All about electronic valves and hi-fi
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.
I tried the below configuration and the results are very promising. Sound is as good as previously implemented SiC cathode bias.
Now looking at the TH and frequency response:
The stage performs as expected. I built some prototype PCB for the SiC filament bias. One board per channel. If there is any interest, I will run a bigger batch
Sounds great, as always.
Cheers
Ale
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.
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.
Continue reading “Ba DHT Preamp Build”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:
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.
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:
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.
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)”
I have 2 pairs of gyrator boards for sale. They are fully functional ( I tested all of them in a 6e5p driver at 200 Vpeak to peak) and stuffed with my best selection of components:
Another DHT Preamp on the way with Rod Coleman regs. I will do a A-B comparison with some Tentlabs modules, it’s an interesting experiment to undertake. The preamp has the classic gyrator Rev07 and Source Follower boards…
Many times I get emails from DIY Audio builders who embark on building a DHT preamp when they don’t need gain, but instead what they need is a simple line stage to drive their amplifiers and interconnect cables effectively. Then they come back asking: “can I reduce the gain of the 01a or 4P1L preamps?”
For those who don’t need the gain, here is an interesting idea which brings together several design decisions which makes the DHT sound to its best. The challenge with many of the best sounding DHTs of low-mu is that is very hard to implement with filament bias. I’ve done a driver with a 46 in filament bias which was a crazy idea. I could turn of the heating with the amp running! It was a nice experiment though. With exception of the 71a and some other few DHTs, if you’re looking for good anode current and low ra, you’re in trouble. The 300B, 45/46, 50 and some other variations can’t be used in filament bias.
Subject to your religious beliefs in audio, you may not want to add a capacitor in the cathode, like me. I won’t dive into this discussion which is pointless as is a personal decision. If you continue reading this is simply because you value the sound difference in the DHT without a capacitor bypass in the cathode. Keep reading then…
Finally I managed to build a stable version of the 2P29L preamp. The wooden modular version inspired on DHTRob designs paid off. Here it is:
The result is fantastic. The 2P29L is a superb DHT, quiet and it sounds amazing. The circuit is similar to the original one but with a slightly different operating point due to the valves themselves:
I’m still running them at 20mA. This time the anode voltage has to be increased to 150V to achieve this. Consistent across the 2 valves used.
C2 was modified to fit a gift I received from my friend Vyacheslav. Instead of the usual 220nF, I fit a bigger 470nF FT-3 version which improves the LF response.
The frequency response is very good, as I published before. For the curious ones, here are some further pictures of the build:
I can’t say more than what I said before. This is a fantastic DHT, it provides a great detailed sound and is quiet. No microphonic noise and the dynamics are great. One of my favourite clearly.
Last time I wrote about the 4P1L in screen mode. It was great to see some DIYAudio member (Blitz) to post about his work on the 4P1L with screen as anode. I call it screen mode but probably is incorrect.
His post about G3 structure remind me to post this, I have tested it but never blogged about it. Yes having G3 as part of the anode structure will increase conductance and will form a nice “mesh” anode. Here it is how I implemented:
The pin 4 (G3) is now connected to 3 (G2) to form the anode. I reduced the anode voltage down to 110V to get 10mA. It could be increased, sure within the Pd limits.
The response is very good:
Here you have the distortion at 1kHz:
Well, I wrote about it before. The 4P1L is one of my favourite valves. In this mode it sounds great, with a particular clear detail in the treble. I like this valve and will play it for some time to get further impressions.