gyrator PCB – Page 2 – Bartola® Valves

6e6p-dr Headphone Amp – part 3

6e6p-dr HP amp in action – cap coupled output

I’ve been running with this HP amplifier for a few weeks now and I have to say I’m delighted with it. I matched a pair of 6e6p-dr on my eTracer and after fiddling with the operating point a bit, I settled for Ia=25mA and Va=180V. Anode voltage will depend on the valve as said before, the Russian valve parameters tend to be all over the place. Anyhow, expect anode voltage to be around 170 to 185V. The beauty of the gyrator PCB is that you can adjust the anode voltage with the trimpot.

45 SE Amplifier – revisited

A unique sound

If you have a small room and high-efficiency speakers, then keep reading. If not, you can still enjoy reading about probably the best sounding output valve in my view. The 45. I wrote about it few times and have to say, it’s still one of the best. Better than a 300B but unfortunately can do only 2W. You can use it in Push-Pull and is superb. Have a look at this.

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Pentode Differential Pair (Hybrid Mu-follower)

Pentode drivers are very interesting. You can get excellent results out of them. Here is my version of a differential pair using pentodes. I want to try them out in the 45 PP design I wrote previously Continue reading “Pentode Differential Pair (Hybrid Mu-follower)”

Hybrid Mu-follower (aka Gyrator) Rev08 PCB Update

It’s been far too long since I last posted on this blog. With the limited spare time I’ve got these days, I concentrated in setting up the new workshop and system since we moved back to our place. I’m nearly there, so now it’s time to get back to work

I made some updates to the “gyrator” PCB. I’ll stop referencing it gyrator from now on, since the name is misleading. However, it got popular that way. Nevertheless, it’s a hybrid mu-follower circuit but if only if you take the output from the anode, it behaves like a “gyrator” from a frequency response perspective. If you’re interested in this circuit in more detail, please read the lecture I gave last year at ETF.18. You can download it from here.

Back to the board, here are the few changes made:

Moved the trimpot P1 for easier fixing of the board with the M3 standoffs.
Added a gate stopper resistor (R9) to avoid oscillation at low anode current (<10mA) when using high-gm MOSFETs in the lower position J4. This was evident with devices like BSH111BNK
Added an LED (D4) indicator and a series resistor (R8) at the drain of M3. This enables indication of:
1. Normal operation subject to value of R8
2. Source current into load (e.g. like in A2 operation) subject to value of R8.
3. Short output to ground. Depending on duration and current limitation of power supply, this may prevent damaging M3 MOSFET. Not guaranteed, but in some scenarios will work.

Continue reading “Hybrid Mu-follower (aka Gyrator) Rev08 PCB Update”

Aa DHT Preamp (Part I)

Obviously it was time to test the little brother of the “Ba” DHT. In particular, as I have such a low DAC in place now, I need the gain. With nearly a gain of 30, it’s an attractive fellow to work with.

First good sign is that it doesn’t pick up as much hum as the Ba. That’s good, nevertheless I placed a back copper plate which acts as enough earth shielding to keep the Ba quiet.

I worked with my breadboard to find a good “sweetspot”( at least electrically) for this valve. Given the low signal source level, I aimed for a low bias. I played with my fixed bias setup before replacing it with the SiC bias board. I found that 2V/160V was very good in terms of keeping the distortion profile to minimum. Here is what I ended up with:

Only 2 SiC diodes are enough. The source follower PCB is mandatory given the low anode current. I run it at 20mA (hot) to get best results of the stage. Rest of the circuit is very simple, achieving a gain of about 30. Here is the distortion profile:

There is no shielding and you can see some minor IM distortion with mains hum. Harmonic decay is nice with H2 being strongest. THD is very low at 4Vrms which is good sign. Let’s see the frequency response now:

Nearly 140kHz of bandwidth which is plenty for the stage. Great response. I just need to plug this one in and listen to this promising German DHT.

Ba DHT Preamp Build

Just in the last days before the arrival of our second child, I had a spot of luck and managed to get some free time to work on my DHT preamp experiments.

A pair of NOS Siemens “Ba” valves under initial testing

Originally I was put off by the hum pickup susceptibility of the Ba German DHT. However, I decided it was time to listen to the design I worked and experimented in my workshop time ago. I remember listening to this valve and was very pleased with its sound.

