AB763 with extra gain stage

[pula58]

I love the clean sound of a Fender amp!. Is it possible, or, has anyone here tried, to add a Dumble-like overdrive (two stage) between output of v1B and the input of the Fender reverb circuit? That is, with overdrive switched-out it is a straight-ahead AB763, but, with overdrive switched-in the overdrive circuit sits between the output of the 2nd triode of the Fender circuit (V1B) and the input to the reverb stage.

I love reverb, and gotta have it..and other reverb circuits, to my ears, have not sounded or felt the same as the Fender circuit.

[ayan]

I love the clean sound of a Fender amp!. Is it possible, or, has anyone here tried, to add a Dumble-like overdrive (two stage) between output of v1B and the input of the Fender reverb circuit? That is, with overdrive switched-out it is a straight-ahead AB763, but, with overdrive switched-in the overdrive circuit sits between the output of the 2nd triode of the Fender circuit (V1B) and the input to the reverb stage.

I love reverb, and gotta have it..and other reverb circuits, to my ears, have not sounded or felt the same as the Fender circuit.

That is basically what Mesa Boogie did on their Mark IIC, IIC+, III, IV and maybe even one of the modes in the V. Careful, the part of the circuitry added for the reverb (i.e., mixer network and recovery stage) will totally change the feel of the Dumble tone. The sound will be more compressed and farty with the extra stage and parts.

Gil

[thyx]

An honorable attempt...that's how we learn. The best way to know is to try it. I'd consider putting in some kind of a separate "send" control going to the reverb driver (typically a 12AT7 in a BF Fender) from your OD circuit. The levels coming out of your OD circuit will likely overdrive (and distort) at the tank input. Saturation of the tank input = good. Distortion of the tank input = bad.

[talbany]

Personally I would tap the reverb off V1b (before OD) and return back in at the PI/MV using summing resistors..This is how Dumble did it in his later ODSR's

Tony

[ayan]

This was my first attempt at Dumble-izing an AB763 amp. It actually sounded really good, if I may say so.

Gil

[marcos]

I would recommend changing the mid pot to 25 k to get a little closer to the Dumble tonestack.Just my 2 cents

Marcos

[markusw]

This was my first attempt at Dumble-izing an AB763 amp. It actually sounded really good, if I may say so.

Gil

Looks cool
Thanks for posting!
What's the idea of adding the 1n cap to gnd at the reverb pot?

Peace,

Markus

[ayan]

I found it mellowed out the sound of the reverb a bit.

Gil

This was my first attempt at Dumble-izing an AB763 amp. It actually sounded really good, if I may say so.

Gil

Looks cool
Thanks for posting!
What's the idea of adding the 1n cap to gnd at the reverb pot?

Peace,

Markus

[markusw]

I found it mellowed out the sound of the reverb a bit.

Gil

Thanks for your answer!
Do I get it right that the low pass filter (Reverb pot + 1n) shifts to lower frequencies as you turn the reverb down?

Peace,

Markus

[ayan]

Gee, I really don't remember, that was 13 years ago. I eventually returned the amp to stock and sold it in 1999 when I built my first Dumble clone from the ground up.

Cheers,

Gil

I found it mellowed out the sound of the reverb a bit.

Gil

Thanks for your answer!
Do I get it right that the low pass filter (Reverb pot + 1n) shifts to lower frequencies as you turn the reverb down?

Peace,

Markus

[thyx]

This was my first attempt at Dumble-izing an AB763 amp. It actually sounded really good, if I may say so.

Gil

Interesting. What was the idea behind using such large values of resistance just prior to the OD master?

[ayan]

This was my first attempt at Dumble-izing an AB763 amp. It actually sounded really good, if I may say so.

Gil

Interesting. What was the idea behind using such large values of resistance just prior to the OD master?

Great question. The value of the large (4M) resistor was from an actual Dumble which did not have a master volume for the lead channel. The bypass resistor, 560pF, was given to me in error. When I tried that circuit, it almost killed me it was so bright. So I tweaked it, I had the 3.3M pot from the Fender amp and thought I would make a "harmonics" control with it; it worked really well. That amp was extremely unstable, you would let go of the strings of the guitar and it would take off all by itself.

Gil