AB165 second gain stage

[jreese2001]

I had some questions about the second gain stage in the AB165 circuit. I've been modding this amp pretty heavily trying to get "my sound" out of it.

At this point, the amp sounds pretty good, but my only complaint is that the low end is too flabby when I play notes on the E and A strings. It just lacks definition and just sounds like mud. If I lower the bass or up the treble, then the high strings sound too bright.

First question, any suggestions on how to deal with this (I've been switching out coupling caps and such already, which helped a little, but I think I'm at the end of the rope on that one).

Also, I tried removing the local feedback look on the second gain stage. Wow, the tone that I wanted was there, in terms of EQ. However, there was WAY too much gain for my tastes.. and also WAY too much noise... hum, RF interference, hiss... you name it.

I am wondering, first of all, why did so much noise and hum enter the picture when I pulled out the loop, is it just a product of the extra gain? I tried some other things to reduce the gain and keep the tone after that point, like pulling the cathode cap on that gain stage, but nothing got rid of the noise.

Anyone have any suggestions on how to tackle this?
Thanks much,
Josh

[Satamax]

http://acruhl.freeshell.org/mga/main..._vs_AB165.html

[jreese2001]

Satamax, thanks for the reply. I should have mentioned that I've actually done almost all of those mods. The PI is the blackface aa864 version (along with the blackface NFB configuration), also got rid of the hum balance control and put the real bias circuit in. Took out the weird 220K power tube plate resistor stuff. I have .047uF caps going to the power tubes, and a 500pf PI coupling cap, to even further control the bass. As I play with these values, it seems to keep getting brighter on the high strings but still the flab is there on the low strings, so I suspected that the problem was from something before the PI. The normal channel is the original AB165 version, and it had this flab, so then I suspected the second gain stage.

From a theory perspective, what things do you have to watch out for when connecting two gain stages together?

[Rob Mercure]

JR,

Can't tell if you've done it but the first thing I would do is lover the value of the stage's cathode resistor bypass capacitor. Fender's 25uf have always been over generous passing frequencies far below that a guitar can generate (can't find my shelving tables since HD crash) with the factory cut off being around 30-40 Hz if my memory serves me well (Bruce, you out there - I know you've got this memorized). In any case you can lower the cap to 5-10 uf and see if you like the result - some have gone as low as 1-3 uf to match thier needs. Generally the dirtier you get the more you need to eliminate low/high "extremes" <grin> of guitar signal.

Rob

[Bruce / Mission Amps]

JR,

Can't tell if you've done it but the first thing I would do is lover the value of the stage's cathode resistor bypass capacitor. Fender's 25uf have always been over generous passing frequencies far below that a guitar can generate (can't find my shelving tables since HD crash) with the factory cut off being around 30-40 Hz if my memory serves me well (Bruce, you out there - I know you've got this memorized). In any case you can lower the cap to 5-10 uf and see if you like the result - some have gone as low as 1-3 uf to match thier needs. Generally the dirtier you get the more you need to eliminate low/high "extremes" <grin> of guitar signal.

Rob

Here's the chart I drew up a few years ago....

[jreese2001]

Bruce and Rob, thanks a lot, this is great info. I haven't tried playing with those values yet so this gives me some more experimenting to do! I will post the results once I get a chance.