This is the end of the beginning. Today is the last article of our topic: power amps (part one). Yes, there will be a part two, with plenty of interesting action, later in this series. But for now let us have look at sound practices issue 10, to CES Report by Joe Roberts, and in the same issue, State of the Art: 1935, also selected by Joe.
In the CES article, Joe first describes how tube were back in vogue in 1996 and it is not the retarded mainstream circuits going around since the 70s. Next there is a bit about Marantz reissues. Then things start to get interesting. In quick succession Joe introduces the Marantz T-1; the works of Nobu Shishido; the Plenteous amplifier. All with schematics to put our teeth into. Then things simmer down again with some product descriptions.
teaser quote: ‘The T-1 reflects a vision of audiomaniac splendor germinated within the upper strata of the Japanese experimenter community where the quest for good sound is not constrained by narrow concepts of “up-to-date” design practice.’
‘State of the Art: 1935’ really says it all. It consists of 2 pages from transformer catalogues: one with eight circuits from Thordarson, one complete 2A3 circuit from UTC.
bonus tracks
More transformer coupled action in issue 1 (!) on the third page of Meet the Tube: the mighty 845. And a nice selection of Shishido circuits can be found online.
my take
Two issues before last week’s article, Joe puts out what could be called ‘the transformer issue,’ the first demonstration of transformer coupling in sound practices on a grand scale. As I described last week, it was the sign of the times. It is of course no coincidence that the 1935 material gets reproduced, it connects the dots, back to a time when transformer coupling was the normal way to achieve hi-spec results—even if you compensate for the bias of a transformer manufacturer. What is remarkable about the three amps described in the the CES article is that a) they are/were products and b) they are not retro.
The T-1 amp has of course a lot in common with the UTC schematics in issue 1 and 10, but the tube line-up is one of a modern, inspired triode-lover. The current sources for the filaments and the microprocessor control solidly make it a 1996 amplifier. Note that different from the UTC plan, in the first stage of this amp the 300Bs are loaded by plate resistors which is bridge loaded by the interstage transformer, with a cap to block DC. There is no feedback in this amp, although it looks like a product that was destined for the high-end salon, coupled to some wholly unreasonable high-end speakers with complex cross-overs.
The works of Shishido really have nothing to do with the past. They inspire to stop being afraid of the zero-volt grid line and explore class A2 territory, with its higher currents, lower plate voltages and lower output transformer impedances. They have lead me to my 814 amp and A2 operating points. Some notes to go with the schematic: the output tube is pulling grid current at its DC operating point and this offsets in part the DC plate current of the driver in the interstage transformer, this is the trick behind IIT; I have never found out on the internet why Shishido used voltage doublers so often in his power supplies; however it is known that he used a LM317 for the bias supply out of sheer convenience; the negative feedback (both of them, X and Y) is necessary to get some damping factor out of this amp, loading a 5K plate impedance tube with 1K5 is not going to give you any.
The Plentious supports the modern audiophile sport of tube rolling. Also unusual is the cascade of two transformers between the stages. The first one is the easiest to understand for me. A 600 Ohm volume control sounds so much better than a 10K—let alone a 100K or 1Meg one—if you can drive it. So even without the direct line input, it makes sense to use a step-down on the 3A/167M tube to drive the attenuator, then use another transformer to step up the signal to the grid of the 46. For the second instance I can only think of that they split the job of handling DC (plate transformer on the 46) from the job of stepping up or down (grid transformer on the output tube). But then the transformer on the 3A/167M handles DC and steps down—it must.
pop quiz
What are the mystery resistors in the bias supply of the Plenteous? We can see from the numbers that the 2K pot always adjusts a range of 25V, so always 12.5mA is flowing through it. -135 Volts is on the 25µF cap, so the resistors always form 135 / 12.5 = 10K8 bias supply resistance. In the middle of the biassing settings this gives worst-case 5K DCR for the bias supply. Whereas last week we saw that the bias supply was low-DCR, in line with the transformer DCR, this one is not.
That is it for power amps (part one). Next week we will start having a look at the big picture.
Now go and read the article, see you next week.
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