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Old 02-08-2018, 01:14 PM
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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.
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Old 02-08-2018, 07:09 PM
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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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Old 02-08-2018, 07:45 PM
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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
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Old 02-08-2018, 07:58 PM
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Quote:
Originally Posted by jr_tech View Post
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
I don't remember any other set using it for a damper but the first post-war GE, model 801???
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.
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Old 02-08-2018, 09:39 PM
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  • GE 801 as dieseljeep mentioned, as well as the 802, 803, and the 901 projection set
  • All of the RCA projection sets from 1947-1949 using the PTU-1 projection unit
  • Philco 48-1000 chassis (including all of the cabinet variations, etc),
  • Philco 48-2500 projection set
  • DuMont RA-101 sets in all of the cabinet variations
  • DuMont RA-102 in both cabinet variations
  • That weird blonde Remington-Rembrandt set uses it
  • And, if my memory is correct, at least one Farnsworth from that same '46-'49 period uses it.

I'm sure perusing Rider's Vol 1 would turn up others.

Last edited by benman94; 02-08-2018 at 09:42 PM.
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Old 02-08-2018, 09:43 PM
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Quote:
Originally Posted by Electronic M View Post
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).
Well I didn't mention it because RCA never officially sanctioned the 6AS7 as an OTL amp output tube. Still, it is an interesting application.

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...
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Old 02-08-2018, 09:48 PM
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This treatise
http://www.earlytelevision.org/damper.html
indicates that the grids were used to effect a a linearity adjustment.
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Old 02-08-2018, 10:02 PM
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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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Old 02-08-2018, 10:23 PM
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Quote:
Originally Posted by benman94 View Post
Well I didn't mention it because RCA never officially sanctioned the 6AS7 as an OTL amp output tube. Still, it is an interesting application.

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...
One of the RCA tube manuals does give a 6AS7 OTL schematic/parts list...

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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Old 02-08-2018, 10:33 PM
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Quote:
Originally Posted by old_tv_nut View Post
This treatise
http://www.earlytelevision.org/damper.html
indicates that the grids were used to effect a a linearity adjustment.
Interesting reading! I also looked up several of the sets mentioned above and indeed see the grids in a linearly adjustment circuit... I have never encountered a set with this linearity control configuration. Thanks for posting!

jr
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Old 02-09-2018, 08:51 AM
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Quote:
Originally Posted by Electronic M View Post
One of the RCA tube manuals does give a 6AS7 OTL schematic/parts list...
They show a transformer coupled 6AS7 amplifier in RC-16. Mind sharing which tube manual shows it in the schematic section? I highly suspect it is just the Dickie and Macovski circuit I mentioned. D.P. Dickie worked for either RCA or Pacific Mercury out on the West coast. My Uncle Harry golfed with him.

I only ever recall seeing that circuit in Audio Engineering and Orr's handbook however.
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Old 02-09-2018, 08:46 PM
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Quote:
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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.
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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Old 02-09-2018, 09:04 PM
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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...
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Old 02-12-2018, 05:07 PM
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Well to start they are both totally different tubes. The as7 is a simple low mu twin triode while the 6L6 is a high mu and high power pentode capable of 30 watts of plate dissipation (on a single plate) while the as7 only dissipates a grand total of 26 watts (keep in mind 13 per plate). The as7 is a lower power tube and is not nearly as efficient when used as an audio amp tube. For example, running a 6AS7 in a class A1 amp only gives an amplification factor of 2 while a 6L6 in the same setup has an amp factor of 8. We must take into account that a single pentode 6l6 puts out more than double the amplification factor than using two triodes thus making the 6l6 a more efficient and powerful tube with out the manufacturing costs of two separate triodes in one tube. RCA intended for this tube to be used in lower power circuits and had high power audio triodes such as the 6sn7gtb's which at less plate and screen voltage have an amp factor of 20 in a class A1 amp which is perfect for audio use. The as7 was never really meant to be used for audio circuits, its main use was as a series regulator in scopes and other test equipment such as signal generators ans supplies. Nowadays the audio fools see a nice big twin triode and automatically assume it would be great for their amps, none really do their research and look to see if this tube is really ideal as an amp tube.
Just my 2 cents....or more
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Old 02-12-2018, 06:32 PM
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Originally Posted by SwizzyMan View Post
Well to start they are both totally different tubes. The as7 is a simple low mu twin triode while the 6L6 is a high mu and high power pentode capable of 30 watts of plate dissipation (on a single plate) while the as7 only dissipates a grand total of 26 watts (keep in mind 13 per plate). The as7 is a lower power tube and is not nearly as efficient when used as an audio amp tube. For example, running a 6AS7 in a class A1 amp only gives an amplification factor of 2 while a 6L6 in the same setup has an amp factor of 8. We must take into account that a single pentode 6l6 puts out more than double the amplification factor than using two triodes thus making the 6l6 a more efficient and powerful tube with out the manufacturing costs of two separate triodes in one tube. RCA intended for this tube to be used in lower power circuits and had high power audio triodes such as the 6sn7gtb's which at less plate and screen voltage have an amp factor of 20 in a class A1 amp which is perfect for audio use. The as7 was never really meant to be used for audio circuits, its main use was as a series regulator in scopes and other test equipment such as signal generators ans supplies. Nowadays the audio fools see a nice big twin triode and automatically assume it would be great for their amps, none really do their research and look to see if this tube is really ideal as an amp tube.
Just my 2 cents....or more
I'm well aware of the fact that the 6L6 is a beam power tube and the 6AS7G is a dual power triode.

