2P29L – Preamp and driver for 4P1L PSE Amp

A very interesting Russian directly-heated pentode related to 4P1L is the 2P29L. It has a similar mu (μ=9), much higher anode resistance 2.8-3KΩ and transconductance of 3mA/V when triode-strapped. The filament requirements are much lower at 120mA. I picked one valve from my collection to submit it to the mercy of the curve tracer:

The triode curves are really nice:

This valve is as linear as the 4P1L (hooray). As a preamp it can be easily implemented like the 4P1L Gen2 preamp using a gyrator PCB which simplifies the building process:

Running it at 15mA and slightly above the recommended 160V achieve its lowest distortion.

We could also use this valve as a driver for a 4P1L preamp, which comes very handy for filament bias:

Filament Bias: a practical example with 3A5 DHT

Introduction

Recently I was asked about whether I could write on my blog about how to design a filament bias stage. My immediate answer was:

I don’t have much time these days am afraid to write extensive articles (and sometimes to even write-up at all)
Thomas Mayer has written about it (see here). Of course, I completely forgot that Thomas never completed his intended series of posts around filament bias, so I decided to attempt explaining the practical aspects of its design in this blog.

Before you continue reading this post, I suggest you read first Thomas’ article above and get yourself acquainted with DHTs and triode amplification. I’m not going to cover any of that theory which I will give it for granted that the reader is experienced with valve circuits and in particular with the hybrid mu-follower amplification stage with gyrator load.

3A5 DHT example

Continue reading “Filament Bias: a practical example with 3A5 DHT”

Rolling valves

I’m not a big fan of rolling valves. Perhaps it’s likely to do with the fact that I don’t have too much free time these days. However, I do look into burning in the valves. A noticeable change found on my 4P1L preamp after 350-400 hours use. The microphonic noise reduced to a minimum whilst the sound became more rounded. I added an electronic clock (LCD module) to the HT supply to monitor the exact number of hours between any changes I made to the system. It’s very handy. I’ve been running now the same pair of 4P1L valves for over 850 hours and they sound better than before. No further mechanical expansion noise is heard during warm up, something which was noticeable at the early days of use.

The second aspect I’m also in constant monitoring is the impact of the filament starvation in the 01a. Running them at 20% less current than expected is not recommended as the filaments will not operate at expected target temperature. However, after some years of joy, I’ve not seen any issues. I may remove and trace the pair of 01a in use currently to check their health.

Have fun!

DHT day

DHT in excess

We met yesterday at Andy Evan’s with our friend Tony for an interesting set of tests. Firstly we looked at measuring Andy’s 4P1L SE and PSE amplifiers:

4P1L SE LL1682: a great sounding amp overall which sounded as well as it measured. A must amplifier to listen to!
4P1L PSE (O’Netics OTs). This one particularly revealed an issue with the 4P1L driver configuration as it was running out of steam at about 2W before distortion creeped in. I think Andy will look into fixing this shortly. It also showed a slight dip above 10kHz up until 20kHz which may be attributed to the O’Netics.
4P1L PSE which I nicknamed the “Daemon” as it nearly screwed up my measurement gear due to some nasty grounding issues. We decided to give up on testing the response of this amp after this.

Andy’s speakers are Mark Audio Alpair 10s full range in some standing 23L cabinets. They do sound great with a solid bass and detailed treble. Perhaps a bit higher on the treble, but they are worth every penny.

We set the listening session to rotate the amps as well as the preamplifiers:

4P1L Siberian Gen 3
Andy’s 26 preamp, filament bias, LL1692 step down transformer and Rod Coleman regulators.
Tony’s 30sp with Rod Coleman regulators and depletion FET CCS loads
01a preamp Gen2

In my opinion, I think we can draw a conclusion to the 4P1L-4P1L-4P1L configuration. Perhaps it’s the H3 harmonic profile, but it doesn’t sound good – a bit harsh on the treble. The challenge in my view was that one 4P1L stage driver wasn’t sufficient to bring out to life in full Andy’s system. It forced the DAC to swing higher output levels and didn’t sound as good as with the preamps.

Don’t get me wrong here, all the preamps sounded great, however there were subtle differences which showed that 01a was superior in this setup. The 26 was also outstanding as expected, however the 30sp was slightly thin. The three agreed on the evaluation carried out and we concluded that 01a-4P1L-4P1L was a wining formula!

