All about electronic valves and hi-fi
It’s been a long and wet winter. I barely gone out as it was always pouring down buckets of water. Spring is coming, and with that the mood of everyone is changing. A time to enjoy the open air and play music that cheers us up. New project and challenges. but for whatever reason is, I tend to play a different set of records when the weather is good. Can the british weather affect us so much?
I’m really looking forward to the good weather and for the summer to come, oh yes…
To many festivities give us an opportunity to work on belated projects. This year, I couldn’t take time off between Christmas and New Year, but can’t complain as will be taking a week off in January. Either way, I look back and is amazing everything achieved over the year. And I’m grateful to have such an opportunity to realise many of the dreams and ideas. What makes me even happier is the bigger list of projects that I continuously dream about. That is what I call passion and I know many of you feel the same way.
Not only I wish you all a very merry Christmas with your loved ones and a fantastic 2014, but also I hope you can also find time to bring to life all those great audio projects piling up!
Cheers,
Ale
“One chord is fine,” he once said, alluding to his bare-bones guitar style. “Two chords are pushing it. Three chords and you’re into jazz.”
Thanks Lou for so many years of education, joy and great music that definitely inspired many music generations and myself. Hope you are taking a walk on the wild side now…
Bank holiday weekends give us time to think in a fast world. A time where analogue beauty is almost eclipsed by the digital norm. In fact I like both or at least the right blend of them. At the end of the day we can create (as our predecessors did before) with what we have. Some may do good things and some may still produce a pile of rubbish.
For DHT single-ended (SE) topology, I have to admit that I reached to the conclusion that in my opinion either 6C4C or 4P1L are the way forward in terms of sound and cost after not being happy with the option of running the 45 in A2 mode. Both 4P1L and 6C4C sound lovely in SE despite many will say the 300B is unbeatable. Yes, won’t say a ridiculously thing such as 6C4 or 4P1L are the “best DHTs”. We all know that there are many great DHTs out there, but at a cost. Well, if cost is not a problem for you, you can chose great NOS valves from PX4, 50, 300B to 813 or 845. The latter comes with a hidden price: the power supply. I’ve been there as I’m building the 4-65a SE and most of the budget is used in the power supplies. Sound-wise, we did a side-by-side listening tests on many SE and PSE amplifiers and couldn’t find a significant different between 4P1L PSE and 300B stages. This could easily end up in loosing the tangent and falling into an endless debate about topologies, OT, driver-output stage combinations, bla, bla, bla, but in reality you can’t beat a 4P1L PSE in terms of cost and bias flexibility (i.e. you can easily get 5W from a pair of 4P1L as we will see later). I wish I could achieve the output power I like (i.e. 3W) with a 45. A 45 in push-pull is then very attractive but I haven’t listened (or build) it yet.
I have a very decent stash of both 4P1L and 6C4C, so obviously I will be inclined to get the most out of these ladies rather than continuing burning money on other NOS valves . If you are still reading this is simply because you have (or at least considering buying) 4P1Ls or 6C4Cs and you want to build a good amplifier with them.
So how much power can you get out of the 4P1L? Anatoliy did his own tests and was very pleased with the results in terms of sound. I haven’t run the 4P1L in A2 yet but here it would look like in A2:
You can get clean 5W from a pair of 4P1L running at 50mA (each) and biased at 200V. The driving requirements are only 50Vpp and we can see in the diagram above that the positive grid excursion is to just over 10-12V. Obviously the right driver needs to be used to provide the necessary grid current in A2 and also withstand the changes in grid impedance when transitioning from A1 (high impedance) into A2 (low impedance) with minimum distortion.
I don’t currently have an 2K5Ω OT gapped at 100mA, so won’t be looking at running a 4P1L PSE in A2 like this.
Instead, I have a pair of LL1623/60mA which can provide a varied set of transformation ratios: 5K6Ω, 3KΩ and 1K6Ω.
With this OT I could then easily get 2-3W out from a 6C4C or 4P1L PSE as we shall see looking at the loadlines.
The 3A5 DHT is another interesting valve to bear in mind. Many have used them successfully even in a phono stage. Great examples are Dmitry Nizh’s DHT phono stage and Sheldon’s. I will probably use them as well at some point.
Well, many DHT have been dismissed over the years due to their microphonic sensitivity. True, they are more microphonic, but this is manageable. Look at the 4P1L for example. I built a great preamp a couple of years ago and played around with the mechanics of the preamp to quiet it down. And I managed to, succesfully (as many have as well). The 4P1L sounds fantastic, despite so many turning it down because of its natural singing skills 🙂 Now, 4P1L is looked after, many are building DHT preamps and even full DHT amplifiers.
Looking at the specifications of the 3A5, we can see that the heater is attractive to be used with rechargeable batteries. Well, at least 220mA @ 1.4V is manageable. Key specs are:
How linear is this DHT? Let’s look at the curves first:
