Structo
03-06-2009, 10:42 AM
I recently picked up a Hammond organ amp for a project.
Originally I had planned to make use of everything I could with the parts in it, but after getting into it I decide to gut it and just use the power transformer and rectifier socket.
The good news is, was that it came with some vintage tubes, two Sylvania 6V6GT's, two 12AU7's and one 12AX7.
Anyway, I found a schematic on the web that approximated what I wanted.
I did not want the dual inputs of the Tweed Deluxe because you waste half of a tube that is not used.
Here is the schematic I used.
http://i23.photobucket.com/albums/b392/Structo/Hammond%20Deluxe%20Project/Hammond_conversion_schematic_1.jpg
The problem I am having now that I have the amp running is it has way too much gain. The only way I can calm it down is to put a 12AU7 in V1 or V2 which is the phase inverter.
If I use a 12AX7 in both V1 & V2, it will oscillate when the tone control is dimed.
I added a Deluxe style tone control.
I also have installed a global feedback with a presence pot as well.
I've experimented with different plate load and cathode resistors and still can't seem to tame it.
One issue that bugs me is with the 1 Meg volume pot, It's range of sweep is very small.
In other words, when I turn it up, it goes from clean to mean within a fraction of a turn.
I've experimented with the 470K grid leak resistor shown on the plan, trying 100K, 50K, and 10K with no noticeable difference.
So how can I lower the gain in V1 (I really want to be able to use a 12AX7 there)?
I have also played around with values in the PI to reduce gain there as well.
But the volume pot sweep range is probably bugging me the most right now.
The amp.
http://i23.photobucket.com/albums/b392/Structo/Hammond%20Deluxe%20Project/IMG_0034.jpg
Lead dress isn't the best due to cramped quarters but the noise floor is low and it only oscillates if the tone pot and volume are dimed.
I put a 47pf cap across the PI plates to help this.
Originally I had planned to make use of everything I could with the parts in it, but after getting into it I decide to gut it and just use the power transformer and rectifier socket.
The good news is, was that it came with some vintage tubes, two Sylvania 6V6GT's, two 12AU7's and one 12AX7.
Anyway, I found a schematic on the web that approximated what I wanted.
I did not want the dual inputs of the Tweed Deluxe because you waste half of a tube that is not used.
Here is the schematic I used.
http://i23.photobucket.com/albums/b392/Structo/Hammond%20Deluxe%20Project/Hammond_conversion_schematic_1.jpg
The problem I am having now that I have the amp running is it has way too much gain. The only way I can calm it down is to put a 12AU7 in V1 or V2 which is the phase inverter.
If I use a 12AX7 in both V1 & V2, it will oscillate when the tone control is dimed.
I added a Deluxe style tone control.
I also have installed a global feedback with a presence pot as well.
I've experimented with different plate load and cathode resistors and still can't seem to tame it.
One issue that bugs me is with the 1 Meg volume pot, It's range of sweep is very small.
In other words, when I turn it up, it goes from clean to mean within a fraction of a turn.
I've experimented with the 470K grid leak resistor shown on the plan, trying 100K, 50K, and 10K with no noticeable difference.
So how can I lower the gain in V1 (I really want to be able to use a 12AX7 there)?
I have also played around with values in the PI to reduce gain there as well.
But the volume pot sweep range is probably bugging me the most right now.
The amp.
http://i23.photobucket.com/albums/b392/Structo/Hammond%20Deluxe%20Project/IMG_0034.jpg
Lead dress isn't the best due to cramped quarters but the noise floor is low and it only oscillates if the tone pot and volume are dimed.
I put a 47pf cap across the PI plates to help this.