The 01a brought a level of clarity that it was superior. The piano, bass, snare drums, brass and voices we listen to in detail across various test tracks sounded with a level of detail and delicacy which was unique. This was a surprise to all, as we were expecting a system with 2 stages to be the superior combination. I wish the 4P1L could have a gain of 20! 🙂

Here are some few pictures of the messy DHTs spread around:

Andy’s HT supplies

4P1L Gen3 preamp, 01a Gen2 preamp, Tony’s 30sp preamp and Andy’s 26 preamp

Andy’s 4P1L SE Amp

What a mess on the floor!

First tests on the 01a, 4P1L preamps into Andy’s 4P1L SE Amp

wiring 01a, Slagle’s AVC and 4P1L Preamp into Andy’s 4P1L SE

Andy’s 4P1L PSE

Andy’s 4P1L PSE a.k.a “The daemon”

Thanks Andy for hosting a great day.

Here are Andy’s impressions posted in DIYAudio of our great experience testing the preamps and the 4P1L SE and PSE amplifiers:

“I had a very interesting day today with Ale Moglia and a friend of ours, Tony. We auditioned 4 line stages. Amp was a PSE 4P1L, with 4P1L input (plate choke and FT-3 coupling cap). Speakers were Mark Audio Alpair 10s. Full range, 23 litre infinite baffles. We placed them in this rank order:
1. Ale’s 01A
2. 26 into LL1692A stepdown
3. Ale’s 4P1L
4. 30sp

All DHTs and all sounded good – quite alike in having that DHT sound. I think they were all filament bias. Ale’s 4P1L line stage might have suffered from going into two more stages of 4P1L so may be better into a different amp. In that sense it may not be a definitive test, so the jury is out on that. We have known from past experiments that 3 4P1L stages in a row just don’t sound that good.

The 26 preamp was predictably nice, sweet and detailed. Audibly better than 30sp. Just a bit better in this context than Ale’s 4P1L but not far away.

The star without any doubt was Ale’s 01A preamp. It was just stunning. Quite magical. So if it’s a question of building a line stage, this to my ears supersedes the 4P1L. I never expected this – I’ve built 01A preamps in the past, but this is a very clever circuit. you can find it here:

01a Preamp (Gen2) | Bartola Valves

I do urge you to look seriously at this design. It’s a bit special. It was the only one of the 4 we auditioned that actually sounded better in the system than without it. There is enough gain with just the 2 4P1L stages in the amplifier, which I usually drive straight out of a ES9023 DAC. But adding this stage was a better sound. I never expect 3 stages to sound better than 2, but this did.”

(Andy Evans)

Situbes digital panel meter review

I’m a heavy user of fixed-bias output stages. Yes, I do prefer them despite the additional complexity. However, I’m not looking to open a can of worms around this subject. On the contrary, I wanted to report a fantastic product developed by Situbes.

SiDPM Digital “Panel” Meter

Here is the brief description from the website. I suggest you take a look at the datasheet as well:

The SiTubes DPM is a digital “panel” meter packaged in a standard octal tube envelope.

It measures a DC input voltage from 0 to 2V full-scale. Several selectable legends (V, mA, A, etc.) can be selected by an external programming resistor. It is particularly useful to measure supply voltages (such as plate voltage) and tube bias levels (normally plate current) in tube amplifiers.

The DPM can be powered from an AC or DC voltage. The power input is isolated from the measurement input, so floating measurements can be made up to 1500V above or below the power input. This allows high-side current measurements – for example, sensing plate current directly at the tube plate – or the ability to be powered from a supply that is not referred to ground.

My review

Looking at their construction you will realise instantly the high-quality of this product. Impressive finish and presentation. The tube is made of glass and fits very tightly to the 8-pin plastic base.

I did a simple and basic test on my work bench to test this device and its accuracy. In 5 minutes I wired it on my curve tracer to access the pins easily without soldering a test rig. With a 1Ω 1% resistor I configured the device to current mode and placed my 5½ digit bench meter in series for reference. The refresh cycle is very good, more than what you’d need in normal operation. Accuracy with the reference resistor was great. It’s calibrated as provided by the seller and error was below 1mA up about 200mA which is the planned use case for me. The OLED display has the right brightness for day operation. It’s just great.

You can use them easily to measure anode/cathode current, grid bias or supply voltages in multiple configurations.

They are pricey, but worth every penny. A top quality product which I’m keen to use shortly in one of my next builds.

813 triode SE with 4P1L Pentode

A monster DHT amp

Lately I haven’t had any time for audio work unfortunately. Changing nappies to a 4 week old baby whilst working long hours is tough. I can get the odd 30 minute here and there and every time I try to get upstairs to the workshop something pops up. Never mind, hopefully things will get easier in the near future.

I’ve been asked about the 4P1L pentode driver. It’s been a long time since I did those tests and never got around to listen to the driver sound. Tests were promising but never managed to include this driver on my amp.

