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Why was the 6AS7 dropped as a TV damper?
The 6AS7 ultimately ended up being a tube without a purpose after TV manufacturers (including RCA) stopped using it as a TV damper, after RCA stopped promoting it as an audio output valve to which it was very well suited (arguably better for high fidelity applications than the 6L6), and stopped being used as a pass element in regulated power supplies.
It was in all probability shunned by RCA for audio use because it would directly compete with the 6L6 which RCA had been promoting since 1936. Still, a push pull 6AS7G with cathode bias could produce 10 watts or so at only 2 percent total harmonic distortion, right in line with a pair of cathode biased 6L6s while remaining about as efficient as a pair of 2A3s, 6B4Gs, or 6A5Gs. Add in that the distortion characteristics of a triode are much more benign, and the plate resistance so much lower, than the 6L6 and the 6AS7G looks rather attractive by comparison. The grid swing necessary is ungodly, but it is possible to drive a 6AS7 in push pull with itself to clipping with a single 6SN7 R-C coupled to the 6AS7 grids. Take a look at this schematic from RCA's Ham Tips, circa September-October of 1948: http://n4trb.com/AmateurRadio/RCA_Ha...amtips0803.pdf Also see this speech amplifier developed in the year prior: http://n4trb.com/AmateurRadio/RCA_Ha...amtips0704.pdf Clearly the 6AS7 works admirably in audio service; RCA didn't abandon it as an audio tube for any practical reason relating to the valve itself. In pass element service, the 6AS7 was simply surpassed by even higher perveance tubes. Why use two or three 6AS7s when one of the newer types will suffice? But in TV damper service, I see no logical reason to choose a 5V4G or any of the dedicated 6 volt damper diodes over the 6AS7. Any ideas? Last edited by benman94; 02-08-2018 at 01:29 PM. |
#2
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Price perhaps? There was a good bit more metal on the plates than your standard 5V4...I have a hard time imagining a 6AS7 as being cheaper.
Also, your forgetting IMO the most interesting audio application of the part: Output transformerless Amplification. I designed/built an OTL amp based on those tubes several years ago...It has been my main amp ever since (it is that good).
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#3
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What sets used the 6AS7G as a damper? I would like to see if it was just connected as a diode or if the grid control actually served some purpose.
Sure, a diode or double diode would be cheaper to fabricate than a double triode. Bean counters usually win. jr |
#4
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IIRC, the RCA 648*** projection set used one as a damper. I just scrapped a mid-60's Techtronics that had one. I is a "G" type branded RCA, with a mid-60's code date. |
#5
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How stable is your OTL? I've toyed with the idea of building one, a variation of the Dickie and Macovski circuit from 1954, but I really don't like the idea of running the 6AS7s fixed bias, nor do I like the idea of having B+ at the speaker terminals... |
Audiokarma |
#6
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I went with a reverse Futterman circuit with some negative feedback in the driver stage. With proper adjustment, I was able to achieve almost no DC voltage across the speaker terminals (someday I may rework the bais adjustment circuit to require less finesse). After a year of heavy service, I rechecked the bias balance and tube emission and there did not seem to be a change. My amp is a 2/4 channel with the rear outputs AC drive path and feedback switched over to parallel with the front when in stereo mode. I was very conservative with DC quiescent current in the outputs...I expect output tube life to be close to heater life. My output rails are only +/-60V so there is not much more risk to the speakers in the event of a rail short than some transistor amps. I built that amp during a summer break in college when buying the outputs at $5 a pop, the chassis sheet metal, wood, and pots (most everything else was from my junk box) was a great stretch of my budget. The power supply evolved with the amp...Eventually utilizing SS regulators for nearly all B+ and B- rails. I could not afford the right power trans so I engineered around the junk box transformers I had....That box has every valid internal mounting point utilized to it's utmost....Probably weighs as much as an RCA 630.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#7
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I only ever recall seeing that circuit in Audio Engineering and Orr's handbook however. |
#8
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With +60/-60 rail voltages I would feel a hell of a lot better about such a topology. The one 6AS7G amp. I'm familiar with uses +140/-140 volt rails. That makes me nervous and for good reason. It also calls for a potentially lethal hot chassis design. No power transformer, no output transformer, hell, no inductors at all! A bit scary to have something like that plugged right into the wall... |
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OTLs are IMO the best of both worlds in amps; combining and balancing the redeeming traits of the characteristic sounds of tube and SS gear.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#10
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I like Tom's idea , the cheaper the better for the Bean counters and a kinda big triode VS a smaller diode wouldn't fly . While the TV's screens themselves kept getting bigger , smaller & cheaper circuitry was the driving force behind such design changes since the beginning of mass market electronics . Yep , price gets my vote .
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Audiokarma |
#11
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I'm sure perusing Rider's Vol 1 would turn up others. Last edited by benman94; 02-08-2018 at 09:42 PM. |
#12
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This treatise
http://www.earlytelevision.org/damper.html indicates that the grids were used to effect a a linearity adjustment. |
#13
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jr |
#14
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I also wonder if it was dropped simply because greater inverse plate voltage and peak current were needed as screens got larger.
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#15
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Price comes down with increased manufacturing. It is possible that the 6AS7G could have dropped in price if RCA had cranked them out in greater quantity. The 6L6 was initially a somewhat expensive tube and was notoriously difficult to built in production quantities. RCA worked out the kinks and by the late 40s it was about $0.50 to $1.00.
Of course ramping up production on the 6AS7G would have required greater demand, which would have required manufacturers, or an insane number of amateurs, to actually use it. It seems to me the 6AS7G was the right tube, at the wrong time, or rather, the wrong price. As far as audio use goes, RCA missed a golden opportunity to lock down the Hi-Fi market early on by really pushing that 10 watter they had designed. If they had marketed that design and the 6AS7G in the same manner that the Williamson and KT66 were promoted in England and Australia, and subsequently the US, I strongly suspect the Williamson wouldn't have been nearly so popular in this country. But RCA was a corporate giant interested in selling radios and television sets. While they were competent at building excellent Hi-Fi equipment, it wasn't their bread and butter, and certainly wasn't on their radar in any real sense... |
Audiokarma |
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