Twin Reverb brass ground plate questions

[lazza.jazza]

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

[Michael Smith]

What year is your amp? Do the pots all appear to be original? They could be spacers so the pot shafts don't stick out of the front panel too far (if they are replacements)

[lazza.jazza]

What year is your amp? Do the pots all appear to be original? They could be spacers so the pot shafts don't stick out of the front panel too far (if they are replacements)

Hi Michael,
Thanks for the reply.
This amp has been severely butchered. It is (I think) 1975 vintage and was originally a silverface with master vol which somebody has attempted to convert to a blackface.
Looks like an assortment of pots/caps/resistors etc. It is an original but apart from the transformers I don't think anything else is original.
The insulators are very thin so I don't think that they were spacers.
I have decided to carry out a complete refurb as the eyelet boards also look terrible.
Just swapped out the filter caps to start.

The question re the brass ground was because there seem to be so many differing views on it efficacy. Just wanted comments from actual users/owners.
Thanks again,
Larry

[Michael Smith]

Hi Michael,
Thanks for the reply.
This amp has been severely butchered. It is (I think) 1975 vintage and was originally a silverface with master vol which somebody has attempted to convert to a blackface.
Looks like an assortment of pots/caps/resistors etc. It is an original but apart from the transformers I don't think anything else is original.
The insulators are very thin so I don't think that they were spacers.
I have decided to carry out a complete refurb as the eyelet boards also look terrible.
Just swapped out the filter caps to start.

The question re the brass ground was because there seem to be so many differing views on it efficacy. Just wanted comments from actual users/owners.
Thanks again,
Larry

I looked at a couple of my silverface amps, and it's my opinion that the entire brass strip making contact with the chassis is the ground point. I think the blobs of solder at the edge of the brass plate and the chassis are just to hold the brass plate in place until the pots are installed and tightened down.

It's a shame your amp was butchered, but a worthwhile restoration project, as long as you can do the work yourself. Lyle at PsionicAudio on YT recently had a video where he stripped all of the components from the eyelet board, cleaned the wax from it and rebuilt it. It may have been the rusty Bassman series of videos, but I could be mixing that up with another amp, since he does so many.

It would be nice to see some photos of your amp as the work progresses.

[phantomman]

I looked at a couple of my silverface amps, and it's my opinion that the entire brass strip making contact with the chassis is the ground point. I think the blobs of solder at the edge of the brass plate and the chassis are just to hold the brass plate in place until the pots are installed and tightened down.

+1

I think the purpose of the brass plate is to suppress any possibility of ground loops.

[lazza.jazza]

Hi again folks,
Sorry for the delay but have managed to sort this amplifier out.
After completely removing all internal parts and cleaning the chassis, I reassembled everything. This included removal of all components from the eyelet board and cleaning that as well.
After re-assembly I was confronted with exactly the same hum issue
I proceeded to carry out a thorough voltage check and discovered some curious readings all over the place which dd not agree with the published specifications. My probe then rested on an unpopulated eyelet which read as 8 volts!
I replaced the eyelet board with a new one from mojotone and the hum was gone.
Part of the previous cleanup included heating the old eyelet board to remove the wax and then flattening it out. I was sure this would have removed any moisture that I had read could be a problem with these but clearly it did not.
Subsequent readings of power rails and test points agreed (usually to within 5 volts) with the published specs.
I also hard wired the 240 volt primary tap as I had had my suspicions about the state of the voltage selector. On disassembly it proved to be very flakey.
I was going to attach a couple of pictures of the rewiring and finished project but I don't seem to have this entitlement.
It is now looking presentable but more importantly sounds like only a TR sounds. Have you noticed that nobody ever complains that they aren't loud enough :)
Thanks for the interest and help with this - great forum full of great people.

[phantomman]

Have you noticed that nobody ever complains that they aren't loud enough :)

Most gripes about Twin Reverbs concern the weight. Doubly so when fitted with JBL's, Altec's, SRO's, etc.

[lazza.jazza]

The sound is all that matters even if the amp feels like it has been bolted to the floor at the end of a gig.