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 Post subject: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Thu 08, 2018 8:21 pm 
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Joined: Apr Tue 17, 2012 7:58 pm
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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 from 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?

(I put this in the vintage television forum because my question is regarding the valve's use as a damper, if there is a more appropriate forum, the moderators have my apologies. Please move it.)


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Thu 08, 2018 9:04 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Lafayette, CO
With the 6AS7, you have grids whose bias can be timed. It makes for that elusive perfect linearity. Dig up a schematic for the GE 901 (a little known apex in American technology of the 1940's). It also uses a 5V4. And the "boost" supply comes from the b+, not the dampers. No 6L6's in that set...maybe GE didn't like RCA's proprietary parts. Craig


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Fri 09, 2018 5:49 am 
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Location: pensacola fl
Hi all. Quit possibly the fact that the tube has a high heater requirement so cost is higher with bigger transformer. Also diode circuit simpler and cheaper. Other components also cheaper like flybacks. The 6AS7 therefore does not lend itself to series heater strings.


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Sat 10, 2018 1:17 am 
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Location: Redlands CA
What sets used this tube? I've never encountered it.


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Sat 10, 2018 2:14 am 
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Location: Arlington, TX, USA
I'm pretty sure the RCA projection sets (8PCS41,etc.) used the 6AS7.


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Sat 10, 2018 2:38 am 
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Okay, I have one of those projection set but didn't pay much attention to what damper was in it.


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Sat 10, 2018 4:22 am 
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What chassis used a triode when a diode was called for?
The 6AS7G doesn't have the heater-cathode breakdown voltage to give it any benefit over, say, a 5V4 or similar, which required its own very-well-insulated heater winding.

6AX4 and the later 6AU4 addressed this issue in the octal domain.


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Sat 10, 2018 4:24 am 
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A 6AS7 was used as a damper in the Dumont RA-102 (https://antiqueradio.org/DuMontRA-102Cl ... vision.htm).

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Sat 10, 2018 5:15 am 
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Here is an incomplete list of some of the sets known to use a 6AS7G damper:

GE 801 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 from '49-'50
And, if my memory is correct, at least one Farnsworth from that same '46-'49 period uses it.

I'm sure a careful search through Vols. 1 and 2 of Rider's would turn up more.

According to this treatise written by Hugo Holden, and brought to my attention by Wayne Bretl, the use of the 6AS7G allows for additional control over the horizontal linearity. This seems a worthy enough improvement to warrant the use of the 6AS7G in damper service to me, but I suppose that at the end of the day the answer is that the bean counters won out. Why use a relatively expensive dual triode where a rectifier already in widespread use would work?


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Sat 10, 2018 5:28 am 
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The big ugly projection United States Television Corp. sets commonly used in bars, restaurants, and in at least one case, a funeral home, also used the 6AS7G in damper service.


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Sun 11, 2018 1:58 am 
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Thanks guys, I just learned something. We don't have anything that old where I come from!

It seems to me it could also have been used to stabilise the HV and width, in a similar way to the widely-copied Philips 110 degree design using a VDR that first appeared in about 1958.


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 Post subject: Re: Why was the 6AS7 dropped as a TV damper?
PostPosted: Feb Sun 11, 2018 4:42 am 
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Joined: May Sun 07, 2017 11:35 am
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Well there you go.....!
Attachment:
6AS7 as damper.jpg
6AS7 as damper.jpg [ 74.56 KiB | Viewed 913 times ]


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