Brighten tone of Ampeg B-15 "Big M" amp

[martin manning]

Without that additional 270k resistor, setting the volume to minimum on Martin's circuit would also short the other channel to ground. I'll leave those resistors in.

That was my intent, it was just easy to throw the load on in that way. Substituting 2x 270k for the load and taking the output from the junction lowers the overall response by 6dB, but otherwise it's the same.

[Paul Lebow]

Yes. I was just deciding where to make the connection. Without the 270k coupling resistor one would have to run the modded channel at full volume in order to use the un-modded channel - would add some hum. Will try it both ways. The 6db may be too much - its not a high gain amp even with the feedback removed.

Thanks
Paul

[Paul Lebow]

So I finally got around to modding using the "Big Muff" single tone control suggested. Aside from some slight deviations in capacitance - 1nf instead of 1.5nf due to availability and wiring the tone pot backwards, the circuit seems to be behave. The over cut in max volume compared to the original doesn't let the amp distort like the original circuit, even with negative feedback to the inverter completely disconnected. The tone variation is very subtle compared to the original, which is surprising given the LTspice bode plots.

FWIW I made a Power Point partial layout just showing the mods to Ch 2 if anyone is interested. Not easy working on the chassis - very cramped and the Ampeg tech tightly wrapped wires to the pots. Brain surgery.

Also attached are screenshots of my versions of the LTspice circuits for the original and the Big Muff mod.

Thanks for all the advice!

Paul

[martin manning]

Did the brightness improve? Is the reduced gain a deal breaker for this mod?

[Paul Lebow]

Maybe it's my old ears but Its hard to tell. I still have ch 1 which can be pushed to get some breakup. Ch 2 is just on the edge with my Texas Specials pickups. I wouldn't call it a deal breaker - nice to have the option on ch 2. I think the mod is more balanced with the curves swiveling around the 1 KHz point. Most of the guitar tone is predominantly in that range. The original circuit has too much interaction between volume and tone. I tend to just set the amp volume to max and adjust the guitar volume instead. A great learning experience

[Paul Lebow]

Rule of thumb for wiring - avoid soldering when 5 hours jet-lagged. (Scotland is amazing).

I miswired several connections - I have no idea how the amp even worked at all. After correcting the tone variation is much more pronounced as the Bode plots show. Still a lower output but very usable.

Paul