5F1 Ground Scheme

[rplovanich]

Please check out the attached drawing and give me your thoughts..Thanks

[echuta13]

That's almost exactly how I did my last one. I don't know what type of jacks you are using... I use Switchcraft, and had an issue with the output jack last time. I had to use isolation washers due to a ground loop that I missed.

[rplovanich]

Thanks for the reply. I am using Switchcraft also. I will use your suggestihankson...T

[tubeswell]

You have the general concept of a split ground. But I would only run the pre-amp grounds to one input jack only, otherwise you will end up with an unwanted ground loop. The other input jack does not need grounding if you follow the standard 5F1 schematic wiring (where only one of the switching jacks is grounded). You should also ground the output tube cathode ground with the reservoir filter cap ground, not the pre-amp filter cap ground.

[rplovanich]

Thanks Tubswell... output tube cathode ground with the reservoir filter cap ground... At which point is this? I will eliminate one of the input jack grounds, I will make the changes on my drawing and re-submit....Thanks Again

[pvsage]

Thanks Tubswell... output tube cathode ground with the reservoir filter cap ground... At which point is this? I will eliminate one of the input jack grounds, I will make the changes on my drawing and re-submit....Thanks Again

I think "reservoir filter cap ground" means the 16uF and first 8uF cap - the filter/decoupler caps for the power tube plate and screen grid rails. "Output tube cathode ground" is the ground side of the 6V6 cathode bias resistor and bypass cap. For what it's worth, Watts Tube Audio (turretboards.com) just runs a wire directly from the cathode ground to the cap for the screen grid rail in their Valve Junior replacement boards, creating a mini-bus.

Related question for the Peanut Gallery: Anything wrong with putting the output tube cathode bias resistor and bypass cap between the 8uF decouplers and running the wire to the cathode under the 22K resistor? This seems to make a little more sense to me - it puts the preamp rail closer to the preamp circuit.

[tubeswell]

Yep the reservoir cap is the first filter cap after the rectifier. So named because it is a reservoir for +ve charge which it stores and alternately releases between the peaks of the rectified high voltage pulses of +ve charge coming from the rectifier, in order to create a relatively smooth and steady DC for the B+ supply.

[rplovanich]

Thanks Guys

[lmolter]

Can I deduce from this discussion that I should not connect all the filter caps' minus leads to a common bus and ground this? In my cap pan, I did just this, but I have not yet wired the rest of the power supply elements yet. It's not too late to make changes. In the same regard, I _was_ wondering how and where to ground the cathodes of the output tubes. It seemed wrong to just ground them to the nearest screw. What's the accepted method?

Takin' it slow and careful with my 5F6A build.

Larry

[tubeswell]

Can I deduce from this discussion that I should not connect all the filter caps' minus leads to a common bus and ground this? In my cap pan, I did just this, but I have not yet wired the rest of the power supply elements yet. It's not too late to make changes. In the same regard, I _was_ wondering how and where to ground the cathodes of the output tubes. It seemed wrong to just ground them to the nearest screw. What's the accepted method?

Takin' it slow and careful with my 5F6A build.

Larry

Well I don't there's any hard and fast rule about it. I have found that if I keep the pre-amp filter ground with the other pre-amp grounds, running each ground through a length of return wire to the same point on the chassis at one of the input socket grounds - and keep this kaboodle separate from the reservoir and screen node cap grounds, which I keep together with the other (non-pre-amp) grounds at one of the Pt bolts at the other end of the chassis, my amps have hardly any hum. Other people use a common bus wire, and others use a brass grounding plate and ground the different grounds more or less in accordance with the classic layout diagrams at different localities in the chassis (maybe I'll try that in one of my next builds). Either way, the point is to reduce the resistance between different parts of the ground so as to avoid greating a potential voltage between the grounds that can generate hum.

[pvsage]

What tubeswell said.

I assume you're using this schematic?
http://schematicheaven.com/fenderamp...5f6a_schem.pdf
This pdf includes the classic Fender layout on page 2.

Basically, you want to ground the three 20uF caps at the far right of the page with the power tubes and the 8uF cap with the preamp cathodes. You want to connect the power amp ground to the same point as the mains ground - usually the transformer mounting bolt closest to the power cord entry. You usually also want to ground the OT secondary to this same point.

I'm primarily a single-ended amp guy, so I'm not sure whether the phase inverter is grounded with the power amp or the preamp. Hopefully someone else can shed some light on that sub-subject. I'm fairly certain you'd want to ground the PI with its decoupling cap (the remaining 20uF high-voltage cap); I'm just not sure where that particular ground should tie into the chassis.

IF you go with a bus ground, it is usually a good idea to use the largest-gauge wire for the bus that you can find in order to minimize resistance, which can contribute to ground loop noise.

[tubeswell]

I'm primarily a single-ended amp guy, so I'm not sure whether the phase inverter is grounded with the power amp or the preamp. Hopefully someone else can shed some light on that sub-subject.

Hi pvsage, FWIW when I built my 5G9, I grounded the PI (a LTP) with the pre-amp grounds as I recall, and the trem all by itself at the trem Ft/Sw socket ground