Blair wrote:I bought 8 of these on a wild hair after reading on DIYAUDIO about sweeps. I was going to use these with the Sowter outputs I purchased, but I'm having some hesitations. I will probably go with a more conventional tube for the Sowter OPTs.
I am interested in the screen drive operation of this type of tube. Has anyone done anything with this tube? TubeLab over there says he got 125W into 3.3K with a pair in screen drive. I'm curious if maybe a 3.5-4k 100W Edcor output transformer with since screen drive circuit sounds about right for 100W or so. Also, has anyone ever compared distortion figures of "hifi" tubes Vazquez sweeps in audio amplification?
Thanks!
Blair
Haven't done anything with 6CB5s specifically. I did, however, put some 6BQ6GAs and 6BQ6GTBs to good use. Same deal: TV HD power finals designed for wide screen B & W TVs. The 6BQ6GA gives excellent sonic performance, making mostly H3, and very little of the nastier, higher order harmonics. In that regard, they're like 6V6s that can put out 40W. All you really need is some gNFB to take the edge off and get the woofers under control.
As for local NFB, the 6BQ6GA/GTB doesn't require it. They're plenty linear as it is, and don't make the nastier harmonics. I wouldn't bother with OPTs that include Ultralinear tertiaries. Save on that unnecessary expense.
The 6CB5 is probably similar though having a much higher Pd rating. TV screens got bigger, with sharper deflection angles, and demands on horizontal deflection subsystems kept increasing. Color TV produced some real beasts of HD finals. These types are easy loads, and can produce lotsawatts at reasonable rail voltages, and lowish load resistances. That will definitely mean custom OPTs (The 6BQ6 project used an off-the-shelf OPT originally intended for Class A 6L6s, and so comes in at 30W, not the 40W the 6BQ6s could pump into it. That OPT did include Ultralinear tertiaries that I'm not using.)
I have a project in mind for some 36LW6s that came my way recently. These being of the last generation of octal HD types for big screen color TVs, and they are beasts indeed. 95W with grid drive easily, and north of 200W with screen drive, and even here, screen drive is a good deal easier than driving audio triodes for a fraction of those watts. MOSFET drive is certainly up to the job.
"TubeLab over there says he got 125W into 3.3K with a pair in screen drive".
But for how long? Tubelab likes to drive 'em right to the poof-point, and often beyond. I'd try to run 'em a bit more conservatively, but you could still do ~100W or so.