Dumble mods to AB763 circuits

[dunner84]

I will be starting my 1 channel ab763 with no trem as soon as my turrets arrive. I sorted out a lot of kinks and drew up a layout only to find that Sluckey has already built this amp and posted his work. It is very handy to have a reference that is known to work!

I have been doing a lot of reading about dumble amps lately and found some schematics, as well as some mods to make the ab763 preamp work like a dumble. I have listened to a lot of clips, and I have concluded that I prefer the ab763 clean tones. My amp will have a raw control, so I have an option for a bit more gain, but I really love that overdriven dumble sound.

I am wondering how the dumble overdrive stage would sound following the ab763 preamp. Does anyone have any experience with it?

I like the idea of having it as a switchable option for more drive. I noticed that the Dumble preamp has an extra grid stopper on the first tube, as well as a rc type circuit between the plate and the grid. I assume this is to help tame oscillations before entering the next gain stage. Are these additions necessary if I want to add the dumble overdrive stage to my ab763? will they alter the stock tone?

[dunner84]

Here are the preamp mods.. The only thing I am curious about here is whether or not the mods on the first preamp tube are necessary.

[tubenit]

The Dumble tone has a lot to do with what's happening in the preamp.  Notice that Dumble preamps emphasize the mids in the tone stack and then have lower value cathode cap values.   (5uf compared to Fender 22uf)

I don't think you will get a Dumble tone IF you are faithful to the Fender preamp with 3 gain stage prior to the Dumble OD channel.  I think you
will find less smoothness and more grit in the tone trying that.

So another option would be to have a Fender preamp and an entire Dumble preamp and OD going into the LTPI and use 220k mixing resistors to separate the two distinct Fender/Dumble preamps.  Then you would use an A/B pedal to switch between the two amps.

With respect, Tubenit

[tubenit]

And lastly, I think a Dumble tone stack and reduced cathode cap values in a Deluxe Reverb would sound great especially with a 5751 in V1.

I think you could get Fenderish cleans and still have a mid-boost switch and a PAB switch that would head you into Dumblish overdrive tones.

This is a schematic to an amp that I'd really like to build.

With respect, Tubenit

[darkbluemurder]

I am wondering how the dumble overdrive stage would sound following the ab763 preamp. Does anyone have any experience with it?

My ex-AB165 Bassman was once set up like this - the first two stages set up like AB763 and after that two additional gain stages and a master volume control. It worked quite well even though it did not exactly sound like a Dumble.


I have now a Dumble kind tone stack in that amp so it should be closer to one of the real things. However at the moment I don't use this amp as it developed some problems - I cannot get a clean tone out of it anymore at any volume, not even with the clean section.


Cheers Stephan

[Mike_J]

I am wondering how the dumble overdrive stage would sound following the ab763 preamp. Does anyone have any experience with it?

My ex-AB165 Bassman was once set up like this - the first two stages set up like AB763 and after that two additional gain stages and a master volume control. It worked quite well even though it did not exactly sound like a Dumble.


I have now a Dumble kind tone stack in that amp so it should be closer to one of the real things. However at the moment I don't use this amp as it developed some problems - I cannot get a clean tone out of it anymore at any volume, not even with the clean section.


Cheers Stephan

The Dumble Overdrive Special had as high as 330uF electrolytics on the B+ and a lot of filtering on the screens.  It did not have tube rectification either.  A lower filtered amp with a rectifier tube would probably have quite a different feel I would think.  Probably not bad but different.


Thanks
Mike