Saturday, 18 February 2023

The Lineage - essential SET amplifier development

We needed a good single ended triode amplifier to demonstrate what our speakers are capable of and so it was time to use the development concepts from the decades of triode amplifier building that had been sitting on the shelf for some years... The concept was a distillation of essentials. These essentials were not an economisation but a focus toward the purpose of fidelity. To go where a 300B simply cannot.

Once a tube spelunker pokes around enough chasing the darkness of convention and hugely expensive magazine touted amplifiers one discovers that power sucks - the nuance out of playback, that transmitter tubes are dogs with high miller capacitance, huge transformer losses and require extra stages just to drive them and it becomes clear that high efficiency speakers and very low power amplifiers can do what nothing else does - portray the low level detail which preserves the gestalt* of a musical event and makes music something quite special to replay and enjoy. It takes courage to stop watching the candle shadows on the cave wall and emerge into the wild sunshine and I am grateful to have been brought up with live music - a bullshit barometer - which provided no choice but to emerge.

The 4P1L has been one of my favourite tubes for some time after replacing the 45 and 46 tubes which got me into SETs to begin with decades ago. After competing with tube collectors and hoarders it was just too much fun to find a tube that was equivalent of the 45/46 in low level detail and ambient aura that was overlooked, abundant and inexpensive. It is a direct heated pentode, a Russian copy of the RL2 / 4P6 Wehrmacht tube on which there is no information in the old German tube lexicons - they are listed there but with no information. Top Secret? I was curious how it would do in parallel single ended triode operation having used it for years Sakuma stye amp, 4P1L interstage 4P1L, in straight single ended configuration. The 2P29L was chosen as initial drive tube keeping the eventual, almost century old, 226 globe tubes in reserve until after the amp was stable. This tube is also a direct heated pentode clone, this time of the CO-257 direct heated pentode, also used in triode mode.

So what does this design do different? All DHT, low power = small transformers, two stages = low losses. An abundance of inductance in the PSU, some of which is in the ground leg a la CMC. Attenuators are not at the input but used later to increase current drive and slew rate. No cables to drive avoiding transformer and cables losses in the critical section between stages. Schottky, choke loaded supplies for Rod Coleman's excellent cathode regulators and yes, these were tried with traditional EI AC transformers before finding toroids sounded better, filament bias, and overall a plethora of iron - more than any theorist/engineer would endorse.

This highlights the key element of testing with ears and trusting what they indicate. Music is a series of dynamics, shock waves, over a broad spectrum of frequencies and cannot be measured with single frequency tones or square waves. Another note is that there are no politics or economy in parts selection: if it works - its stays, if it don't - it goes not matter the expense or theoretical attachment... Measurements bow to the human factor which is what music is from and for.

Many of these concepts were derived from the trifecta of collaboration over decades between Peter Fowler, Bill Jaensch and myself: we three geezers.

On to the build:

Parts collection initiated.


Shuffling parts around for layout design. As one can see here the henries of the PSU, Ale's SiC bias boards and Dave's most excellent attenuators.

The beginning of tie down.

Wiring initiated with ground bus.

Provisional power supply...

Clip leads are fun! Here beginning with Ale's gyrator plate load. 

Then on to amorphous core choke plate loads from NP ... and the next iteration of the PSU.

The beauty of breadboards is that a new device or circuit alteration can be changed in minutes.

I became rather fond of the little filament PSU sculptures almost naming them after movie robots... before going to choke loading for the filament supplies. 

This provisional supply used a high voltage Schottky bridge for the B+ which eventually became a mercury vapour rectifier tube half wave. The damper diode is in the ground leg replaces a SS slow start PCB - not only to provide slow ramp up of the B+ but it does more as per JC Morrison's inspirations.

After almost a year on the breadboard with many, many iterations this was the final configuration of the circuit that became preferred. All six supplies are now fully choke loaded with toroids and R cores and the B+ went to mercury vapour rectifier. Nothing, not even TV damper diodes come close to what a MVR sounds like. Yes, I have heard the nonsense that one cannot hear the rectifier in a properly designed PSU. That tidbit of wisdom is for people who do not have good ears IMHO. Hybrid rectification need not apply as they sound predominately like its weaker links. 

The driver section "evolved" to conventional interstage. The "natural motion" of a transformer plate load - "reactor" - as the old text books describe it - sounds the most natural to my ears all theorising aside. Also plastic PP caps and electrolytic have begun to be been minimised and the favourite GE97F series oil caps beginning to populate filtration.

Dennis Boyle's transformers (Ace Amplifer) and interstage are used up to this point with
M-6 Orthosil@ 29 gauge .014” thick laminations. He tested cores of Orthosil@ Super Orthosil@, Permasil@, 40% and 60% Nickel in .009”, 0.10” .014” and .185” lamination thicknesses in both power and audio signal transformers. His findings were that the M-6 .014” EI cores offered the lowest distortion with a wide bandwidth. This was measured and they sound surprisingly good.

That said, Finemet and Dave Slagle's 49% Nickel come in later and clearly do some things that M6 does not. More to follow as we discover some nice things about nickel that the ear and nervous system can easily and pleasantly discern. 

A few more posts on this amplifier will follow - including pictures with less spaghetti...

 * gestalt -

  1. A physical, biological, psychological, or symbolic configuration or pattern of elements so unified as a whole that its properties cannot be derived from a simple summation of its parts.
  1. A collection of physical, biological, psychological or symbolicentities that creates a unifiedconcept, configuration or pattern which is greater than the sum of its parts (of a character, personality, or being)
  1. a shape, a form

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