Vox AC30 Top Boost grounding question.

[Charlie Wilson]

Hello, I am working on a older Vox AC30 with an added Top Boost and was wondering where the best(quietist) grounding location for the Top Boost 32uf filter cap is?
CW

[Roe]

see the paper of valve wizard.

[pdf64]

There should be a handy chassis grounding eyelet by V1, the normal and bright channel input stage? That works fine IME, and pretty much complies with Merlin et al’s multiple star ground suggestions.

[Charlie Wilson]

Thanks, yeah I tried clipping the ground end of that cap all over the place and about the same noise level. I think AC and heater noise is really the issue with this amp. I found that the routing of the power cord hot was causing some of it.
CW

[pdf64]

You’ve got the heaters balanced?
I prefer to omit the cathode bypass from the TB stage, as the extra gain has more drawbacks (instability, raised noise floor) than benefits.

[Charlie Wilson]

Thanks pdf64. The amp is a 1964 AC30 with an added top boost and I don't think it was factory installed. It is a bit of a mess and to get to almost anything I would have to rebuild the thing. I found the two .022 caps were bad and the slope 100k resistor measured 230k( )so after replacing those the top boost sounds pretty good. The Guitar shop I fix amps for sold it, and then asked me to make it sound better. So after replacing the filter capacitors, fixing the top boost, and installing a footswitch I'm calling it done. I ended up grounding the 32uf top boost cap at the same tag the double 8uf caps are grounded at.
CW

[jumpbluesdude]

I tried adding a top boost when I found a very accurate kit years ago. It just didn’t work well. Sooo much noise and very sensitive. I know if I stuck with it, I could make it work, but I wondered if it was worth the trouble. I still have the kit and could take another swipe at it, but I’ll have to wait until I’m in the mood for pain. I think there is a spot near the voltage selector you really have to message a shield into for it to really shine.

[Mark]

When I look at the layout of the AC-30, I think the earthing is a mess. I find the earths are in series rather than in parallel. The Vib/Trem channel earths are included in the earths for Brilliant/Normal channel and P.I. stage. I certainly wouldn’t call myself an authority on earthing, but I would try and keep earths associated with their power supply node earth point.

The choice of 8uF filter caps for the Brillant/Normal and P.I. stage seemed odd to me too. I did substitute a 16uF cap in the course of fault finding and I can’t say I heard any tonal difference between a 8uF and 16uF cap, but I wasn’t running the amp into distortion where the screen grids are pulling down the voltage on the later supply rails. This could be a factor in amps overdriven sound, I cannot say for sure though.

[Mark]

One thing I found on the non-top boost amps, the cathode connection of the first normal and brilliant channels (with split cathodes as the treble models are) must be grounded in same spot. I found I had hum in the normal channel if I moved the cathode earthing point.