If we directly compare a cathode biased pair of 6L6s and a single 6L6 working in push pull with itself, the difference between the valves is not as terrible as you seem to suggest. Assume we want an output stage of approximately 10 clean watts. Assume the 6L6s are cathode biased with a plate voltage of 266 volts, a screen voltage of 266 volts, and 16 volts are being dropped across the bias resistor(s). For zero signal, the total screen and plate current is 130 mA, meaning you're drawing 34.58 watts from your 266 volt rail. Assume the 6AS7G is in push pull with itself with a plate voltage of 375 volts and 125 volts are being dropped across the cathode resistors. For the zero signal condition, you would be drawing 100 mA from the 375 volt rail, or 37.5 watts. That's only a difference of 2.92 watts. So in terms of plate/screen dissipation, you really aren't gaining much by opting to use the 6L6s.

Now let's consider the max signal condition:
For the 6L6s, total screen and plate draw from the 266 volt rail is 145 mA, or 38.57 watts. For the 6AS7G, total plate draw is 106 mA from the 375 volt rail, or 39.75 watts. Now the difference is only 1.18 watts. The 6L6 now looks even less attractive.

The 6L6s are theoretically giving about 13 watts, the 6AS7G is theoretically giving about 11. Both stages at 2% THD. The total heater current drawn by the 6L6s is 1.8 amps at 6.3 volts. The 6AS7G is drawing 2.5 amps. So you're giving up another 4.41 watts by using the 6AS7G over the 6L6s.

So what are you getting by using the 6AS7G? More benign distortion characteristics, and MUCH better damping factor. In fact, a 6AS7G in push pull with itself has a plate resistance far lower than the 2A3. That's impressive as hell. The plate resistance of a 6L6 is 22,500 ohms. If I'm designing a Hi-Fi amplifier, you better believe I'd trade away 1-3 watts of B+ draw, and another 4.4 watts of heater power, for excellent damping factor and intermodulation distortion that won't make one's ears bleed. If you had bothered to peruse the articles I had posted from RCA's Ham Tips, you would have found all of these advantages laid out in black and white by J.H. Owens, editor of Ham Tips, and D.P. Heacock of RCA's Tube Application Engineering Group. In fact, it was the incredibly low plate resistance that was exploited to make a better driver for class B modulators.

This all comes at a cost of requiring an ungodly drive voltage. The 6L6 needs ~36 volts peak grid to grid, but the 6AS7G needs ~250 volts peak grid to grid. That said, this is a small price to pay, and with such a high allowable grid leak resistance, a consequence of the design of the valve, you could probably coax a 6SL7 driver stage R-C coupled to the grids of the 6AS7 into swinging the required drive voltage. A mu of 2 isn't so terrible for an output triode. The venerable 2A3 has a mu of 4.2.

I really don't think you understand the concept of amplification factor. First of all, the amplification factor you quoted of 8 for a 6L6 is for when the valve is triode strapped. It is much higher when operated as a normal beam output tube. Output devices tend to have relatively low amplification factors. You aren't trying to get copious amounts of gain from the output stage; you can always get more gain from a 12AX7 or 6SF5 or 6SL7 gain stage out front. You're trying to develop a signal of high enough voltage at sufficient current on the primary side of the output transformer so that you can drive a 16 or 8 or 4 ohm voice coil. Consider a 16 ohm voice coil being driven by 10 watts. You need ~8.9 volts RMS at ~1.1 amps. The total power delivered by the device is not correlated with its amplification factor. If it were, we'd all be using 6SF5 single ended triode amps

As far as being used as a pass element in power supplies goes, yes, the 6AS7G was used in such capacity, but it was not the main goal in producing the valve. In fact, it is a rather marginal pass element. In some cases a 5881 or 6Y6G was used in pass service instead of the 6AS7G. In any case, the 6AS7G, like the 2A3 which preceded it in pass service, was rather quickly discarded in favour of even higher perveance valves.

The valve was built originally for TV damper service, this is known fact. This is what my original post was about, and indeed it was the point of this thread. The audiophiles are using it, and for a damn good reason too. Perhaps in time you'll figure out what it is.

Last edited by benman94; 02-12-2018 at 06:37 PM.
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