bradm
Junior Member
Posts: 74
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Post by bradm on Mar 27, 2018 15:20:36 GMT -7
So, I'd re-tubed my Galaxie...added a PPIMV to it...(kept the Presence control. the PPIMV was parked in the extension speaker jack spot, I just use a 1x12). After all was said & done, I still had a very low level 60 cycle hum that bugged me to no end. You couldn't hear it while playing, but mute the strings and there it was. Turn the volume down...still there...turn the PPIMV off and the noise was gone.
Typically it was a low enough level that I'd suspect power caps, but I'd already replaced them (which lowered the hum level just a bit). The next step? Grabbed a meter probe and grounded the banana jack to the chassis and started probing pot backs and jack grounds and no difference. One thing I did note, pot case grounds were at the opposite end of the chassis connection vs. the power caps ground. So, I soldered a 3" piece of 20 gauge wire from the pot cases' grounding wire direct to the power caps' ground...hum completely gone.
Word of caution, you can't necessarily do this for all amps. Some don't directly ground their speaker jack and it won't be advisable, too. With this amp, the chassis is the ground reference throughout the amp, so probing ground & chassis connections that should be connected was fine to do. I might have resolved the issue by unscrewing and raising up the board and verifying the chassis connection from the power caps to the under-board chassis connection, or loosening the pots and spraying DeOxit between the pots and the inner chassis... but it wasn't worth it when a short, above board wire connection silenced it.
If any of you are suffering some very low level hum and are just living with it? You don't need to be. The doc builds some silent amps...but, like all metals, they all will suffer from oxidation issues over time. If you're tech-capable, it's a simple fix. If not, send it to the doc for a checkup!
Cheers! Brad
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Post by bgkyt1 on Mar 30, 2018 9:31:40 GMT -7
i'm a novice but, it's common for modern amps to ground the preamp section near the input jack and the power section near the power tran.
this might not apply to your situation, but in general:
if an amp, over time, ends up with ground issues that could be caused by loose bolts or oxidation, and can be fixed by cleaning and tightening parts, the solution shouldn't be to solder jumper grounds to other ground points. (unless you're at a gig and that's the quickest short-term fix). if the amp was ever later serviced and those parts were cleaned and tightened, there would then be a ground loop issue and that jumper would have to be removed.
soldering new wires to a stock amp to fix hum implies there was a design (or build) problem.
if an amp that had no ground issues was modified and now has ground issues, then it's possible the mod introduced the problem.
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bradm
Junior Member
Posts: 74
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Post by bradm on Mar 30, 2018 11:29:36 GMT -7
Here are the facts. 1 - The hum was there before the 'mod'. (A cross-line PPIMV is a very simple install in this amp...the solid-core twisted wire is securely elevated and not introducing any issue in & of itself). 2 - The fact that the pot cases being grounded does indeed suggest an oxidation issue is the likely culprit. However, soldering one wire to redirect is MUCH simpler than either removing and or cleaning all the pots on the front panel (given they're all also soldered together). 3 - the bottom line is...yes, the pots "could" be cleaned. But, that issue is doomed to repeat itself. This is what happens when you rely on the chassis to be the 'star' in your ground system as aluminum, in this case, is known to oxidize and produce these results. That's just the way it is. The wonderful thing about aluminum...it's a WHOLE lot lighter than steel! I'll take the oxidation issues and redirect the grounding scheme with soldered copper wiring to compensate...thus, alleviating the issue. It's amazingly quiet...and has some amazing tone (as the Doc has proven to do)...time to jam!
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Post by zpilot on Mar 30, 2018 16:18:13 GMT -7
Doc recently shared this:
I believe he picked up some of this knowledge from working with medical equipment where grounding is a VERY critical issue. I was QA Manager for a company that, among other things, manufactured medical devices so I've dealt with the FDA requirements on this. Guitar amps are nothing compared to that. I trust that grounding through the aluminum chassis has something to do with how lively my Dr Z amps are.
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bradm
Junior Member
Posts: 74
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Post by bradm on Mar 31, 2018 14:57:19 GMT -7
Oh, I'm not doubting the chassis makes a great ground. But, the issue still remains that we typically don't use medical grade components when spec'ing everything else. Case in point...look how the pots are constructed. In many cases, the back plate is simply attached with 4 tabs bent and touching the front using small points of contact and not soldered.
I can tell you with certainty that one Jaz issue I resolved was due to an oxidation issue regarding the speaker jack not properly completing the ground due to oxidation. Once I removed the jack and steel-wooled the aluminum chassis' interior and reassembled...hum gone. How did I discover it? Just a simple pressure point to the jack enough to notice there was a difference in hum level.
With most Marshall amps, for example, they're not grounded through the chassis for completing the path...it's a hard-soldered wire back. Instead, their issues arise often at the FX loop jacks and, again...we have an oxidation issue. No amp escapes this somewhere at some point. However, when using multiple points of contact that aren't bonded except by pressure and with dissimilar metals, you have a greater potential for this to happen. It just takes time, but it will happen.
In medical equipment, I'm guessing there are a whole lot more preventative steps taken to prevent this from happening in the life of the equipment. Right? The issue I'm making with an aluminum chassis is...when it gets to the point of becoming an issue...you can't see it.
Now, in regard to grounding through the chassis and how lively it is? I don't buy that. What I will say is the Doc has (and still) puts a LOT of time into research to make some circuits that prove out to work well whether you're a single coil or a hum bucker player...THAT is hard to do time after time. And, i do trust the reliability of his circuit board construction...to date, it's the most consistent I've jumped into the middle of.
I have also read many threads now on the boards here regarding hum (do a search)...and some live with small amounts of it. Those really low levels that are overcome with just soft playing? You don't have to live with. Some do and some have voiced "they expect it". I heartily disagree. I've worked on 7-8 Z amps of varying models to date and all of them were made to be hum-free in regard to anything you can hear. You also have to expect that when these amps are leaving his hands...they are just as quiet as when I jump into them...but, the components are new...oxidation is just beginning and typically realized by the time I see some (years later).
In some cases, it was a jack here or there...in some cases, a quick wire from pot cases to filter caps secured the ground connection that the pots can't keep with their pressure connections.
I love the amps and will continue to keep my Galaxie (which is VERY rare for me to keep any amp for very long). I just needed to make a couple tweaks to keep it in top shape for a longer period.
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