Balanced Load Design

[audiobot]

Don, Don, Don,

Deep hole? committable? Your sounding like my mom. How bout an 807? I've been to the Turner site before and like it a lot, but I need to refresh myself on it every couple of months. Gives me something to do today I guess. I have a Lundahl 1620 that I could play with on the bench for a month or so before it goes to it's final use in another amp, that is sort of why I was interested in the full monty balanced load.

I am still wondering about the ins and outs of direct heated filaments vs cathodes in this circuit. Seems to offer some roadblocks to my weak mind.

Thanks,
Michael

[smoking-amp]

I have just attended a private audience with the Pope, and he conveys his blessings on the 807 balanced load design.

Seriously, you should get some other opinions on this undertaking, maybe some have tried or contemplated such a project. I cannot speak from any experience here, just opinions. I'm sure some can give pointers on how to actually proceeed. Good luck.

Don

[smoking-amp]

I just took a look at the Lundahl site and noticed they have a LL1620 CFB version with 25% CFB tap capability, you don't happen to have this version? This probably is low enough % to get one out of the bootstrapping territory for the driver stage.

a note:
I once thought of another twist on bootstrapping. Usually this is just a resistor from the output plates (crossed over) back to the driver plates with the idea of a constant voltage across it so it acts as a constant current source load. Then we hear the comment that it is really the same as positive feedback, so will increase distortion.

How about putting a real SS constant current source in place of the resistor. Hard to imagine that causing distortion with a constant current output. Of course, it will eat up a little voltage headroom, but has nearly constant voltage across it. And might be spectacular if something fails. But this IS DIY. Just a thought.

Don

[audiobot]

Wow, you don't suppose the Holy See could be persuaded to jump up a couple of prime numbers, say to 813? Could settle for 811 maybe.

No I don't have the cfb version of the 1620. I don't think it was available when I originally purchased mine. Looks like it would be fun to play with though.

By the way, how does one calculate the output impedance of the balanced load setup?

Where does the bootstapping come in? Seems Mac used a regular low voltage driver through an interstage for their 6L6 version. Wonder what their distortion and output impedance numbers are without the global feedback loop?

Are you sure about your final stage mu estimates? Seems like it would be more like 1/2 the mu of the outputs operated in the regular fashion. I could of course be wrong as I so very often am.

Thanks,
Michael

[SY]

813 isn't prime. (annoying pedant award for 2005 goes to SY)

[audiobot]

257 as in HK

M.

[smoking-amp]

Cost ya ten Hail Marys for the 811 and 20 for the 813. Cost ya some real money for anything else. Just don't use any forbidden 6_6 tubes.

Output impedance: Hmmm, my 1st Scientific Wild A__ Guess for pentodes would be 1/Gm transformed across the CFB to output turns ratio squared, but don't quote me on that. Could check it with a Spice simulator I guess.

Most of the Mac schematics I've seen used bootstrapping on the drivers. An interstage xfmr. for voltage gain would be a way to avoid that, but can forget about global feedback then.

Mu estimates: Hmmm, use a Spice simulator or solve all those nasty simultaneous equations.

Been thinking about plate cap leads since you mentioned the 807. There are some neat electroluminescent flexible strips available now, they glow with assorted colors. I've seen where they were sewn into clothes for party costumes. Use battery powered inverters for power I think. Might be a natural fit around tube voltages.

Anyway, could spirally wrap them around the plate cap leads to make an out of this world ambience. No doubt we'll be seeing some glowing speaker wires anyday now. I've been thinking of putting some on my hiking clothes since I seem to end up making it out of the woods well after dark a lot lately (lost). Keep the coyotes away, scare the natives, that sort of thing.

Don

[audiobot]

I've been trying to spice the circuit, but my skills are near nil. Will try some more.

How does the glowy stuff hold up to heat? I thought about getting some off ebay a while back, seemed like it might be fun to play with.

M.

[smoking-amp]

"How does the glowy stuff hold up to heat?"

Hmmm, good point. Must be made from plastic, probably cheap stuff too. Probably have to put some holes around the tube socket and pressurize the chassis with a fan. Maybe could put a clear panel on the front of the amplifier and then "wire" it up inside with fake glowing wires. Glowing interconnect cables? How about an exposed layer of glowing "wires" on the output transformer. Clear transformer end bells. Make it pulse with the volume. We need some tubes with phosphorescent coated plates.