Fender Twin Reverb brass grounding plate questions

[minnievirginny]

Hello Folks, I am hoping for some advice regarding the refitting and wiring of the brass grounding bus on my amp (Fender Twin Reverb) which runs behind the control pots/input sockets. I am rather confused by: 1 What appear to be insulation washers between the brass plate and the chassis. They are mounted on the pot thread. I am trying to fathom the sense in the apparent attempt to isolate the brass from the chassis and the fact that the pot, once fitted and tightened will clearly connect the chassis to the plate electrically. 2 My amp had the brass plate soldered to the chassis about half way along but in all of the images I have studied this is not the case and the general advic seems to be to ground the plate as far from the Main power section grounding as possible (i.e. near/on the NORMAL input sockets). If anybody has any advice on this and, ideally, a comprehensive diagram of the correct grounding layout I would be very obliged to hear/see it. I have studied the schematics and see the various locations of the ground wires leaving the pots etc. but the images don't seem to specifically show to which points they should be attached. Thanks for having a look at this Larry

[Chuck H]

Some will tell you that the original idea was to sepatate the signal ground from the AC ground. And maybe it was. As you've already noted the metal pot cases make this effort moot. So, not that I know this for certain but...

It seems more likely that the brass and steel are isolated for two other possible reasons. One would be bi-metal reactions and possible oxide corrosion avoidance (stretch). The other would simply be to keep direct metal contact from acting as a heat sink and making soldering more difficult. It's possible all three reasons exist for the isolation washers and that the prototype for the brass plate system was actually electrically isolated. Then a less than perfect version of the same system was implemented for production relative to costs. In other words, the isolation washers may be nothing more than an artifact of the proto circuit but since the amps were working fine as built this was never reexamined. That sort of thing happens a lot. No reason not to actually isolate the brass plate if you want to. It would be more ideal.

Grounds in those amps from the factory are generally taken from wherever they occur on the board right to the plate. And yes, the ground for the brass plate to the chassis should be close to the input jacks. Though, again as you noted, the brass plate is pretty much grounded at every pot mount. Grounding near the input makes sense if you do isolate the ground plate.

[x-pro]

It's simple. Currents wandering along the device body induce noise (buzzing, ringing, crackling) on highly sensitive circuit elements. According to the logic of tracing, grounding, as correctly pointed out, is done at the input. But sometimes it does not give a positive result at the installation. Something will get through from the noise. Therefore, the installer at the stage of assembly, finds the point of the best flow of interference to the body. It's quite a painstaking task.
This is why insulators appear in places where customarily there are none.

[Chuck H]

It's simple. Currents wandering along the device body induce noise (buzzing, ringing, crackling) on highly sensitive circuit elements. According to the logic of tracing, grounding, as correctly pointed out, is done at the input. But sometimes it does not give a positive result at the installation. Something will get through from the noise. Therefore, the installer at the stage of assembly, finds the point of the best flow of interference to the body. It's quite a painstaking task.
This is why insulators appear in places where customarily there are none.

As noted, the isulators are not actually insulating anything. This was not a question about grounding practices exactly. The question, as I interpret it, is why are the insulators there when they don't insulate and is there another purpose for those insulators.

[g1]

I must be misunderstanding something. Insulating washers between the chassis and the brass plate? Fiber, plastic? What keeps them from falling down during assembly before the pots are installed?

[x-pro]

Maybe the nut that screws onto the pot threads is not making electrical contact with the front panel due to the oxide film on it.

[Tom Phillips]

Photos of the subject amp showing the details discussed would be useful. By inspecting the photos one or more of the members may recognize the workmanship as original Fender or an aftermarket mod attempt. We would also then know which of the many versions of "Twin Reverb" we are dealing with.

