Hammond AO-43 Treble Roll-off

[wilsonsk]

Hi,


I threw together a little practice amp a couple of years ago.  I used a basically unmolested Hammond AO-43 (L-111) power amp with reverb, and drove it with a Presonus Tube-Pre followed by a Presonus EQ-3B parametric. I put it all in an old Crate 1-12 cab and it actually sounded pretty good, I even gigged with it a time or two. I had put it away for a year or so, and when I fired it up, i have basically little to no treble response, the Presonus is a 3 band parametric, and the treble band makes virtually no difference in sound. I assume the Presonus was the cuplrit and connected a tube preamp I had laying around, and that was the same, I plugged my guitar directly in to the AO-43, still no highs.


There is no tone stack on an AO-43, but there are 2 gain stages and the phase inverter. I haven't even gotten as far as poking around yet, just wondering if there is a particular direction I should be looking.


Thanks in advance.

[sluckey]

Refer to the attached schematic. Where did you put the input jack?

[wilsonsk]

I am going in the RCA jack where the signal path typically comes from the vibrato amp. This had been working well for some time, I probably gigged with it or rehearsed with it pretty steady for a year or so. Then it sat for a year or so, and after i dug it out, it has gradually produce less and less treble. I haven't gone through tube sockets deoxit yet or anything, later tonight I'll try a few very high level things.

[wilsonsk]

Well, I probably asked for help too soon, I hit each of the tube's pins and sockets with deoxit, they are all the original Hammond tubes, and the pins are just plain black, that brought the highs back, I also swapped one out that was microphonic, although I tried 3 or 4 others (12AX7s) and they all were, and this socket has a shield so possibly that is normal? Regardless, in the tube swapping, replacing the microphonic one, while even thought the replacement was as well, sounded a bit rounder.


Too late to make a long story short, but everything sounds good, that really does make a nice little practice amp or small club amp, I have another AO-43, I may just make another one with a 2 channel preamp for clean and overdrive.

[sluckey]

The AO-43 is a good donor amp. There have been several 43 conversions on this forum in the past year. I built a Plexi 6V6 with one and I have two more sitting on a shelf just waiting for a good idea to come along.

[wilsonsk]

I also have a field coil AO-29, 2 AO-15s as well, any ideas for those?


This is going off topic, I'm going to start a new thread.....

[jojokeo]

The AO-43 is a good donor amp. There have been several 43 conversions on this forum in the past year. I built a Plexi 6V6 with one and I have two more sitting on a shelf just waiting for a good idea to come along.

+1 sluckey