All about electronic valves and hi-fi
On the last post, I shared this great circuit. Now, with the Holiday season and being locked down I somehow find the time to build it. A very quick process as it’s all modular. I’ve got all PCBs that can be interconnected like LEGO, that’s what I have always in mind when I design a new PCB.
It’s been a while since I last share some of the experiments. I’ve done a lot with different drivers topologies and pentodes lately, yet not much time to sit down and do a proper write-up.
It’s been a long and weird year with COVID-19. This evidently has kept me a bit distracted and changed priorities (as probably has done to you as well). Anyhow, here are the belated curves I was asked to publish.
The SRS-551 is a lovely powerful transmitting pentode from RTF. Similar (or close enough) to RS1003 and F3a. Much more accessible in price is the SRS-551. I have a nice stash of NOS valves somewhere lost in my attic storage. Either way, they sounded nice and could definitely live in my setup. The valve was intended for audio modulation in FM transmitters so it’s very reliable and linear device. Distortion was extremely low when I measured it but most importantly it could work really well with local feedback to squeeze out 14W in Single-Ended at nearly 490V/100mA bias point with an LL1623 OPT. The distortion was below 0.7% predominantly H2.
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.
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.
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:
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.
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:
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.
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:
