AB165 question

[trem]

In the third gain stage, where just one triode is used, there is what looks like a negative feedback circuit. The signal leaves Pin 6, goes through a 470k resistor, and back into Pin 7. What is going on here that made Fender add this?
Thank You


http://www.el34world.com/charts/Sche...SMAN_AB165.pdf

[martin manning]

Not a gain stage but a mixer stage, and a much more elegant way of combining two channels than a pair of 220k resistors. Kind of out-of-character for Leo, really, isn't it? No, wait, that's CBS's doing... Leo would never have done that, and the other half of the dual triode is just wasted!

The NFB gives it broad/flat frequency response, linearity, and low output impedance. Aiken's tech info contains a good description of the circuit.

MPM

[hasserl]

Actually the two channels are mixed thru 220k resistors prior to that stage.

[martin manning]

Okay, better than JUST a pair of 220K resistors, where there is a loss in signal strength and interaction of the signal amplitudes. Adding the mixer stage makes the summing junction a virtual ground, which is better for isolating the inputs.

MPM

[akimball442]

It is a gain stage, but it's not a huge amount of gain. The 470K resistor at the input shunts some of the signal to ground, and of course the 470K global feedback resistor further reduces the gain. The later Bassman 50 amp reduces these resistors to 220K, which lowers the gain of the stage further yet. At this point, it's almost a buffer stage.

You can replace the entire stage with a cap and clean the amp up a little. On the other hand, aside from the obvious bass channel, this stage sort of gives the AB165 some of its unique characteristics compared with some of the other blackface amps. There's a lot more to it though, the big output transformer and lack of a tube rectifier certainly contribute.

The amp is very easy to do reversible mods to, because you don't have to punch a hole in the chassis for an extra tube, Fender was nice enough to provide it for you.

[Baxtercat]

I lament selling the 165 I had decades ago. Pretty darn nice amps as-is.

Well, I'm in love with these more than before.

The other gtr. player in my band brought his to me last week saying 'Do what you want with it'. I know more now and so I lifted several resistors and caps [standard Internet and old VG magazine mods].

Wonderful fixes... Great 'twang' now.

I gigged with it [and one EVM-12] all weekend.
Amen to Bassmen! - these simple heads sound so solid.