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
Vox AC30 Top Boost grounding question.
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Re: Vox AC30 Top Boost grounding question.
see the paper of valve wizard.
Last edited by Roe on Mon Nov 30, 2020 8:28 am, edited 1 time in total.
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Re: Vox AC30 Top Boost grounding question.
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.
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Re: Vox AC30 Top Boost grounding question.
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
CW
Re: Vox AC30 Top Boost grounding question.
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.
I prefer to omit the cathode bypass from the TB stage, as the extra gain has more drawbacks (instability, raised noise floor) than benefits.
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Re: Vox AC30 Top Boost grounding question.
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
CW
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Re: Vox AC30 Top Boost grounding question.
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.
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Re: Vox AC30 Top Boost grounding question.
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.
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.
Yours Sincerely
Mark Abbott
Mark Abbott
Re: Vox AC30 Top Boost grounding question.
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.
Yours Sincerely
Mark Abbott
Mark Abbott