300B SE Amp – here we go

A Brave New World

Surely you’re as tired as I’m with COVID-19. One of the best things I can do to distract my mind is to keep myself away from social media. Every stone you turn, there is COVID or a statement about it. I won’t moan as I have a job for now and a healthy family. Some members of my family were infected but nothing major. I can only say is that the world has change. And so my day to day life looking after the young family whilst working is a real challenge. Starting my fifth week of lockdown, I have to distract somehow my mind at times, otherwise will go mad.

A New Concept

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Modular approach (Part I)

As time is very limited these days, I’m focused in continuing my modular building approach in LEGO style. I have developed several PCB modules which are flexible to be used in multiple amplifier and pre-amplifier designs. Now, I used the power of PCBs to build some additional supporting modules to speed up my breadboarding over the IKEA boards. Not the most elegant approach, but building becomes a very fast process this way.

You will see what I’m saying when you see a few of the following additions:

Turret and 2mm female connectors in a strip

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Monolith Magnetics S9

Now fitted on the 4P1L PSE stage:

Tuning the system for ETF

This year I entered the shootout competition and will bring my DHT system to the European Triode Festival in France. It’s comprised of the ER801a stage plus the 01a (if extra gain is needed) and the 4P1L PSE output stage

I will have to swap out the amorphous OPT for the Monolith Magnetic ones as the speaker load is 5R.

It’s going to be interesting!

300B Amps: an early breadboard

Browsing my schematic archive I found this early implementation from some years ago on the 300B:

The amp design is straight forward. Let’s start from the output stage. The 300B is run hot at 33W (376V/90mA) with a fixed bias of about -78V. I used a pair of LL1623/90mA OPTs which I had wired on 3K:8 mode.

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“Schade” SE Amp Example

Introduction

On my last post I covered how the gyrator PCB can be used in a pentode driver. The pentode driver is the best candidate in a “plate to plate” / shunt feedback or “Schade” feedback amplifier which is the name typically used in the DIYAudio world. The triode doesn’t work well here as you need high gain and low distortion with a load which can get quite low (due to the feedback effect of the feedback resistor). I’m not going to cover the subject as it has been covered (and discussed) extensively before by many people, so I suggest you do a bit of research yourself if you are interested in the subject and want to learn more.

A Study example

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6Э5П Shunt Cascode Driver

Introduction

The main challenge when implementing valve amplifiers using transmitting valves or valves which require a significant voltage swing (e.g. 300B, 45, etc.) is the driver. Getting the driver right is not easy. You’re asking for a single stage to swing 150 to 200Vpp at minimum distortion. There are some ways you can achieve this:

Implementing 2 stage voltage amplification. Here is where we find a lot of bad designs and poor results. Sometimes the 300B gets a bad reputation due to a wimpy or poor driver. Many designs out there use 2 stages of 6SN7 for example. Nothing wrong about using the 6SN7, however when you cascade 2 stages the sound is muddled at low levels. Harmonic profiles may be encouraging but they simply don’t sound great.
Implementing a high-mu driver stage. There are several high-mu drivers out there than can swing plenty of volts. 6Э5П, 6Э6П, 6j52P, 6j49p-DR, E280F, C3g, etc. They work well, specially if you couple them with a gyrator, you can achieve hi gain. If you opt for degenerating the cathode resistor, the gyrator still provides a low output impedance to avoid degrading it due to the degeneration resistor. I’m a big fan of this approach. The only disadvantage is that you need a buffer/line-stage capable of driving the Miller capacitance. I have a nice preamp/line stage so this isn’t a problem to me.
Implementing a pentode driver. Pentode don’t suffer from Miller capacitance. However, you need to find the right driver, not all sound well in my experience. I like the 4P1L and C3g. You can use a gyrator load with pentodes as well. Some folks complain about the pentode harmonic signature. I think this is a question of personal taste.
Implementing a shunt cascode driver. Hey, this is what this post is about! There are several benefits already discussed at length on this topology. If you need high gain and minimum capacitance load (e.g. Miller) as you have a DAC output for example, this is what you should look into. The Shunt Cascode operates the triode in a vertical load line (not horizontal like the CCS or gyrator).

Design

You should start by reading this extensive blog post. That will provide you with a lot of information around the shunt cascode and how it works. Back in 2013 I started playing with the 6Э5П in this topology. It was quite promising. Now, I have revisited and built this driver to see how it really performed.

The design is very similar to what we discussed back then. I shall proceed in describing the circuit, in particular the changes made. The driver is still the marvellous 6Э5П. There are few valves out there that I don’t like as much as I do with the 6Э5П. I measured the curves long time ago when I started with the curve tracer project. I also tested the 6Э5П and 6Э6П extensively. I do love the 6Э6П as well, it’s one of my favourite drivers.

The 6Э5П is biased at about 200V/30mA with a degeneration cathode resistor of 120Ω. As the gain of this stage isn’t dependent on the μ of the valve, then is good to do this to improve the linearity of the driver. M2 forms a CCS with Rmu. It provides the current to the 6Э5П as well as the current to the common base stage formed by Q1 and Q2. The gain of this stage is gm times R5. The gm is the valve’s transconductance The collector current of the MPSA92 is kept low to ensure distortion is minimised as well as its operated under SOA. D3 provides a protection to the darlington pair when is reversed biased.

The gain of this stage was measured to be x140 (or 43dB). That equals to a degenerated transconductance of 5mA/V with a cathode resistor of 120Ω and a gain resistor for 27kΩ.

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Alpair 10M / FT96H speakers

It’s been a long time since I haven’t tweaked my speakers. After more than 9 years I decided to change the drivers after falling in love with the Alpair 10M from Mark Audio. I listened to my friend Andy’s system (4P1L PSE driving the Alpairs) and decided to get hold of them.

A simple upgrade

As I don’t have much time left for DIY audio these days, I needed a simple solution. I couldn’t build a new set of speakers despite the love I have for some horn-type designs. Bringing new speakers was out of the question, so I had to modify my existing boxes to replace the FE167E. Sadly they didn’t fit straight on, so my friend Tony made me a pair of adapter boards to fit these. Made of MDF I painted them in grey:

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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.