Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on January 28, 2024, 01:07:00 pm
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Hi all,
In the project page for the Hoffman 6G15 reverb unit, it’s advised to disconnect green earth wire from chassis ground if hum is a problem.
While the unit will then be grounded through amp, I worry what could happen if house ac or the amp was wired incorrectly. What’s the preferable solution to getting rid of the 6g15 ground loop hum without lifting chassis ground?
Thanks.
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Relevant thread; go to Tubeswell's post, #4, for a drawing.
https://el34world.com/Forum/index.php?topic=30592.msg337450#msg337450
I built one with more or less the same ground scheme, very quiet.
Take note of Merlin's "hum block" on the circuit ground to chassis ground.
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63 Reverb Reissue Service Manual (https://el34world.com/charts/Schematics/files/Fender/Fender_63_reverb_manual.pdf)
Read the Theory of Operation section.
Check out the schematic; be sure you know the difference between the chassis-ground and signal-ground symbols (https://www.eeweb.com/wp-content/uploads/articles-quizzes-quiz-ground-answer-1305526105.png). Fender uses these, and only the chassis-ground is connected to the chassis & 3rd wire of the power cord.
Look over in the power supply section of the schematic under C13 & C14; don't ground pots to the chassis, and use the bits in this section of the schematic to separate signal-ground from chassis-ground.
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Here's how I did it on my latest 6G15 build. It's dead quiet as far as noise and hum goes.
Not sure if it makes sense by looking at the drawing.
EDIT: HT center tap is not connected.
/Max
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Relevant thread; go to Tubeswell's post, #4, for a drawing.
Thanks, found the drawing and looking it over. I am going to start implementing this but a few questions about how this translates to physical building. The 'ground buss' apears to be multiple red lines .... Do I just put in one buss from one end of the chassis to the other, and add green points to it?
I'm just confused as the thing labeled "buss bar" as shown in pic doesn't appear to be a bar at all but more like a star ground, unless I'm looking at the photo wrong.
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Thanks, found the drawing and looking it over. I am going to start implementing this but a few questions about how this translates to physical building. The 'ground buss' apears to be multiple red lines .... Do I just put in one buss from one end of the chassis to the other, and add green points to it?
I'm just confused as the thing labeled "buss bar" as shown in pic doesn't appear to be a bar at all but more like a star ground, unless I'm looking at the photo wrong.
Open to interpretation, I suppose, best I can do is show you what I did, trying to follow that scheme.
Nutshell; bus bar near the pots for pre-grounds, with wire run from there to a small main ground bar near the chassis ground (but isolated from chassis), that included Merlin's hum-block. Then the single connection to chassis ground through the hum-block. Giant orange drop in the hum-block circuit was a stand-in until a smaller part arrived
I did not have a heater CT.
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Thanks for the photos. I am getting ready to start putting together a unit with this gronding scheme.
One question:
On the diagram posted earlier in this thread, the four diodes on the rectifier are paralleled with .01uf/1KV caps. I do not see these caps on your photos, and they are not in the 6G15 reissue schematic either.
Can anyone tell me the necessity of them? Should I add them? No?
Thanks. Pic attached of the reissue schematic showing rectifier.
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I learned (after this build) that those are sometimes added to lessen diode switching noise. I haven't noticed extra noise in my unit, but guessing it's one of those things that probably can't hurt either.
I first encountered those on a '79 Pro Reverb I was rebuilding where they were part of the original design.
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That's great to know, thanks. For now I guees I'll leae them out as I don't have any 1KV caps. Will report back with pics of build, thank you again!
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On the diagram posted earlier in this thread, the four diodes on the rectifier are paralleled with .01uf/1KV caps. I do not see these caps on your photos, and they are not in the 6G15 reissue schematic either.
These 'snubber' caps are often added simply as a matter-of-course in any design with a SS diode rectifier, as it's considered good practice.
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If those diodes are switching the ground situation is already intolerable.
> often added simply as a matter-of-course
Just because it is a habit does not mean it is a good idea.
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I deliberately kept the diagram simple so that it clearly shows that the high current ground returns are grouped separately at the other end of the buss bar (from the low current returns and the hum loop blocker). If I’d drawn it all going a single ground return point for everything, it might get inferred that deliberate separation of high current returns from low current returns is inconsequential - when it isn’t.
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Well most of the layout has been done. I tried to follow that drawing posted earlier, running the higher current stuff apart until it got close to the ground lift circuitry (diodes, resistor and cap to chassis ground).
Any thoughts? I won't lie, running the grounds was odd. Of course, I isolated the in and out jacks, and reverb jacks at rear.
Thanks.
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Very nice indeed!