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?

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).
https://farm5.staticflickr.com/4609/...189effce_n.jpg

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. :scratch2:

Sure, a diode or double diode would be cheaper to fabricate than a double triode. Bean counters usually win. :thumbsdn:

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. :scratch2:
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.

• 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.

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...

This treatise
http://www.earlytelevision.org/damper.html
indicates that the grids were used to effect a a linearity adjustment.

I also wonder if it was dropped simply because greater inverse plate voltage and peak current were needed as screens got larger.

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.

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

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.

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 .

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...

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 :lmao:

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.

You've inspired me to try my hand at a Futterman style OTL. It would be nice to stop forking over so much money to Brian Sowter at Sowter Transformer...

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...

The output's purpose has usually been more focused on efficiently coupling the needed power to the speaker instead of raw signal gain....If it weren't then there would be no preamp/driver stage tubes in all the amps made, and all one would have is outputs.

If you swap the grid coupling caps and put them on the opposite output tube grids from the Futterman design (IIRC this was the called the Technics variant) you will have better output impedance match to low impedances. Back a few years when I was working on mine I found a site that proved it mathematically...Wish I still had the link.

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.

Maybe this has something to do with it - source: 1955 Radio Shack Catalog

Fair enough. It wasn't a cheap tube so in the case of damper service it was the bean counters that won out. But, for a push pull amp, you only need 1 6AS7G vs 2 6L6s. The cost in tubes is still comparable.

Fair enough, but a pair of 6L6 was usually used in 20-30W amps, for the 10W class there was the 6V6.

And triodes are hard to drive, needing more stages. Triodes in general lost favor over time, even the 2A3 is found in hardly any hifi era products.

The 2A3 and its variants were used, at least initially. The 6B4G was found in the Sun Radio amps, the 6A5G in the Peerless, the 2A3 in the Brook amplifiers. They weren't terribly common post-1950, but they had adherents. With the popularity of the Williamson, I would hazard a wild ass guess and say that a majority of Hi-Fi amps used between 1947 and say 1953 or 1954 used either triodes or triode strapped beam power tubes.

I'm not going to argue the merits of the triode vs pentode/beam power tube. That is an ongoing battle that has been beaten to death from the dawn of Hi-Fi. I think it is sufficient to say that if the demand of triode output stages didn't exist, hundreds of thousands of Williamsons, with their triode strapped KT66s, 807s, and 6L6s wouldn't have been built. You wouldn't have had articles in Audio Engineering magazine extolling the virtues of the triode strapped 6550s circa 1954. I don't fall into either camp. I think you can build an excellent sounding amp with a pair of 6V6s and a pile of garbage with a pair of 300Bs. The circuit matters far more than the output device of choice.

Sure the 6V6 could do 10 watts in a push pull pair... at 5% THD, with a shitty damping factor, and virtually requiring some sort of negative feedback. A push pull pair of 6L6s could get you ten cleaner watts without much effort. A 6AS7G or pair of 6B4Gs or 6A5Gs could have done even better yet, and with less effort...

There's a reason Paul Klipsch demonstrated his Klipschorn in the early years with a single Brook 12A and not a push pull 6F6 or 6V6 amplifier. It is very easy to build an excellent sounding push pull triode amp. It is much harder to achieve the same level of quality with pentodes/beam power tubes.

Ok true, but the culture of the time was to use as much negative feedback as would make the amplifier unstable, and then back it off a bit. Given this sort of treatment, beam power tubes formed the basis of high fidelity amps from the early 50s until the end of the tube era. Triodes were still around but no longer mainstream territory.

The triode revival of the 90s was another story. I do remember in the early 90s hifi world made a stereo amp with a 6080 (industrial 6AS7). There have been others too, and also the OTLs.

I run single ended 2A3 in one system, and push pull 807 in another. I like em both

I'm a fan of negative feedback myself, and built it into my OTL...It is not needed at lower volumes, but if I want to crank it to 11 the feedback adds some headroom before distortion.

This is the entire basis of the Williamson, which despite the beam power tube output valves, is in essence just another triode amplifier. If you're tying the screen grid to the plate, you have a triode.

I would say that post-1954, neither the straight beam power tube arrangement nor triode strapping was terribly popular in comparison to the distributed loading arrangement popularized by Hafler and Keros.

No matter what camp someone is in, commit to one and don't go the distributed loading route. It is quite simply a mixture of the worst characteristics of triodes with the worst characteristics of the beam power tube/pentode.

The main advantage of ultralinear, so far as I can tell, aside from damping factor, is that you can use a screen voltage the same as the plate voltage, and avoid complexity in the power supply. Beam power tubes really work better when Vg2<Vp, but this of course asks for a more complex and expensive power supply, regulated screens being best.

I don't really like it either, but it does offer a lot of value for money, in terms of performance versus cost and complexity.

There's a paper from Williamson in which he rips into the so-called ultralinear design and proves mathematically that under certain sets of conditions, it affords more distortion than a properly built beam power tube output stage with regulated screens, or a comparable output stage with triodes or triode strapped tubes.

