Reverb Circuit on Wonderland/Fender 75 advice please

[B_Lowoff]

Hi All

First post here - I am in awe of the knowledge and generosity here, and a little apprehensive to ask my questions as I am very much a newbie to the dark arts of amplifier building.

I have converted a Fender 75 amplifier to a circuit that closely follows the JM Wonderland amplifier. I have use as much of the original amp as possible, but used the typical TW/AN/Ceriatone layout. Everything worked out great so far, I'm just in the first trials with my test 12" cab. The original cab has a 15" speaker, which I will put the chassis back into for final testing.

My question is about the reverb circuit in this. I can see it mostly follows the AB763 scheme with some smaller values for the cathode bypass caps. The Fender 75, although not great in several aspects, did have fantastic reverb - on 10 it was cavernous!. The new circuit needs both the send and return pots on 10 to get anything like an average depth.

I am going back through the build, testing every component again to ensure there are no wrong values, but if that proves fruitless, I'm wondering what would be good places to start adjusting the circuit for a deeper effect. Is it the mixing resistors?

My working schematic is included below. Many thanks in advance.

[dbharris]

What model reverb tank are you using?

-Dan

[pdf64]

On the face of things, the NFB on V4b will be hamstringing its gain. I suggest to try breaking the anode to grid feedback and reassess.

[Guy77]

Hi All

First post here - I am in awe of the knowledge and generosity here, and a little apprehensive to ask my questions as I am very much a newbie to the dark arts of amplifier building.

I have converted a Fender 75 amplifier to a circuit that closely follows the JM Wonderland amplifier. I have use as much of the original amp as possible, but used the typical TW/AN/Ceriatone layout. Everything worked out great so far, I'm just in the first trials with my test 12" cab. The original cab has a 15" speaker, which I will put the chassis back into for final testing.

My question is about the reverb circuit in this. I can see it mostly follows the AB763 scheme with some smaller values for the cathode bypass caps. The Fender 75, although not great in several aspects, did have fantastic reverb - on 10 it was cavernous!. The new circuit needs both the send and return pots on 10 to get anything like an average depth.

I am going back through the build, testing every component again to ensure there are no wrong values, but if that proves fruitless, I'm wondering what would be good places to start adjusting the circuit for a deeper effect. Is it the mixing resistors?

My working schematic is included below. Many thanks in advance.

Hi, change your reverb send pot to 1MA and place a .002uf cap across your 100K reverb return pot and see if you like that amount of intensity.

Cheers
G

[B_Lowoff]

[/quote]


Hi, change your reverb send pot to 1MA and place a .002uf cap across your 100K reverb return pot and see if you like that amount of intensity.

Cheers
G
[/quote]

Thanks Guy - this did the trick. I haven't felt the need to lift the local feedback yet, Pete. I did try parallelling a 200r resistor with the input grid resistor on V4 as well, but it increased noise. I think with the change to the 1M pot, it's good.

It's a fantastic sounding amp - I'm very pleased with it.

[Guy77]

Hi, change your reverb send pot to 1MA and place a .002uf cap across your 100K reverb return pot and see if you like that amount of intensity.

Cheers
G
[/quote]

Thanks Guy - this did the trick. I haven't felt the need to lift the local feedback yet, Paul. I did try parallelling a 200r resistor with the input grid resistor on V4 as well, but it increased noise. I think with the change to the 1M pot, it's good.

It's a fantastic sounding amp - I'm very pleased with it.
[/quote]

Glad it worked out for you. I do this conversion often and they always like it .

Cheers
Guy