AB763 tremolo hardly noticeable...

[bruno]

I recently built an ab763 amp, with the hoffman layout, I changed the horizontal resistor to 150k, added a 22uf cap across the 4.7k resistor, changed the left 0.1uf cap to a 0.047uf, I have around 470 volts on the plates of the 6L6s and the amp is biased around 45ma. Still the tremolo is barely noticeable, the effect can only be heard when the intensity pot is past 3/4 of the taper. Any ideas on what I could try next?

thanks

[rzenc]

I have around 470 volts on the plates of the 6L6s and the amp is biased around 45ma. Still the tremolo is barely noticeable, the effect can only be heard when the intensity pot is past 3/4 of the taper. Any ideas on what I could try next?
thanks

Is it for one tube or both?
I recently build it and the bias vary tremolo had more effect with the amp biased on the cold side.
Also, you can try different tubes in order to find a higher gain tube, which may improve it's performance, JJ ECC803S worked very nice.

Hope this helps.
Best Regards,

Rzenc

[sluckey]

Change the bias.

[jim]

What Sluckey is saying that the bias modulated trem will work better if the output tubes are not biased as hot as you like it.  You might also want to try adding a bypass cap about 22mf/50V or so on the cathode resistor connected to pin three first.  I think it is 4.7K.   Good Luck, Jim

[jim]

OOps, you already tried the bypass cap so reduce drive on the output or try a stonger trem tube.  Jim

[Fresh_Start]

Take a look at the second page of the attached schematic.  Change the cap going to ground between your 150K and the .1uf going to the tremolo pot from .1uf to .047uf.

The second attachment shows all of the tweaks for bias vary tremolo with a 6L6.

There's also a long thread in the Archives on this subject.

When biasing this amp, you need to strike a balance between hot, juicy bias and the point where the tremolo works.  However, the Super Reverb variant I built worked fine with 472 volts on the plates and 43 / 48 ma going through the cathodes.  Tremolo is lush.

No one has mentioned trying a higher gain tube in the tremolo driver spot.  I recently had a low gain 12AX7 which simply wouldn't jiggle a real Blackface Princeton Reverb at all.  Remember, it doesn't matter if the tube is noisy because it's "signal" doesn't go into your signal path - it just jiggles the bias voltage.

Cheers,

Chip