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.

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Sweep buffer

I’ve been using my sound card and Pete Millett’s interface for testing. However, the limitation on high frequency response is due to the sound card. A cheap, but yet effective option, is to use the Chinese digital oscilloscope Instrustar ISDS2062B which comes with built-in DDS signal generator with sweep capability. With this little piece of technology, you can sweep up to 10Mhz. The resolution is 12bit, not great for FFT but good enough for a FR analysis. For frequencies above 20kHz, you can use this device to look into things. For FFT and THD analysis, I will keep using the sound card and audio interface as usual.

When I test the device, I found that the DDS output didn’t have the stones to drive loads at HF. Therefore I went back to my workshop and built the following sweep driver. This was based on the great SSM2019 that my friend Mogens sent me:

The circuit is very simple. It’s operated from a pair of 9V batteries. So far, I’ve tested with the 20dB (actually 19dB gain due to 1kΩ resistor at hand) and 0dB gain modes.

The response is good enough for my purposes:

I can get 1.5Mhz @ 19dB or 3Mhz @ 0dB gain HF response which is great. I now need to test it again with a real load.

801a DHT Triode Spice Model

My friend Bela, who is an experienced DHT user and very fond of the 801a, asked me if I could help in developing an LTSpice model for the venerable 801a. I have several 10/10Y but unfortunately no 801a at hand to trace so I used the data sheet curves which are a bit challenging due to resolution/scaling of the characteristic curves:

I had to compromise the model fit as if you want accuracy in A1 region the matching is less than perfect in A2. Still, the model is not perfect but is very good for simulation purposes.

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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:

Slew Rate (Part IV) and the Tale of the three Source Followers

Some of you may be a bit fed up already with these slew rate posts, however I find this fascinating as is taking me through different routes of experimentation.

On my last tests, I abused the DN2540 to an extent which meant the dead of it. So I ended up adding the appropriate back to back protection zeners on the gate:

Depletion FEET with protection zeners — Depletion FET with protection zeners

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Slew Rate (Part III)

Introduction

Are you writing again about the “Slew” thing? There is a lot to be said about the slew rate still. In fact, this post is about some real life measurements and experiences had in the past few days.

This week I went to my friend’s (Tony) place to listen to his 01a with the additional follower discussed previously. Tony complained about that the addition of the follower changed the sound of the 01a. Something was lost, detail on the treble, airiness. I brought up again my test gear and surprisingly we found some H3 level which wasn’t there on the previous preamp. Odd, could that be the FET follower? In the end after several tests, I checked the operating point (you should never assume they should be as expected) and found that the follower wasn’t operating at the optimal level, source current was low and also the 01a was running out of place. I recalibrated the gyrator and voila. The sound of the 01a, came back!

Unfortunately we ran out of time and I did not take any measurements again after re-calibration.

The H3 bug

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Flexible HT Power Supply (Part IV)

It was time to submit the power supply to a quick test:

I used a 1:1 transformer to measure the supply noise using this jig I built time ago. The test load was a simple pair of 4K7Ω 50W clad resistors. All wiring was done with alligator clips so induced hum was obviously picked up.

The 50Hz artefact is induced noise from the chokes and circuit being close enough and no shielding was in place for any of the measurement cables. The 100Hz and other byproducts of the rectification is really low below 3mV The overall inband noise is just above 50mV which is mainly the induced noise of the supply being so close to the test load.

So far, so good.

Flexible HT Power Supply (Part III)

After some further efforts, I managed to complete the power supply. And it works, which is always rewarding when you get it right first time:

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6Z49P-DR/6J49P-DR Pentode

A forgotten Russian pentode

There’s not much information around the 6Ж49П-ДР or 6Z49P-DR / 6J49P-DR pentode. This pentode was designed for high-frequency operation and durability. It has relatively high transconductance and high mu when is triode connected. Hence, an interesting device for a phono stage given the low capacitances in place as well. The 6Z49P is close to his brothers 6Z51P and the famous 6Z52P albeit it has different characteristics.

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