After some further efforts, I managed to complete the power supply. And it works, which is always rewarding when you get it right first time:
Tag: 4P1L PSE
Flexible HT Power Supply (Part II)
Here are some picture of my slow progress on this flexible power supply. I started with the layout of the parts before any marks were made on the top plates:
Once I was happy with the layout, I submitted myself to the mercy of the most dreadful job on earth which is metalwork!
Flexible HT supply
The return of the Jedi
Time has come when breadboarding and HT is not longer an option. With a baby around, I was forced to remove most of the valve equipment. However, the return of the Jedi is closer than you expect. With very little time, albeit at continuous pace when I get a free 30min here or there every weekend, I keep very focused on my objective. The 814 SE monster amp is close to retirement (probably before Christmas) and although I have a fantastic LME amp already working, I need to get the valves back on the scene providing I can prove I’ve complied with “heath and safety” – as my wife well put it.
Well, I had a nice attempt of a wooden frame made out of pine wood. Yes, I’m really bad at woodworking but I’m getting better. The top plates are 4mm thick. I have added a nice set of 2 100mA meters at the front of the frame. A 300VA custom-wounded multi-tap HT transformer from JMS which gives me full flexibility from 100 to 500V in 25V steps. The supply is choke input (LL1638) and filtered with a set of nice 50uF oil caps and 20H chokes. Damper valves used with some nice SiC to form a hybrid rectifier.
Top plates are protected with paper before drilling!
This looks to be quite neat in my view!
The idea is to use this supply for my new 4P1L PSE, 300B and DHT Schade experiments with 6P21S/47 and 1624 valves. A lot planned and so little time for this unfortunately.
Cheers, Ale
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. 30spAll 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)
4P1L – LL2746 driver test
After a recent discussion in the DYI Audio forum about the 4P1L drivers, I decided to do some quick tests on an idea I had around to use a step up transformer (1:4) – 4P1L and step up interstage transformer (1:2) to drive a 300B or similar using the 4P1L in filament bias.
First suspicion is on whether the 4P1L has the grunt to drive a capacitive load which would be a real challenge in a 1:2 step up as load capacitance is multiplied by 4 when impedance is reduced by a factor of N^2=4.
I built a test rig with the 4P1L in filament bias using a 15Ω wire-wound filament resistor and connected the filaments in parallel to obtain easily a nice bias voltage with 650mA of filament current. Also lower Rf will improve the low frequency response as helps keeping low the output impedance:
The valve was biased at Ia=30mA / Va=160V and grid bias is about -10.2V. A 10KΩ resistor was added as a primary Zobel as per recommendation of the datasheet. Then it was replaced by a 25kΩ potentiometer (P1) and the right value was found by looking at the frequency response.
Initial tests showed a very good response at 1kHz with only 0.24% THD @200Vpp output. The gain is approximately 16. The mu of the 4P1L with paralleled filaments is around 8 and lower than when used in series which is approximately 9-10. Albeit the results were promising initially, the real test of this stage is by looking at high frequency response where the capacitance will makes it real pain.
4P1L PSE/PPP module
Like LEGO bricks, I thought about building this module for a pair of loctal sockets for 4P1L. It can be used in PSE or PPP…
4P1L pair ready for testing
4P1L pair module
4P1L triode SPICE model
After playing for some time with Dmitry’s great DHT composite triode models, I looked at refining the model by matching my own set of curves of the 4P1L in triode-mode. Here is my take on it:
The Shunt Cascode Driver
A heavy-weight driver
Rod Coleman came up with a brilliant design recently which baptised as “shunt cascode” driver. For those who cannot stand a pinch of sand in their circuits, I suggest you skip this post now. This hybrid circuit is actually a folded cascode if we consider the book terminology. What makes attractive of this design is its outstanding performance against the classic multistage designs aimed at achieving a large drive signal for output stages such as 300B, 6C4C/2A3, etc. I personally haven’t build it yet but according to Rod the sound is superb.
Before building a stage which will replace my current 45 SE driver, I thought it made sense to analyse the circuit and understand why is claimed to be such a great alternative for today’s designs.
I’ve got the (SE) power!
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.
4P1L PSE load line
A pair of 4P1L can be easily matched, so 4P1L PSE is a great cost-effective option to deliver +3W single-ended warm sound in A1. Having investigated filament bias, harmonic content, now is time to look at this configuration in a bit more detail.
I have at hand a nice LL1623/60mA which can be configured to 3KΩ:8Ω. After looking at the loadline here is what I think it should play well to deliver 3W:
- Va=250V, Ia=60mA, Vgk=-22.9V
- The pair of 4P1L will equate to mu=8, gm=12mA/V and Ra=690Ω
- Vg= 41.6 Vpp
A driver with some headroom to provide at least 80Vpp should be fine for this SE amplifier. Without looking at harmonic cancellation, this stage should deliver 3W at about THD=1%. Clearly proven that I will not readapt the 45 for A2 🙂
EDIT – 17th March 2013
Just realised after reading Imzen’s comment that the maximum Pa used is incorrect. 4P1L is a 9W device when triode-connected. So here is the correct loadline for a 5K OT:
As we can see, it’s better to run this valve in PSE as you will get just 1W in SE with 1.5% THD when biased at 220V/40mA…