Flexible HT supply

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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

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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:

  1. 4P1L SE LL1682: a great sounding amp overall which sounded as well as it measured. A must amplifier to listen to!
  2. 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.
  3. 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:

  1. 4P1L Siberian Gen 3
  2. Andy’s 26 preamp, filament bias, LL1692 step down transformer and Rod Coleman regulators.
  3. Tony’s 30sp with Rod Coleman regulators and depletion FET CCS loads
  4. 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:

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. 30sp

All 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)



6GE5 Pentode Model

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A quick trace of a NOS Tung-Sol 6GE5 measuring around 113%:6ge5 test

Derived the Spice models using Dekker’s ExtractModel tool. The uTracer has a limit of 200mA so it’s not possible to trace the extended set of curves for this valve. However, this is not a limitation to produce a good model.

Here is the triode matching:

6ge5 triode

6ge5 triode

I run a set of curves at various screen voltages from 100V up to 200V at 25V steps to generate the pentode model The saturation effect is visible on higher currents above 140mA as you can see below:

6ge5 Vs=100V

6ge5 Vs=100V

6ge5 screen current at Vs=100V

6ge5 screen current at Vs=100V

6GE5 Vs=200V

6GE5 Vs=200V

Below is the triode model, you can dow Continue reading

CCS: not everything that glitters is gold (Part I)

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This is the first instalment of a series of blog posts around CCS for valve circuits. Hope you enjoy it as much as I did with the experiments conducted as a result of my interest in CCS-driven circuits.

The depletion cascoded CCS

It’s been long time since I’ve done some circuit analysis and algebra, hopefully I’ve got this right. Seems to get to the expected result, so hey: I’ve done it ok.

The analysis of this circuit starts by using the T-model of the MOSFET. I’ve omitted the parasitic capacitances to simplify the analysis. I leave you the challenge to add them in though. If we look at the typical self-biased depletion FET CCS we can find the output impedance by doing the following formulae crunching:

CCS zout formulae1In summary, the output impedance looking from the source side is:

Zout\approx Rs+\left ( 1+Rs\cdot G_{m} \right )\cdot r_{o} \approx Rs\cdot G_{m}\cdot r_{o} = Rs\cdot\mu

Continue reading