Ale Moglia

EL152 Pentode Spice Model

Some time ago I got hold of a nice stash of Telefunken EL152. These German pentodes are amazing. After playing with the RL12P35P and then obviously GU-50 (which is a copy of the LS-50), the EL152 was a nice valve to have at hand as it’s actually the LS-50 in a different bottle.

The B-10V socket is quite tricky as it seems like it was designed for the EL/FL-152 and similar Telefunken valves. Anyhow, managed to get some new ceramic ones to trace the curves and generate a Spice model. Hope you find this useful.

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Fixed Bias Board

Long time ago I built a series variable voltage regulator for 600V. It worked flawlessly and survived all sorts of abuses as is on my bench HT supply.

With the same circuit design, I developed the final stackable PCB (see previous post here) with this regulator:

Looks more complicated than it is. The single-supply Op Amp (LM358) needs a low voltage supply. I derived this from a simple CCS (DN2540) and a pair of 12V Zener diodes. The voltage reference is the famous TL431A and with P1 you can adjust the output voltage. The feedback resistor pair (R14 and R15) senses the output. C6 is for frequency compensation. The MOSFETs used are ST3LN80K5 which have built in protection Zener diodes, so none of the ones shown in the diagram are actually needed. T4 provides current protection to the pass device T2.

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RL12P35 SE Amp – part I

I have plenty of these German WWII pentodes. I’ve been saving them for a few projects. Quite likely for a push-pull amp given the OB speaker project I’m cooking. Either way, I have a nice set of CFB SE as well as PP OPTs from Toroidy custom made. They are very good transformers.

So, if you want a simple SE amplifier that can do 8W, here is a good contraption to go after:

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PP with local feedback: bias PCBs

I’ve been doing some push-pull output stage experiments with either cathode feedback (CFB) via the output transformer (Toroidy custom-made OPT) and parallel-parallel feedback (aka plate-to-grid or a la “Schade”) with PMOS driving the cathode. See some funny experiments here on the driver side.

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Pentode driver with feedback (Part I)

It was about time to get my hands on this driver experiment. I’ve been trying to find the time for a while and could only make it due to the obliged COVID-19 isolation upon return from holidays.

The idea is simple. I wanted to use a pentode driver to swing large volts (e.g. 200Vpp) whilst retaining the triode-like characteristic from harmonic perspective and low distortion. A nice challenge and fun to work on.

Have to say that the parallel/parallel feedback (also referred to as “Schade feedback” by some in audio) when applied locally in the output stage, does sound very nice and is a very nice way of implementing high-gm pentodes used for vertical service in TV. They can produce very low distortion and sound amazing when implemented correctly. I’m not covering this now, as it has been dwelled on for some time by many good people out there.

The circuit topology is as follows:

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EML300B Mesh – initial test

My friend Mirek sent me from Czech Republic a few valves for testing, including a pair of precious EML300B Mesh valves. I managed to slot them into my system before departing for holidays. I only listened to them for a few hours, so these are only initial impressions.

I like the sound and was expecting the additional level of detail and sound of the mesh plate. Worth addition clearly. I wouldn’t say it’s a significant step improvement, just minor, subtle details are clearer. You won’t go wrong with the standard EML300B. The Mesh is a nice upgrade but you will need to think (as always) where it best to put the money on. I’d invest in the iron and overall circuit before you get to throw more money on the output valves. That’s my view.

Anyhow, great job Emission Labs for this superb valve.

300B SE Amp: 47 Driver

Going DHT end to end

As previously mentioned, I played around with the 46 driver. I love it sound and is a great valve. However, there were 2 reasons that pushed me to switch to the 47. Firstly, I wanted an extra tad of gain. Secondly, I have a nice stash of RCA 247 globe which measure extremely well. I’ve been reserving it for a 47 PP amp with local feedback (a la Pimm) and hopefully will get to in the future. Anyhow, the 47 in triode mode has a mu of about 8 which in combination with the SUT, gives me good gain to drive my 300B. After tweaking on the bench the stage for optimal swing and distortion performance, I ended up with the following circuit:

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300B SE Amp: 46 Driver (Part II)

The 300B amplifier journey continues as planned. A bit lower than expected, however small steps being made. Recently I mentioned about the 46 driver.

I made some progress on the LL7903 step-up transformer which is wired on 1:8.

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300B SE Amplifier write-up

Finally I managed to get the time to write this up. Hope you enjoy it, you can find it here.

300B SE Amp: 46 Driver (Part I)

And time arrived to start experimenting with the different drivers. Why? Well, the whole point of the latest modular design is that I can easily (I hope) make quick changes and experiment.

The D3a driver board can be replaced with a new one. In fact I will be building a pair of stacked boards (which are in essence ground planes) to hold the SUT as well as a pentode screen bias regulator and the driver board.

First incarnation will be on the 46 DHT. However, drilled the board to fit either loctal (e.g. 4P1L, C3g, etc.) as well as noval for other 9-pin drivers I have in mind.

The build process is fast. I have the ground plane PCBs, which I drill (M3) to fit multiple plastic hex standoff to hold either PCBs or different turret/2mm connectors to plug in/out the board as needed. This LEGO-like approach is amazing, very happy with the flexibility and speed of work I can get with this.

Stay tuned.