This is a long overdue post which I never had the time to write about. I was hoping to get my measurement gear down to where my system is to take a final FR sweep analysis of my 4P1L PSE amplifier, but never got around to do it. However, after the recent posts in DIY Audio, it was time I shared the measurements made and my listening impressions of the NP Acoustic Transformers.
CCS PCB Ready
I received the first batch of PCBs after several tweaks during the prototype phase. I’m very pleased with the result:
Emission Labs 20AM: first tests
After tweaking my test rig to fit a variable supply for fixed bias, I proceeded with the tests with the EML20AM valve. 
Valve test rig with fixed bias
My test rig for DHT/IHT stages (and even Pentodes) has evolved over the years. Lately I settled with some nice modifications to allow testing the majority of valves I have. I use a modular socket system, nothing fancy and can add/remove a source follower stage at the output. There is also a screen regulator in case pentodes are submitted under the mercy of the jig.
Here is the simplified diagram. I added a nice fixed bias supply formed by a SMPS PCB board which delivers up to 400V, however the output is dial to about 100V. Then I use a Swenson Regulator to knock down the noise by about 100dB. A simple pot provides the voltage needed between 0 and -100V. It can be tweaked for whatever range you need. The pot is 20T wirewound so allows a fine adjustment on the bias:
For the curious builders, 
here is the rig mounted on a piece of floor board:
300B Un-bypassed Rk Bias Line Stage
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…
Swenson+ HT regulator test
So the developing is progressing. This time I tested the first PCB prototype of my enhanced “Swenson” regulator which I baptised Swenson+. It’s a great circuit and performs really well. Although component selection and track cree page and clearance is for about 1kV, I think it should safely operate at least to 800V. Not really thinking to use it at those voltage levels, but rather as phono stage supply instead, which requires much lower voltages.
As usual the focus of the PCB is to provide as much flexibility in component selection (SMD and TH) as well as use cases.
Still some layout refining and further testing but so far so good! Here is a taster which shows at least 60dB rejection of the incoming power supply noise which wipes out all rectified-related harmonics (100Hz etc). The 100dB 50Hz and byproducts is likely to come from my workbench:
More to come soon (hopefully)
EML 20-AM DHT curves and model
There is one DHT which attracted me from Emission Labs which is the EML20. If you’re looking for a mid-mu DHT valve these days, this one is the way to go. In order to meet with the two key requirements of linearity and low microphonic noise, EML made a great effort in producing this valve.
For me it’s a great candidate as a driver or for a Spud amp for headphones. I will likely use them in several places, but will start with a preamp stage, as you would expect from me anyway.
First step: tracing the EML20AM
Tail CCS PCB prototype
Recently I’ve spent some time designing some PCBs for my own use and why not to share them as well to the DIY audio community. Currently I have a flexible CCS board for anode loads and a tail CCS, which is the one I will write about on this post.
I came up with a 3cm x 3cm PCB board which provides the maximum flexibility for a tail CCS circuit. Below is the diagram:
A tale about DHT Supplies
I’ve been using my choke-input DHT filament supplies for many years. I’ve got many of them as you’d expect. Something I do really hate is to build power supplies. It’s dull and boring, but they’re a necessary evil. I’m afraid I have to admit.
Recently I experienced the dreaded smoke of capacitors blowing up. It’s actually funny in hindsight, however at the moment of the fireworks you don’t laugh. I killed one Coleman regulator as well with my experiments on a flexible DHT supply. That made me revise the design and the stress put on the components, in particular when you’re using filament bias in anger, as I do.
Anyhow, over the last 3 months I’ve been playing with SMPS supplies to try to get the commercial available ones quiet enough to be used with a DHT preamp. I started with classic filter stages like CLCLC, morphed to gyrator filtering (which wasted a lot of heat) and then resisted using LDO regulators which I knew it would do the trick. However, getting rid of the HF noise is a daunting task.
Actually, I have a variable HT SMPS supply built which sounds really good and is extremely quiet. It can deliver 2 channels of 600V/100mA. For a filament supply, the SMPS challenge is of a major league game. You can get the noise to about 1mVrms but the harmonics spread well over the mid and high range. Big chokes have also a big leakage capacitance which makes the choke not that effective at filtering off this HF noise. 1mVrms in 600V is fine, however 1mV in a 16V supply which si feeding the filament bias resistor is a problem.
After giving up, my patience these days isn’t the best I have to admit. In particular when time available for DIY audio is very limited. So I said to myself: “sod the SMPS, I will get a nice pair of custom made transformers with multiple taps and job done”. And that is what I precisely did. JMS transformers in the UK provide an amazing service. I’ve mentioned them in the past several times. I get all my power transformers from them these days. I ordered a pair of split bobbin, with outer copper screens and multiple taps to cater for all the voltage ranges needed for my output stages. From a 4P1L to a 300B.
As I always use choke input supplies for filaments, I used the LL2733s I have in stock and carelessly wired it on 400mH (series bobbins) which provides a huge voltage peak output when the minimum choke input current isn’t in place. This happens at start up, the voltage will raise to the level of cap input supply when the filament is just starting up thanks to the gentle raise of the Coleman regulator. The result is the high voltage peak output which can damage the capacitors.
The solution was to wind the input choke with the two bobbins in parallel to get inductance down to 100mH. Also the resistance is reduced significantly which avoid wasting too much voltage drop on the choke. Also adding a tuning cap (C3) to make the supply to operate in a hybrid mode between choke and cap input is great to dial in the output voltage.
Here is my supply which I use for the VT-25 and the 4P1L preamps:
eTracer Build and Review
Tracing valves: an obsession
Since my early days of valves and DIY audio, I developed an obsession around testing and tracing valves. This led me to design and build my analogue curve tracer which I used for many years successfully until I build my uTracer, which was a great innovation in curve tracing. I do have many valve testers (some which I made myself) so why building another one?
Well, Chris Chang from Essues Technologies developed a fantastic new digital curve tracer for valves, the eTracer. There are a few things which will grab anyone’s attention on this curve tracer. Firstly, the power supplies can accommodate a large range of valves which the uTracer can’t. HT can go as high as 750V @ 300mA and the grid supply down to -170V! This is exactly what you need to test your transmitting valves or even a 300B. Secondly, the tracing speed is surprisingly fast. This is a nice feature, specially when you want to trace pentodes at various screen voltages to develop a Spice model for example.
