'64 Vox AC-30/6 with Top Boost add-on

[Laurent Brondel]

I just put together this top boost add-on for my '64 AC-30/6, it sounds awesome but there is a bit of increased hum and noise, I assume it is to be expected with an extra gain stage, but maybe there's a better way. I tried to keep all leads as short as possible, both in and out from the module are shielded on the pot side.
Should have I placed the 33uF el. cap on the chassis, next to the dual 8uF, sharing the same ground like on the original Vox instructions? What can I do to improve the signal/noise ratio? And yes, I am getting correct knobs next week.

[pdf64]

I suggest that you lift the cathode bypass off the tb common cathode stage and re-assess. With it in place, there’s too much gain, which raises the noise floor and makes the other nasties apparent.

[xtian]

Your hum, no doubt, comes from your multiple ground points. I see the middle lug of your three-position standoff is grounded to the chassis, and at least two wires are grounded there: red and brown. That same brown wire appears to be grounded to the main strip board, true?

Where are the new 12AX7's cathodes grounded?

[Laurent Brondel]

I suggest that you lift the cathode bypass off the tb common cathode stage and re-assess. With it in place, there’s too much gain, which raises the noise floor and makes the other nasties apparent.

You mean the 25uF cap (22uF, that’s what I had) on the TB unit?

[Laurent Brondel]

Your hum, no doubt, comes from your multiple ground points. I see the middle lug of your three-position standoff is grounded to the chassis, and at least two wires are grounded there: red and brown. That same brown wire appears to be grounded to the main strip board, true?

Where are the new 12AX7's cathodes grounded?

Everything on the TB unit is grounded on the middle lug of the strip there, and a wire connects that strip to the ground star next to v1 on the chassis (to a short wire coming out of it, there's no room anymore). The TB unit is also in contact with the chassis where it is fastened: could that create a ground loop?

[pdf64]

I suggest that you lift the cathode bypass off the tb common cathode stage and re-assess. With it in place, there’s too much gain, which raises the noise floor and makes the other nasties apparent.

You mean the 25uF cap (22uF, that’s what I had) on the TB unit?

Yes

...Everything on the TB unit is grounded on the middle lug of the strip there, and a wire connects that strip to the ground star next to v1 on the chassis...

To avoid a hum loop, it’s better to leave the mounting tag unused; obviously that will increment the numbers of tags necessary.

[xtian]

Everything on the TB unit is grounded on the middle lug of the strip there, and a wire connects that strip to the ground star next to v1 on the chassis (to a short wire coming out of it, there's no room anymore). The TB unit is also in contact with the chassis where it is fastened: could that create a ground loop?

Yes, that is exactly a ground loop.

[Laurent Brondel]

Thank you for the responses, I removed the bypass cap and wire going to ground on the chassis and it helped a good bit with the noise.
Now the TB is grounded solely from its bracket to the chassis, is that OK?

[pdf64]

If it works then it works. And I guess many originals got done that way.
But personally I’d fit a 4way tagstrip try to control the 0V return path by using a dedicated wire.

[Laurent Brondel]

If it works then it works. And I guess many originals got done that way.
But personally I’d fit a 4way tagstrip try to control the 0V return path by using a dedicated wire.

Will do, thank you for your help!