Why was the 6AS7 dropped as a TV damper?

[benman94]

Originally Posted by Electronic M

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...

[Electronic M]

Originally Posted by benman94

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.

[benman94]

Originally Posted by Electronic M

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.

[benman94]

Originally Posted by Electronic M

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.

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...

[Electronic M]

Originally Posted by benman94

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...

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.