You're 100% right, the UL connection was a cheap way to handle the problem of the screen grids. It also ignores the fact that a great number of output valves simply can't have the screen at the same voltage at the plate. About the craziest example I can think of is the 6146. 200 volts max on the screen, and 600 or 750 volts IIRC on the plate. Yikes! Good for 100 watts, so the entire neighbourhood can hear what you're listening to :D

There's a certain segment of the audiophoolery crowd that eschews feedback of any kind. I prefer to handle it in smaller local loops when possible only for reasons of absolute stability into any load, but still, anything that lowers distortion and widens bandwidth at the expense of gain, which is not exactly hard to come by, is a win-win in my book.

If you spend a lot of time playing with the curves, even the usual audio tubes benefit greatly from a lower screen voltage. And since pentode gain is largely determined by screen voltage, regulation pays big dividends.

My empirical experience supports your conclusion that pure pentode or triode is better than ultralinear.

As for negative feedback, it's a tool. It obviously is a very useful tool, but like any other tool it can be mis-applied. I think lots of approaches to tube amp design have merit.

Here's the article I was looking for from Williamson:

http://www.keith-snook.info/wireless...perlatives.pdf

Cathode feedback windings are great, IF you can drive the output stage. In the case of the typical triode or triode strapped pentode/beam power tube, cathode feedback windings just won't work.

In the case of the pentode/beam power tube, this is certainly an approach that deserves more exploration. But again, it comes at a cost, higher driving voltage at the grids and a much more expensive and unusual OPT.

That's a classic article. Given the number of McIntosh amps sold over the years, and the enduring popularity of the QUAD II, I wouldn't totally write off cathode feedback - but for someone building at home with off the shelf parts, it's not really a realistic option. I'm sure that Sowter could do a one-off, but get our your wallet!

Back to the original topic, I think the 6AS7G was used most widely by the military, in all sorts of regulated power supplies. In my years of collecting I have come across multiples of military surplus 6AS7G and 6080, but not even one in a consumer targeted box or package. I even built a regulated power supply using one, and it worked really well for what it was.

One strike against the 6AS7 for audio is it's not that linear - sure, all the second harmonic is cancelled by push-pull, so it still does decent with distortion measurements this way, but have a look at the characteristic curves, they are not evenly spaced at all, and lots of "knee" near cutoff. In comparison to a linear triode such as a 6SN7 or a 2A3, they are very compromised.

The 6AS7 is not as linear, no, but the distortion that is does generate is mostly second and fourth harmonics, which get cancelled away as you said. In simulations using LTSpice, I'm able to get lower distortion from push pull 6AS7Gs than 2A3s, and for the simple reason that the 2A3, while producing less distortion overall, does produce more 3rd and 5th order.

You also have to keep in mind that the 2A3 is a filamentary type tube. There is much more 60 cycle hum to contend with than the 6AS7G, and that will mix with the actual audio signal to produce more IM distortion. The 6B4G is even worse, the 6A5G and 2A3H types are the only 2A3 variants, aside from the 6AS7G, that I know of that are relatively free of this pitfall.

Attached are some plate curves, the 2A3, 6AS7G, and 300B in that order, generated from Ayumi's extremely accurate tube models; they agree with the plate curves published by RCA and Western Electric. Sorry, they're kind of dark.

Wow a full $3 more (net price) than the 6W4 damper diode , that was a good deal more expensive .

In the context of modern designs, it's possible to use DC on the filaments of tubes like the 2A3, and get rid of that issue. I know some people almost religiously stick to AC heated filaments, but in my 2A3 amp I used DC and am very happy with the result.

The bias is then off somewhat along the length of the filament. If the reference to ground (or the self bias resistor) is halfway between the ends of the filament, then the bias is only shifted by 1.25 volts, which is negligible. If one end of the filament is grounded (or tied to the self bias resistor) now our bias is off at the other end by 2.5 volts, which, with a high perveance type valve, ought to be avoided. Still, for the 2A3 the DC filament situation is manageable.

The situation for the 6A3 and 6B4G is worse. 3.15 volts if the filament is tied to ground or the self bias resistor at the midpoint, a whole 6.3 volts if one end is grounded. This is non-negligable just from looking at the plate curves...

Still, a 2A3 on DC heaters, while more likely to fail prematurely does sound nice.

If we are considering modern advantages, then there is one area where the 6AS7G is far and away superior to most other tubes. It is ironic, considering that it was probably the biggest stumbling block to widespread adoption back in the day: cost!

I purchased the OPT for my 6AS7G amplifier project from Sowter in the UK today. The total cost was £250 or so with shipping.

I also managed to buy NOS JAN 6AS7G, in the ST envelope, for $10 a piece today. I bought 20. Assuming I have to replace the 6AS7G once every year or two, I'm looking at a 20 to 40 year supply of output tubes.

In order to secure a 20 to 40 year supply 'monoplate' RCA 2A3s, or Sylvania 2A3Ws, or Raytheon 2A3Hs, I would have to sell a kidney...