Many years ago I made a Single Ended amplifier that bordered on the ridiculous. It measured 650mm by 650mm (one monobloc channel) and required two men to lift it.
The text from my web site said:
"The V1505 and it's equivalent the 212E has been described as the ultimate audio valve.
An imposing 270 mm high and 90 mm in diameter it is a spectacular tube by any standards.
A valve of this quality demands preceding driver stages and power supplies of the highest calibre.
I decided that I would make absolutely no compromises in any way in the design and building of this amp. Each monoblock amplifier (two for stereo!) measures 650 mm x 650 mm.
There are 5 power transformers, 4 chokes and 5 signal-circuitry transformers on each chassis.
It is finished in hammertone silver and black and with the Ediswan V1505 valve alight is a beautiful statement of industrial art.
It certainly causes comment and wonderment from all who view it.
CIRCUITRY I chose what I believed would be the best valves for each section of the amplifier- a 3A/167(WE437) for the input stage( but recently went back to a SRPP 6SN7 ) transformer coupled to a 1930's vintage, directly heated 50 valve as the driver, transformer coupled to the V1505.
I don't know why transformer coupling sounds so good; it may be the oldest method of coupling valves, but it's re-emergence as the premier audio coupling device is long overdue. (Thanks to Mr Shishido and Mr Sakuma)
It has been suggested that transformer coupling's low harmonic distortion is partly responsible for it's sound.
The output transformer is a Japanese Tango, very expensive- very good!
The power supply uses four U19 directly heated rectifiers for the V1505 plate supply, buffered by a GZ37 in the ground rail for soft start. Another GZ37 provides voltage for the 3A/167 and 50 and a 6X4 provides adjustable bias for the V1505. Each V1505 and 50 can be kept in perfect bias with individual bias controls and meters."
It performed very well, both on the oscilloscope and to the ear, but it exhausted me building it, and it was so impracticable that I didnt build a matching monobloc for stereo.
But it was a good learning vehicle, and I resolved to one day build a more practical version.
Over the years I assembled the parts needed and I am very close to putting everything together.
First, let me explain the naming of this extraordinary amplifier.
The amp uses very ancient and extremely rare tubes that were designed for and used by both the British Royal Air Force (RAF) and the Royal Australian Air Force (RAAF) as transmitting tubes in their military aeroplanes.
Around the same era, Rolls-Royce made the monstrous 12 cylinder Merlin motor which was famously used in the Supramarine Spitfire fighter airplane which figured prominently in the Battle of Britain during the Second World War.
The Merlin motor was used in other fighter and bomber planes and was even used in some USAF fighters, including the Mustang.
Putting such a huge motor into the relatively small frame of the Spitfire really highlights the engineering expertise of that era, and explains the amazing performance of the Spitfire, due to its very high power to weight ratio.
There are very few Spitfires still flying, and likewise very few of the tubes I'm using in this amplifier are now available, so the name Merlin appeals to me .
This will be a long project , hopefully taking around 6 months to build, and I will do regular updates on the progress as I go along.
If you are interested in the truly exotic , and an amplifier that is so rare and unique that only one set of monblocs will ever be built , then please enjoy the journey.
So to start, the Merlin will be a single ended amp of around 40 watts.
Lets firstly get into a description of the tubes used .
The input tube is the STC made, Northern Electric branded 4310A . STC (Standard Telephone Company) was the British arm of Western Electric (WE) which initially used Western Electric designed tubes before they become more independent from WE.
The 4310A is the same as the WE 310A , a pentode tube used in the famous 1930's era WE91 amplifier, which used a 310A driving a WE300B triode. Fabulous tone.
This tube is so rare that its been almost impossible to even find the correct documentation for it.
VT25 is normally the military designation for the number 10 triode , a very old 1920's triode which later evolved into the 801A.
There is also a VT25A , which starts to get confusing , but these versions are well documented , and while they are getting expensive , still obtainable.
The Australian tube I'm using has a box plate much bigger than a 10/801A which suggests its an entirely different tube, desite it having a VT25 stamp on its base.
There are suggestions that it is a CV1025 . Documentation of the CV1025 claims a mu of 0.4 and Rp of 300 ohms.
This is unlikely and its possible the tube is a DET25.
Using it in circuit will determine what it is, but regardless it will make a very fine driver for the output tube, which is the also very rare Ediswan V1505.
Similar to the WE 212E , this huge tube is a high voltage triode that can output high wattage single ended sound.
The minimum plate voltage for the V1505 is 1200 volts, which is getting into the scary side of voltages, good insulation and careful design and construction will be needed to ensure safe useage.
The output transformer needs to be of the highest quality to match the tubes used, and in keeping with the rare and expensive theme , I'm using a pair of Tango X-10S output transformers.
These are designed for 211 triodes, and the 10k impedance is high for a V1505, but using an 8 ohm speaker on the transformers 16 ohm tap should be fine.
As I use high efficiency horn speakers I'm not after high power levels , and a higher load will deliver even lower distortion.
Here's a mock up of the chassis , they are built in mirror pairs so this is the left channel.
Power transformer and chokes to go alongside the output transformer.
There will be a pair of ammeters and a level control in the 3 round bevels on front panel.
Transformers and some other bits to arrive, come back in a month or so, to see how I'm going.
And in stereo.
