All about electronic valves and hi-fi
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:
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.
Finally I managed to get the time to write this up. Hope you enjoy it, you can find it here.
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.
My friend Rob was right. The UX120 sound is unique. Perhaps is due to its cylindrical anode, thoriated-tungsten filament. Anyhow, he was also right that they pick hum and are extremely microphonic!
Well, not a candidate for a preamp indeed. However, with a low mu of over 3, it just added an extra gain kick into my 300B system:
The modification of the Mule was quick an easy. Fitted back in the UX4 sockets, adjusted the SiC array to 6V (5.8V drop) on filament bias. The hybrid mu-follower boards were adjusted down to 85V and 7-8mA. They are DC coupled into a pair of Source Follower boards biased at 15mA
Also I fitted the copper screen board on the back as I had in my Ba/Aa preamps. This works perfectly and when grounded there is no hum picked by the valve. No need to cover the valve at all.
The sound is amazingly good. Level of clarity and detail which is unique of an DHT. Very similar to the 01a sound in my view. Just great. I had to put the volume pot at the output as the microphonic noise is terrible. Yet, level is set to be usable (very usable) in my system. Love this sound indeed.
The 300B still is driven by the D3a. I will experiment with them in the future as have few things to do before changing them.
My latest 300B amplifier brought me again closer to the D3a. I have to say it’s an exceptional driver for this amp as well as it can perform in a phono stage at same excellent level.
I took the opportunity to trace again one Siemens D3a NOS boxed as “POS 1.176 Q 31-X 601”. This one was handpicked as it measured at 102% (31mA) in triode mode with a Gm of 39mS.
I wanted to develop a pentode model for phono experiments so put this lovely valve back in the eTracer and used the Extract Model tool from Derk Reefman to develop the model below.
