1964 Fender Spring Reverb Unit 6G15

[Yeatzee]

Encouraged by Sluckey to start a thread on this project. Decided to open up my '64 Spring Reverb Unit and see what's what and get it dialed in for when I start up gigs again in the Fall.

Same as my Vibrolux Reverb series, I'll be documenting every step on YT. Part 1 is up:

i=noe-5YrGI3-nw4jN

What was bizarre to me was the 3 prong wiring in it from the previous owner/tech, which had the earth going to a terminal strip connected to the chassis with two diodes in between the wire and ground. I'd never heard or seen that before, and the several people I asked about it hadn't either. So I removed it and soldered the earth to the chassis like I always do with 3 prong conversions. Well, it turns out there was a reason for it! Ground loop and hum when connected to another amp apparently can occur, and for that reason people don't even do 3 prong conversions on these I guess?

So looking for some insight into what options I have. I've heard I can buy an iso box from Lehle or the like for $100, which I really don't want to have to splurge on right now. What else? Cut the green but leave the 3 prong? Re-install a 2 prong? Seeing as it's a clean vintage unit, I don't want to completely dismantle it to do the ground scheme found on the reissue / modern kits.

Of course, when I fire it up I could find the hum is minimal and not worry about it 

One other idea came to mind, my fender's have courtesy outlets on them. What would happen if I installed a 3 prong courtesy outlet and then plugged the reverb into that? Would that be better or worse as far as ground loops are concerned? Not sure I understand how they work just yet.

[tdvt]

I've not played an original, though I understand that circuit it fussy about grounding & shielding, especially with the reverb tank so close to the tubes & chassis.

With that reputation in mind, when I built one last winter, I built the entire unit with isolated grounds right from the start, & also incorporated Merlin's hum block circuit for the single point where the amp itself connects to AC earth.

Result was no noise or hum on first start-up.

Short build thread  HERE 

[sluckey]

Evaluate your hum situation when you get this project back together. No need to rush in if there is no hum issue. I'm not optomistic though. I told you what I did to resolve my Revibe hum issue.

One other idea came to mind, my fender's have courtesy outlets on them. What would happen if I installed a 3 prong courtesy outlet and then plugged the reverb into that?

Interesting idea that's worth pursuing. Send a PM to member p2pAmps. He has a source for grounded courtesy outlets that fit the existing hole on old Fender amps. This also gave me another similar idea that could be worth pursuing... Mount a 10-32 screw to each chassis to act as an earth bonding screw. Make a short 12AWG stranded wire with spade lugs on each end and connect the two chassis together using this bonding strap. This would probably work best if the 6G15 is sitting on top of the main amp such that the bonding strap could be short. You could leave the strap connected to the 6G15 so you only have to fasten the other end to the amp bonding screw.

Meanwhile search the forum for "6G15 ground loop" for lots of discussion on this issue.

[HotBluePlates]

... What was bizarre to me was the 3 prong wiring in it from the previous owner/tech, which had the earth going to a terminal strip connected to the chassis with two diodes in between the wire and ground. ... I removed it and soldered the earth to the chassis ... Ground loop ...

Now you understand what Jeff Gehring was describing here.

Short Short Answer:  Cut the Ground wire of your 3-Prong Cord on the 6G15 Reverb.  Hum gone, and all good.

Short Answer:  you don't need a 3-prong cord on the 6G15 Reverb, because its ground is already connected to the Amp's ground, which then connects via the Amp's 3-Prong Cord to the Ground Pin of the outlet.  As soon as you give the Reverb its own 3-prong cord, you've created a giant ground loop (and hum).

Long Answer:  Put the anti-parallel diodes and resistor between that Cord Ground Wire & the chassis, as (sorta) drawn here.  You will lose most of the safety benefit of the 3-wire cord, except for actual high-voltage on the chassis (which is where it matters most anyway).  And... you really still have the safety ground through the interconnecting cables & the Amp's 3-Wire-Cord.

$$$$$ Answer:  Leave the 3-Prong Cord on the 6G15 Reverb, and also the 3-Prong Cord on the Amp.  Now you have super-redundant safety in case your interconnecting cable's shield/ground gets a bit flaky.  Solve the hum by spending $100-200 on a high-quality audio isolation transformer to break the ground loop on the signal-side (rather than the power-entry side).

[tubeswell]

Here's what I do for grounding a 3-prong mains reverb unit. Done it several times now. Works like a charm

[pdf64]

There’s no way I’d consider plugging my guitar cable into a reverb unit that wasn’t solidly earthed.
For an original vintage Fender, the only safe, no hum options are to either earth the unit properly and use a signal isolation transformer, or use a wireless link between guitar and anything else, combined with powering the gear via a GFCI / RCD protected mains feed.

[Latole]

There’s no way I’d consider plugging my guitar cable into a reverb unit that wasn’t solidly earthed.

I agree.

[sluckey]

Here's what I do for grounding a 3-prong mains reverb unit. Done it several times now. Works like a charm

And that's exactly what Fender did with the reissue unit. Easy to do on paper or a new build. Very involved to do on an original 6G15. Impossible to do if you are concerned with maintaining originality.

[sluckey]

Take a look at this link for another method of stuffing old e-caps. Little more work but looks much more authentic than foam rubber. Don't know why he split the cardboard tube though. No need to do that.

     https://ampgarage.com/forum/viewtopic.php?p=454812#p454812

[Latole]

Take a look at this link for another method of stuffing old e-caps. Little more work but looks much more authentic than foam rubber. Don't know why he split the cardboard tube though. No need to do that.

     https://ampgarage.com/forum/viewtopic.php?p=454812#p454812

We must be registered to see pictures

[sluckey]

So sign up. It's painless. And worth the effort. You'll get to see some beautiful amp building projects. And you'll recognize some names from this forum as well.

[Yeatzee]

Take a look at this link for another method of stuffing old e-caps. Little more work but looks much more authentic than foam rubber. Don't know why he split the cardboard tube though. No need to do that.

     https://ampgarage.com/forum/viewtopic.php?p=454812#p454812

Roger that, requested to join! You're too quick I didn't have a chance to post part 2 yet lol

[Yeatzee]

Part 2

i=aYsTP44jq2wrunQd

[Latole]

So sign up. It's painless. And worth the effort. You'll get to see some beautiful amp building projects. And you'll recognize some names from this forum as well.

I follow too many forums.

[sluckey]

Roger that, requested to join! You're too quick I didn't have a chance to post part 2 yet lol

Sure you did. That's why I made this post. 

[sluckey]

So sign up. It's painless. And worth the effort. You'll get to see some beautiful amp building projects. And you'll recognize some names from this forum as well.

I follow too many forums.

Yes, we know. It's OK. That message was not meant for you.

[Yeatzee]

Take a look at this link for another method of stuffing old e-caps. Little more work but looks much more authentic than foam rubber. Don't know why he split the cardboard tube though. No need to do that.

     https://ampgarage.com/forum/viewtopic.php?p=454812#p454812

Got accepted into the forum, super interesting so he cuts the old caps (how messy is that?) in half and then places the new cap inside the old shell. Doesn't show how he removes the leads from the lips / installs the new leads. Maybe it's obvious when you take one apart.

[Latole]

How to with pictures;

https://hackaday.com/2022/07/30/fix-old-caps-but-keep-that-can-capacitor-look/

https://lensprojects.com/2019/02/15/recapping-can-capacitors-in-antique-electronics/