Category: 300B Amp

Gassy nightmare

Ok, title isn’t very appealing. I know, however it truly reflects what I experienced over the past few days. Let me clarify before I start: a gassy tube developed a nice fault-finding journey. You always learn a lot from all this, for sure. Luckily, due to the holiday season, I had the time to work on this. I’d rather have spent it otherwise, but my 300B amplifier was dead.

6BY5-GA damper

I had a gassy 6BY5-GA damper rectifier tube. I should have changed it, but I was lazy and I paid the price for it. When turned on the 300B HT power supply, it will flash blue for a sec or two and the gas ionisation will disappear after a minute or two when tube was warm enough and operated as expected. Problem was that my stash of these damper tubes was up in the storage, who knows where.

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EF86 – 300B XLS SE Amplifier build

Here is a very nice build from Dan Kercher on his contraption of the 300B amplifier with auto bias. The driver is an EF86 with SiC diode biasing, the topology is a hybrid mu-follower and a source follower drives the output stage. Filaments are DC with Rod Coleman’s board:

Dan Kercher’s EF86 – 300B XLS SE Amplifier build

More details can be found below:

Hi Ale, 

I wanted to thank you for all of your PCBs.  I just finished a project that uses your Gyrators, SiC bias boards, and SiC Source Followers.  I'm so happy with the results.  Details posted on Audio Asylum.  

https://www.audioasylum.com/forums/tubediy/messages/27/276814.html


Cheers, 

Dan 

300B/845 Amp: Driver Options

For more than 6 months I’ve been playing with a lot of satisfaction my 300B amplifier with the 47 driver. Since then, I’ve made 2 modifications:

  1. Major one was the introduction of my Open-Baffle (OB) speakers. These are the “Liionidas” based on the LiiAudio C-15 and W-15 drivers. I need to do a proper write-up of these, but have to admit their sound has taken my system to another level. They are simply superb. Detail and dynamics are beyond expectations. Nothing less to be expected from the designer (Leon Huijgen) :

Initial test of the Liionidas OB speakers

  1. Second modification was minor, I removed the Zobel network from the input LL7903 to minimise loading to the source. Also, after several tests I found that the impact in the HF resonant peak is minor as the source has some resistance (below 100Ω) which is beneficial. So the 300B amplifier looks like this now:

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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.

EML300B Mesh

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 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.

46 driver in progress

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.