Anyone do isolated inputs on a Fender build?

[Paul G.]

Whenever I do a Fender-style build, no matter how careful I am, there is a small amount of hum at higher volumes. Every one of my Marshall-style builds, on the other hand, is dead nuts quiet all the way up.

I'm thinking it's the grounding scheme? I was thinking of doing a bus bar and Cliff jacks to see. Anybody do this? Am I barking up the wrong tree?

[xtian]

You can DEFINITELY improve on Fender's vintage grounding scheme. As I progressed in my building experience, I had no trouble build Fender style amps that are silent at idle. But that brass grounding bar tacked to the chassis? No! Ground wires soldered to the chassis everywhere? No!

[pompeiisneaks]

Yeah +1 to this. I've read merlin's grounding chapter a couple times, and I love looking at how dumble grounded his amps.

I think at a MINIMUM you should create two separate grounding points that are drilled screws through the chassis (if painted, scraped clean) with a solder lug, and put them at the preamp input area and then join all preamp grounds together into a grounding buss that connects to the pots and over to that ONE point. Then a separate grounding point for the PI, power tubes and the higher current areas.

Dumble uses 5 or 6 depending on the build.

1. preamp
2. Filter Cap ground
3. power tube grounds
4. Center Tap of PT ground and any power rectification ground
5. relay grounds.

These are all separated by a decent distance physically on the chassis so they can't easily create loops to/from one another. Also the fender brass bar tends to get corrosion over time and can cause poor grounds. Using the pot body to chassis connection as a ground also often fails due to corrosion over time, etc.

~Phil

[Colossal]

Yes, I use isolation washers on Switchcraft jacks for Fender builds.

[ViperDoc]

I am a student here, but my last Princeton I built I decided to implement a preamp buss wire on the fiberboard connected at the normal ground points instead of wiring up the backs of the pots. I grounded the buss wire to the input jacks (not quite your question), and the amp is *dead silent*. In the picture, the wire has yet to be grounded to the jack, fyi! In thinking about this, it works very well unless the jack becomes loose. I could see how a separate grounding lug on the chassis would be a better, more fail-safe option. Isolating your inputs then, I believe, would be called for.

[Colossal]

Yeah man, go with a dedicated bolt, solder lug, washer and a nylock nut or toothed nut for your grounds. That way, as you said, if the jack comes loose, and it surely will, your ground will be solid. That's a good looking Princeton. Enjoy that.

[Guy77]

I do the same with all my speaker jacks and effects loop jacks . I use isolated switchcraft and run a wire that grounds all jacks to the same point in the space between the preamp and power tubes on the chassis floor with nylon locking nuts.

Cheers
Guy

[Stevem]

60 HZ hum and 120 HZ reduction require two different courses of action !

When your talking about hum on this build is it 60 HZ, or 120 HZ and with a meter set for AC volts hooked up across the amps output how many millvolts do you read.

Do this test with no input ( be sure the input jack is shorted) and take the readings at idle and full up and please report back.