is the basic configuration of this PCB. With few changes this circuit can be used on many DHTs like 26, 4P1L, 10Y, 30, 30sp, 12, 71a, 45, 46, etc:

If you want to send me your requests please use ONLY the form below:

Gyrator PCB prototype

First sets of PCB arrived today. Thanks to Tom Browne for the great PCB work, and it worked first time round. The PCB can accommodate different FETs like 2SK170, BF862 and J310 to suit most of the requirements

If you’re interested in the final PCB let me know, production run shortly due to large number of request for this circuit for DHT preamps and drivers

Ale

Gyrator bias discussion

A very interesting point was raised on the 4P1L DIYAudio thread around the gyrator circuit using CCS and whether a simple resistor divider was better than the CCS due to the LND150 temperature drift.

I’ve tried both options and I’d say I prefer the CCS despite the variation with temperature for the following reasons (which people may well disagree):

It’s true the LND150 varies a lot with temperature (see attached), however if it’s operated at low current (e.g.<500μA) the variation is small. In a cascoded pair for this circuit the drift in the output voltage is small. Simulated in Spice I get about 6.35mV/°C. The resistor divider will be better of course but you need a smaller values to reduce impact of dR/dT. This creates another problem which is the reduced PSR. With a compromise divider to balance idle current and PSR you can get 5 times less variation with temperature in the circuit under discussion – see below (e.g. 1.4mV/°C)
For a smaller value of resistor divider the PSR is impacted and significantly lower than the CCS. If you don’t have a well filtered supply, the PSR benefits of the gyrator will be reduced due to this. For example, I did some quick comparisons by simulating my 01a preamp. I used a 235KΩ/220KΩ and a 23K5Ω/22KΩ divider options with a typical film decoupling cap of 4.7μF.

The PSR of the CCS is above 100dB whilst the PSR of the resistor divider goes from 56dB (235KΩ/220KΩ divider) down to 37dB (23K5Ω/22KΩ divider).
In practice, I implemented two different circuits as I had a shunt regulator before when I had a resistor divider and now I don’t have any shunt regulator but I use the CCS version.

Looking at the output PSR as the gyrator provides additional rejection to noise. The resistor divider PSR is about 73dB and CCS is 30dB better anyway

The voltage variance is really small with temperature and this circuit in particular isn’t affected by such small drift in my view

4P1L Siberian Gen1 upgrade

Some of the DIYAudio fans have built this version of the 4P1L with great success. There are several upgrades that can be easily implemented to improve this. I haven’t tried this myself, but my recent experience with the Gen3 and the 01 preamp gen2, I think are worth trying:

Replace the gyrator FET for a cascoded pair (M2 and M4 below) to improve PSR
Replace voltage reference by a cascoded LND150 for better HF and PSR response
Optimise the LF pole of the gyrator load by increasing R4 to 4.7 MΩ and reduce C1 to 220nF
Bias 4P1L to about 30mA. This will reduce distortion

Hope this is useful

Ale

01a Preamp (Gen2)

A full write up of this preamp design and build can be found here