300B SE Amp – here we go

A Brave New World

Surely you’re as tired as I’m with COVID-19. One of the best things I can do to distract my mind is to keep myself away from social media. Every stone you turn, there is COVID or a statement about it. I won’t moan as I have a job for now and a healthy family. Some members of my family were infected but nothing major. I can only say is that the world has change. And so my day to day life looking after the young family whilst working is a real challenge. Starting my fifth week of lockdown, I have to distract somehow my mind at times, otherwise will go mad.

A New Concept

Last year at ETF.19 I was discussing with my friend Pete the challenge I had around finding time to build audio gear these days due to the family and business demands and also how much I hated metal work and never been good at (nor had the space for) woodworking. So historically had to rely on other people to build me a nice wood case for my amps and learnt how to design and make my own ultra high-quality top plates, which are expensive so not for a prototype at all. Anyhow, I mentioned that I could only work very fast in building gear when I used the prototype approach over the Ikea chopping boards, an idea I borrowed from my friend DHT Rob.

As a result of that conversation, Pete put me in contact with his friend Simon Mears who is brilliant at woodworking. Check his website when you have some time. Simon is an amazing character, very creative and open to help and contribute with ideas. Sadly never managed to pop around his place to listen to his horns due to the lockdown but is on my to-do-list for when this is over.

I needed to make a cabinet to fit the IKEA chopping boards and hide them from my wife and kids. The cabinet has some rear ventilation and holes for wiring mains and output cables:

The previous picture shows my initial test using the Headphone Amplifier. I can slot at least 4 boards and have some additional supports made for adding extra boards as needed. There should be plenty of space.

The 300B Amp

Ok, why 300B? I had a previous one which I loved and want to get back to it for the simple reason that I want to experiment with different output stages and will start with it.

In principle the design is as follows conceptually:

1. DHT output stage with a (huge) nano-crystalline core transformer.
   1. The transformers are massive and may push me to implement this design in mono-blocks. The are 3K3/8R or 4R. 12W output at 25Hz and can do 5 to 80kHz (-3dB) for 800R source impedance. I will take a picture and post it when I get them out of the storage are in my loft.
   2. Fixed bias with +50 and -200 to -300V supply
   3. Source follower DC coupling
   4. Flexible driver
      1. D3a
      2. 46 with input SUT
      3. Other valves to try!

As the amplifier is modular, I can reuse the power supplies and create different amplifiers easily. This is what I’ve been doing for years with the DHT preamps and allowed me to test multiple versions very quickly.

Here is a view on my first chopping board for the fixed bias supply and 300B filament supplies. I’m in COVID-19 reuse mode so I’m making this amplifier with everything I can find in the workshop. Luckily have some multi-tapped  power transformers made to order by JMS. All with split bobbing and copper screening. They also have multi-taps in the secondary for voltage adjustment and trimming:

The second board will hold an HT supply and another set of filament supplies intended for the driver. The HT supply uses my PCB design for a hybrid rectifier, film cap input and an LCLCC output. Last capacitor in the chain is a WIMA DC-Link 45uF one. Kemet electrolytic caps and a pair of 15H/200mA chokes made many moons ago by Ogonowski. The HT mains transformer is from Thomas Mayer and the 100VA split bobbin filament transformers are from JMS as well. I used them in my 45/46 SE and my 6C4C push-pull amps years ago:

I can only spend about 30min to 1 hour over the weekend when I get a break from the kids. In that time I can quickly do the drilling and get all the mounting work done. I use plastic hex standoff which I drill partially the board (i.e. only 5mm deep, not all the way through the board) with a 2.6mm drill bit. I then use an M3 tapping tool and a bit of oil to allow self tapping the plastic hex standoff without getting it damaged and ensuring a perfect locking. I then reuse whatever standoff I have at hand (brass, plated, plastic) to mount the Speakon connectors which I use for wiring power to the other boards. Also made recently some PCBs to hold octal sockets for the valve rectifier, in-rush surge protector / HT delay, etc.

Enough for now, it’s been long since I devoted a bit of time to this blog but was worth sharing some of the slow project on this amp. Surely some of you may find it inspiring of ideas and may share some of your experience.

Keeping up with the request from readers for help isn’t easy. There is a lot of people out there trying my designs and building several preamps which is very encouraging and rewarding. Happy to be able to share what I learn myself and help others.

For the ones out there who have now more time than usual, please build build and build and then listen, listen and have an audio overdose.

For everyone, keep safe.