ECE4448 L45: Introducing Pentodes and Beam Tetrodes (Guitar Amplification and Effects, Georgia Tech)

[@MoraFermi]

That "kink" area is the home to many weird and wonderful circuits, many of which look like they should not do anything under static analysis.

[@johnforguites4800]

Thanks Aaron!

[@Lantertronics]

You are welcome! :)

[@goodun2974]

Here's a question: I've seen some early single-ended guitar amps (typically using 6V6) that feed the screen from the first power-supply node, and the plate being fed from the second filter node, at a slightly lower voltage. What's the reasoning behind that topology? I have a late 40's Dickerson amp like this, and there are early Silvertone and Airline (Valco) amps with nearly identical schematics. Granted, the voltages in the Dickerson aren't really high like they would be in a BF or SF Fender Champ, but still I would think that this would be bad from a reliability standpoint, and probably doesn't do much for guitar tone either. (These are *not* Ultralinear circuits with additional primary taps on the output transformer. ) .

[@Lantertronics]

I've seen that too and to be honest I have *no clue* what the reasoning behind it is. It seems to violate what I read to be the proper design procedure in all the old tube textbooks. If you figure it out, let me know! :)

[@goodun2974]

@@Lantertronics I also noticed that the tweed Champ schematic in one of your other videos showed the 6V6 screen fed from the same node that fed the output transformer primary, without a dropping resistor between them and without a second filter cap for the screen. Since Fender in the early days often copied schematics that were made available by RCA (in order to help them sell more tubes), and /or licensed from patented Western Electric PA amplifier circuits, one wonders if there was a mistake in one of those other schematics and Fender (and others) copied the circuit as it was drawn and didn't question it.

[@Lantertronics]

@@goodun2974 Hah! That's not an unreasonable theory. There's a lot of discussion about changes in tone stacks in various designs over the years resulting from errors that happened to sound good. ;)

[@goodun2974]

The 5879 is another usefull pentode preamp tube, kind of like a lower gain version of the EF86 and with a different pinout, that is seen in some Gibson amps, and mic preamp stages, especially in old tube PA/paging/ background music amplifiers.

[@goodun2974]

I should add that I have a big Webster PA amp with dual 5U4 rectifiers, 4) 6L6 tubes and 3) 5879 Mic inputs that I am thinking about converting to a guitar amp (the pentode input would make a nice Vox style preamp, and I could always change one of the others over to a 12AX7, for something more ordinary). I drew out as much of the circuit as I could by hand; the power supply distribution and the output stage are a little bit weird to say the least, and it uses some small inductors for the bass and trebe controls. I have hundreds of Sams's schematics for various PA/Mic paging amps, but I don't have one for this particular amp and I couldn't find one online either, although I do have a Dukane PA amp schematic with a similar power supply set up......

[@waynegram8907]

What is that region called when the plate resistance is negative? because it looks like a "negative resistance". What determines the MU of a tube?

[@AnalogDude_]

Just like a transistor, the people that invented these things! Without modern digital multi meters and oscilloscopes, how did they figure that out! these tiny values? very precise delicate analog meters? that "current" flows through mid "air" i mean a vacuum, some guy that stuck 3 metal plates in a glass bulb and measured current? just because? The 2n5457 JFET is a candidate for the 1176?

[@Lantertronics]

Slide rules rule. ;)

[@Lantertronics]

Haven't looked at the 1176 in a while so I can't comment. I'd recommend reading up on everything you can find.