I first met Simon Shilton of SJS Electroacoustics at an audio fair way back in late 1996 and we got chatting generally about things thermionic. At that time I was considering building a valve amp, as I had heard how good they sound myself, and being technically biased (and wanting to save money),I decided this would be the ideal approach.
Simon's cheerful and helpful disposition stayed with me and, armed with some info from him, I went home to mull over the task that lay ahead. After some phone calls to Simon, he sent me a circuit diagram which he said would fit my requirements. I had decided on a 300B Single Ended triode amplifier as, I reasoned, I may as well go for the best within my budget. Simons circuit is based on his design published in his website.
The circuit comprises:
- a GZ37 regulated power supply feeding a 10H choke and Elna capacitors.
- The signal path is through an Elna rotary switch which gives me inputs for CD, tuner, tape and aux.
- Volume is controlled by a Panasonic 10 K log pot.
- EC8010 input valves are used, these are special quality UHF triodes and give a gain of 60.
- Rather than the normal capacitor coupling, Simon suggested transformer coupling direct to the grid of the 300B's for a superior signal transfer.
- Resistors are all either Brown Devils or Holco, capacitors by Black Gate
- A matched pair of Golden Dragon 300B's complete the line up
As far as actually building the amp goes, SJS supplied almost all of the hardware, which saved me sourcing from different suppliers. A full sized chassis layout plan enabled me to get a local engineering company to fabricate the chassis from aluminium, welded and ground corners give plenty of strength as this is one heavy amp! I just had to finish off some holes and do a bit of filing here and there. I settled on gloss black after applying an etch primer.
Everything went well during construction with no mishaps along the way. I used stainless steel cap screws throughout to complement the chrome transformer shrouds.
Some comprehensive start up and testing notes that Simon sent me enabled the most nerve wracking part to go relatively well. No smoke or sparks bode well, so I proceeded to hook up to the speakers, again home built and high efficiency (94dB) to match the amps 9W output.
Initially I had connected the volume pot up incorrectly, as a blast of full volume had me reaching for the main switch. After giving it a rest for the evening and not wanting to rush things, I resumed work on the amp the next morning and was soon playing music!
Everything that one reads/hears about 300B SE amps are true, the midrange is especially sweet and clear, bass can really kick and its surprising how LOUD 9 watts can sound!
I have been enjoying the amp for over two years now and have just obtained a pair of Svetlana 300B's from Amplino in Holland as I could not find any in the UK, I havn't installed them yet but will do so shortly as a move to Singapore got in the way.
Steve Puttock 26 July 1999
Note: Svetlana 300Bs are now available from Border Patrol