OK, next up is the Dinosaur article from sound practices issue 5, written by jc morrison. In it jc introduces the hi-fi interzone and what it means to him, segueing into the all-american mainstream audio world vs. counter-culture. Then starting with what he tried to achieve for himself, he moves on to the trials and tribulations of designing a driver stage for the 50 tube. This is followed by observations about what makes the 50 special in the scheme of things of directly heated triodes and the actual circuit description. jc finishes the article with six guidelines that define his way and invite further mulling.
teaser quote: ‘Hi-Fi always involves a mixing of “separate” worlds: the practices of the creative artist (the musician and the audio software) and the [sic —ps] those of the interpreter (the listener and the audio hardware). This is especially interesting thing to mull over if you are a designer of audio reproduction systems. In the lingo of the cyber-psychoanalytical-pop culture we love so much, audio design is an interzone.’
bonus tracks
Check out sound practices issue 2 (page 4 and 6) for the schematics of the Kondo Ongaku and Neiro amplifiers that jc refers to. And issue 6, the Tuned In! article also by jc, where you can find a ‘New Dinosaur!’ with some additional DHT characterisations and Dinosaur development notes at the top of the third column of the text page.
my take
This article is about delicate balancing acts: between art and technology; between ancient and contemporary technology; between accuracy and elegance; between what you want to achieve and what you have on hand to get there.
If there is something that I took away from this article it is that it is all about attitude. Being part of a counter-culture and ignoring conventional recipes because you are not looking for conventional results. Using what you got (transformers, chassis—down to the number of holes in it, tubes) but taking no prisoners when it comes to the actual design: you gotta love the architecture and the detailed solutions of your audio circuit, and it has to sound the way you want it to.
Another personal experience is that the moment I tried out the 10 as a driver tube, I knew exactly what jc meant with ‘its utter respect for ambiance, nuance and detail.’ It is quite something to behold and I love it. Furthermore, guideline number 4 has been incredibly useful to me: ‘Use the smallest possible component values.’ This goes exactly against the Audio Amateur grain of simply beefing everything up and it moves the balance of power from the black boxes to the music.
Last, from the update in SP6: builders finish an amp, but designers are always moving on. This can be very hard to swallow for some folks, who want a published design to be definitive, so that they just have to solder it together and presto: the search for perfection is over. Sorry, if you have that audio designer streak then it is just a matter of time before that next thought/architecture/topology/twist/goal comes along and it has to be tried out, on the current champ.
pop quiz
So reading about the requirement for the 50 tube (see datasheet reproduced in the article) that the resistance in the grid circuit cannot be more than 10 kilo Ohm (think DC, and bias stability), what is the value of that in the Dinosaur?
In issue 5, the kathode of the 50 is connected directly to earth through the centre tap of the filament winding. This is then the reference point that the grid of the 50 has to be connected to with less than 10k DCR. Both the kathode of the driving tube (through the main power supply) and the 1k2 +47k under it (via the negative supply) do that job. Starting with the kathode follower, there is the 500 and 15k dropping resistors in its power supply (the DCR of the 2H choke, the 6DA4 and the diodes are not significant compared to that) plus the—say—7k of the 6SN7 plate resistance. That combined (22k5) gets reduced by a factor µ+1 by the kathode follower: 22k5 / 21 = 1k07, say 1 kilo Ohm. Compared to that the 1k2 + 47k + 5k + 1k5 of the negative supply route will not contribute anything, so a reassuring 1 kilo Ohm is the answer here.
In issue 6, the bottom of the 1 Ohm resistor under the 50 is the reference point. Here an 18k resistor and the driving 6SN7 share the duty of holding the grid stable with low impedance. The 6SN7 route has 7k of plate resistance + (µ + 1) * kathode bias resistor (21 * 1k) which gives 28k. Now both these routes work in parallel to control the grid, 18k//28k is 11 kilo Ohm, juuust small enough you would say. That is if there was not the driver power supply at the bottom of the schematic. Both our routes connect through the driver power supply to our reference point. Because in that power supply both high voltage and the ‘earth’ reference are interconnected for DC through the diodes and the power transformer secondary, the 36k and 47k resistors in the supply are—paralleled—in series with the 11k above. So the 50 grid sees 11k + 36k//47k (= 20k4) which gives 31k4. And that is 3 times more than the maximum (if I did my sums right…)
Now go and read the article, see you next week.
ps: I forgot to write that there is in issue 6, on page 18 (Orfeo 211 A) a schematic of a 1971 Uesugi circuit with the same ‘Professor Morikawa’ topology as the original Dinosaur and Ongaku.
pps: I knew it I read it somewhere in SP and now I found it: Meet The Tube, The elusive Type 50 (issue 12, page 27). Great little backgrounder on this dinosaur tube.
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