[minnievirginny]

Hello Gentlemen,
Thank you all for your very useful contributions.
ChuckH, Yes your reasoning regarding the existence of those washers makes a heck of a lot of sense and would certainly explain their presence.
G1, I also wodered about how the amp would be assembled given a there doesn't appear to be any means of retaining the washers whilst the pots were being installed. I guessed that the pots would be introduced to the brass plate,then the washers placed over the threads and then the brass plate and washers faced up against the back of the facia but who knows. Washers are some sort of mica like power transistor insulators are made from.
XPro, there is a degree of corrosion which could certainly contribute to the noise (the real reason I dismantled that part of the assembly).
Tom Phillips, I believe this was originally a 1975 Silverface with a Master volume which somebody has attempted to Blackface. apart from the transformers, I don't see any parts which resemble anything original.
I have decided to carry out a complete refurbishment. The exterior looks great and it has a pair of G12K-85's so a worthwhile project I reckon. What could go wrong?......
So far have replaced filter caps.

Thanks again for your comments and help,
Larry

[Tom Phillips]

...I believe this was originally a 1975 Silverface with a Master volume which somebody has attempted to Blackface. apart from the transformers, I don't see any parts which resemble anything original...

Larry,
Given your latest comments, my take is that someone attempted to improve the stock Fender grounding scheme. The stock Fender grounding works pretty well given all the design rules that it seems to violate. There is room for improvement. However, a part way job can result in worse performance and a complete rework of the grounding is a significant task.
By the way. Fender did often include a solder connection between the brass strip and the chassis. Attached is a gut shot of a stock condition 1974 Super reverb showing one such connection.
Good luck with your project.
Cheers,
Tom

[Chuck H]

Attached is a gut shot of a stock condition 1974 Super reverb showing one such connection.

Strange the attachment doesn't show (even in your text) until I instigated a quote on your post. Then I can see there's an ATTACH order and this is what appears:

[Chuck H]

Ok. That's bizzare. I copied the attachment text and it won't print or appear on my post either.?. Not even the text!?! But when I back through the line I placed it on there is a blue dot that demonstrates on that line as I back through it. Definitely a glitch here.

[Tom Phillips]

Strange the attachment doesn't show (even in your text) until I instigated a quote on your post. Then I can see there's an ATTACH order and this is what appears...

There was some weirdness when I made the post. First the post disappeared when I clicked the preview button and I couldn't find the Post Reply button. I eventually got everything back after using the browser back arrow and then the Restore previously saved text feature.

[minnievirginny]

Hello again,
Thanks for the suggestions and comments. Although I was unable to open the attachment I did a little exploring and found images of soldered brass-to-chassis examples although to know that this was original is useful and confirms that Fender was following this practice.
Yes Tom, I think the previous owner decided to alter stock everything!

My next job is to search around the workshop floor to find the circlip which has sprung off my solder sucker .

[Chuck H]

My next job is to search around the workshop floor to find the circlip which has sprung off my solder sucker .

Joy! Sometimes if I drop something metal (screw, ring, washer, c-clip) I use a speaker magnet side down and wave it around close to the floor.

[Leo_Gnardo]

I did a little exploring and found images of soldered brass-to-chassis examples although to know that this was original is useful and confirms that Fender was following this practice.

Excellent.

I wouldn't sweat having any isolating washers between brass plate and chassis. While you have it apart, I'm sure you've cleaned up any debris & corrosion between brass and chassis. With a strong soldering iron it's easy enough to solder brass to chassis in one or more spots but wait until you have at least some of your pots and jacks mounted, it would be a shame to solder that then discover the pot/jack holes aren't squarely lined up. I use a Weller WP80, a large "pencil" iron with a massive chisel tip that effectively transfers heat to chassis metal and whatever you want to solder to it. One thing to avoid is the common Fender practice of running ground lead from pots and such (like the 6K8 "midrange" resistors for instance) to pot bodies. Too iffy a connection for me. Solder those ground leads to brass and you're doing better than factory.

70's Fenders had very thin tooth washers between base of pot bushing and chassis. With a full sized tooth washer, there's little or no bushing thread poking out beyond the faceplate which makes it difficult to secure the pot mounting nut. So, thin tooth washer it is. Or none at all if necessary to get a grip on pot threads with the nut.

Here I just finished working on an early 60's Concert amp. No special anything involving the brass grounding plane. Amp is marvelously devoid of typical hum and buzz, and hiss starts to pick up some as volume controls are advanced past 5. A clear winner in the noise department. I think the fellow who worked on your Twin in the past may have had a bit too much coffee, he was overthinking the situation and trying to solve a problem that wasn't really there. I've never done that ...