Driving transmitting valves is a challenging task. Especially if we want to take them to A2-land (unless they operate in A2 whilst in zero grid bias). Driving big transmitting valves like 211, 805, 845, 813 or GM-70 require a large swing of volts for the driver which should do this linearly. The load is quite demanding in particular when we approach the grid to 0V (or biased positively) and using a triode as driver also puts a daunting task to the previous stage due to the Miller effect. It’s not easy to find triodes that can swing 300Vpp with very low distortion.

Continue reading “813 triode SE with 4P1L Pentode”

Popping the Shunt Voltage Regulator’s clogs

Not a surprise

What I suspected it was going to happen, it did in the end. Although a bit premature and in a bad time. I’m expecting today a friend to come around for a listening session and having no amp wasn’t an option.

To cut a long story short, the Salas SSHV2 shunt regulator has been playing silly buggers for a long time. Since I upgraded the output transformers and readjusted the bias, it looks like I was operating it at the verge of its abilities. The CCS was running at 80-90mA and somehow the stability of the shunt regulator was compromised. Initially was a periodic lost of regulation during warm up, this created an annoying “pop” now and then, later I decided to replace it with a new SSHV2 and blew a pair of DN2540 after the regulator failed to set the output voltage randomly. It worked fine on the test bench, however there is something on my system which is disturbing / interfering with the regulator or the regulator isn’t stable enough at the hot operating conditions I was submitting it to. I have nothing against the SSHV2, in fact, I use it extensively in my preamps. However, I think I’ve found the limit at which it can safely operate. The additional drawback of the SSHV2 is its temperature stability. It’s not great as it drifts when temperature rises.

So the regulator went busted on Thursday evening and I was running out of time. Only Friday was available to fix the amp. Luckily, I was on holidays this week and had the time to fix this, but unfortunately this diverted my energies and time from the 300B amp 🙁 Continue reading “Popping the Shunt Voltage Regulator’s clogs”

300B Amp: the journey starts here

I can’t get away from my big HT power supplies. This time the HT +400V for the 300B amp is here. Lots of Lundahl iron and a lovely hybrid bridge made of mercury rectifiers and damper valves. Yes, I’m in love with the blueish colour of the 816s. Despite they are mercury rectifiers, I know.

I promised the wife I’d get a smaller amp, that was my excuse for the 300B reference system. Still this is not going to be small, however, it will be compared to the 814 SE Amp!

When protection fails

“Don’t Stop Til you Burn Enough!”

Merry Christmas to you all! Christmas Eve ended up with some smoke and the party was over a bit early. We played music really loud last night and got carried away with some dancing around whilst playing “Don’t stop til you get enough” from Michael Jackson. And literally, the 814 SE didn’t get enough!

Let me explain briefly what happened. The 814 output stage has a crowbar protection circuit. It is configured to trigger around 200mA. The crowbar works brilliantly well, however, what is tricky on my design is the fact that you do get proper grid current in A2 operation. This grid current adds to the cathode current and flows through the crowbar sensing resistor. Well, volume was so loud so am sure that when the drum or bass kicked in, the crowbar was triggered. Interestingly enough, the shunt resistor is a 330Ω / 50W piece which should (in theory) blow the 500mA fast fuse. Well, it didn’t. The current peak wasn’t big enough before the output voltage of the 600V supply dropped significantly. Bear in mind that 330Ω should take serious current out of a 600V supply in theory!

The result was evident in a couple of seconds. The shunt resistor went madly hot, burned the plastic stand-off isolator quickly and fell over one of the current meters and burned the plastic cover badly as you can see on the image above. Luckily only one channel crowbar got activated.

When I rushed into the 600V supply mains switch I saw the secondary pair of dampening resistors (100Ω 7W wire-wound) melting and red hot. They were acting as a fuse and obviously preventing the supply to deliver further current.

Of course, the 814 was intact as it was the OT. At least the crowbar did it job, but far too early 🙂

Fixed it this morning after doing the Christmas present stuff. I couldn’t end up without music in Christmas!

Lesson learned here. Crowbar needs proper testing! I will buy smaller fuses – probably a pair of 200mA slow burn will work fine next time.

Happy Christmas!

Merry Christmas!

It is so great to have another Christmas. It gives us an opportunity to wind down and spend proper time with our loves ones. The ones who are physically with us, and remember the ones who are no longer here, but still very present in our memories.

In fact, we all look forward to the holiday period to crack on with our projects. We all have a pile of endless projects and ideas and no better time than Christmas to start working on them.

Continue reading “Merry Christmas!”