DHT Audio Shootout @ London

Here’s the summary of the shootout day we had yesterday at my place with Tony Rees and Andy Evans

Test Music

For testing the pre-amps Andy suggested the following 2 tracks:

Ravel Feria (Cleveland/Boulez)
Janacek The Cunning Little Vixen Act 3 Scene 7 “A vixen’s running through the woods” Prague Theatre/Gregor

The Ravel is orchestrated for a large orchestra and percussion, and listening was particularly for the timbre of strings, brass and woodwind and the clarity of the percussion. The Janacek is for several different voices and chorus, and listening was for the tone and character of the voices and how well they were differentiated from each other. In both cases the overall clarity, life and crispness was considered.
Andy Evans

Although I wasn’t familiarised enough with these 2 tracks myself, I found them to be an excellent choice and a reflection of Andy’s musical experience and knowledge as musician.

Hybrid Mu-follower Output Impedance

Where to start?

Well, I often get the question “what is the output impedance of the gyrator circuit?”. My response has always been “it gets close to 1/gfs as a good approximation”. Recently, I was asked the question again, but this time I decided to crack on the formulae, which is a daunting task for someone who has ditched calculus after obtaining Ms in Engineering degree more than 20 years ago.

For simplification let’s start with a simple hybrid mu-follower stage (a.k.a. Gyrator load) like the following:

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6Э6П-ДР (6e6p-dr) preamp / driver

With a quick mod on one of my active preamps, I managed to implement very quickly the 6Э6П-ДР (6e6p-dr) driver in my system. Here is how the circuit looks like:

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Driving hard (Part I)

Introduction

I’m a firm believer that if you don’t share, you don’t get back and learn. What the point of not sharing what you’ve learned? I asked myself the question again yesterday, just to push me a bit further. Joys of Easter break is that I have the time to sit down and write. At least for a little bit.

Here is the result of my quest of the years to find the best drivers for a SE amplifier. I’d been looking and experimenting with them in terms of best linearity at large volt swings (I mean large when I say 200Vpp), harmonic profile and most importantly the sound contribution.

Why should you bother? Well if you are in the DHT space (otherwise don’t bother reading further) and, unless you are building a 4P1L amplifier, the majority of the output valves require large volt swing. You also need good headroom. Therefore if the driver is clumsy, it will ruin your expensive project. Again, one of the reasons why people claim that their 300B sound bad. Achieving a driver which can perform 200Vpp effectively with minimal distortion and a decaying harmonic content isn’t a simple task.

In one of my recent post, I blogged an example of the GM70 amplifier. Look at the curves below and the demand to get all of the juice:

Yes, you can load it with a steeper load and use a 6KΩ instead of 10KΩ to get more power, but you still need the same volts to get the full swing.

The Driver Topology

I tried it (mostly) all over the years. Transformer coupled, choke loaded, resistor loaded. However, in my experience the best is the gyrator load. You may have a different view, and so you may: well, it’s a free world and I’m not expecting you to agree with me. If you are prepared to accept my point for view, then you can continue reading this post 🙂

The hybrid mu-follower (aka gyrator load) is a very effective topology for a driver. You need sufficient volts at the supply, but that’s not generally a problem. You will need at least 25-50V more than the largest voltage swing. Most of the valves I will review below have a good compromise operating point at about 200V. For a 200Vpp or 250Vpp headroom, this means you need 200V+250V/2+50V = 375V. MOSFETs can work at this level and providing you put them the right heatsink size we’re on business!

Initially some years ago I explored the use of LEDs, diodes and particularly SiC diodes to bias the valves. However, I found later that a bit of cathode degeneration by placing and (unbypassed) resistor was a good choice. This linearise the valve a bit and won’t impact the output impedance of the driver. However, if the resistor is within a reasonable value (smaller than 300R I found in practice), the impact on the Frequency Response (FR) is manageable and also the reduction in gain of the stage.

I will present in this post my favourite contenders for the best drivers. These are:

6e6p-dr: the brother of the famous 6e5p, but a real performer on his own
6z49p-dr: an amazing pentode.
6z52p: high-gm and mu pentode. A replacement for the E810F or D3a for some. This is a pentode that works brilliantly on all fronts. Even at low voltages (thanks JC Labs)
6C45p: this little triode monkey oscillates widely, but can be tamed and works well, so don’t be afraid.
6e5p: this famous Russian beauty doesn’t need introduction
12HL7: the sleeper and best discovery last year.

In all cases I found the sweet spot with fixed bias which allows me to dial-in the right operating point in conjunction with the gyrator setting point. Once the best performance was measured (and listened) it was replaced with an equivalent resistor and re-tested. A tedious job, but worth the efforts.

A note on oscillation

These valves have mostly high gm and gain. You’ve been warned. Don’t even attempt to build with them without special attention on the building aspects. It will oscillate, believe me. You should add grid, anode and screen stoppers. I prefer nice ferrite beads added straight to the socket pins. Continue reading “Driving hard (Part I)”