An amp I just fixed was a disaster waiting to happen...

H

HamishR

Well-known member
As I have mentioned here recently I had a friend's pair of '62 Fender brown amps to repair in my workshop. The brown Super is remarkably original. It had a replaced PT which seems to be quite common but the board appears completely stock other than the electrolytics which have obviously been changed and I updated also.

But as I installed the 240V PT and connected the new 3-pin power cable I discovered that the fuse had been completely bypassed. It looked like it was connected but one end was completely disconnected, hidden by wire. So that was alarming! What kind of idiot bypasses the fuse??

All the PT grounds had been removed from the chassis solder blobs and attached to the PT mounting bolts via eyelets. Now I understand that this is a bad idea because the PT nuts can work loose so all of those connections are now back soldered to the chassis with my big heavy duty iron.

So now the amp is safe and runs even quieter than before. It's amazing how quiet an amp can be when it's grounded properly.
 
HamishR said:
As I have mentioned here recently I had a friend's pair of '62 Fender brown amps to repair in my workshop. The brown Super is remarkably original. It had a replaced PT which seems to be quite common but the board appears completely stock other than the electrolytics which have obviously been changed and I updated also.

But as I installed the 240V PT and connected the new 3-pin power cable I discovered that the fuse had been completely bypassed. It looked like it was connected but one end was completely disconnected, hidden by wire. So that was alarming! What kind of idiot bypasses the fuse??

All the PT grounds had been removed from the chassis solder blobs and attached to the PT mounting bolts via eyelets. Now I understand that this is a bad idea because the PT nuts can work loose so all of those connections are now back soldered to the chassis with my big heavy duty iron.

So now the amp is safe and runs even quieter than before. It's amazing how quiet an amp can be when it's grounded properly.
Click to expand...
The grounding scheme really makes all the difference.

Also, here's my reenactment:
"Hmm, I don't have any fuses on hand. This'll sort it for now ...."
 
HamishR said:
As I have mentioned here recently I had a friend's pair of '62 Fender brown amps to repair in my workshop. The brown Super is remarkably original. It had a replaced PT which seems to be quite common but the board appears completely stock other than the electrolytics which have obviously been changed and I updated also.

But as I installed the 240V PT and connected the new 3-pin power cable I discovered that the fuse had been completely bypassed. It looked like it was connected but one end was completely disconnected, hidden by wire. So that was alarming! What kind of idiot bypasses the fuse??

All the PT grounds had been removed from the chassis solder blobs and attached to the PT mounting bolts via eyelets. Now I understand that this is a bad idea because the PT nuts can work loose so all of those connections are now back soldered to the chassis with my big heavy duty iron.

So now the amp is safe and runs even quieter than before. It's amazing how quiet an amp can be when it's grounded properly.
Click to expand...
At one shop I worked in we had a horn repair section in the room next door. I'd sometimes have my bandmate who worked in there do Fender chassis grounds with an oxacetylene torch, but I still have an old school 300 watt iron meant for stained glass work kicking around just in case I ever take on any more repair work.

Bypassed fuses are just another of many WTF diversions for experienced repair techs, IME. Social Darwinism is alive and well, I reckon. ;)
 
On Dynacord Bass-King I’ve been working on I made this sin you’re talking about, but the my bypass was done for a fuse on heater tap. 😌
 
I've been doing mine via solder tabs on the PT mounting screws.

They can come loose, but a properly torqued connection with a locknut and lock washer would require an act of God to loosen up. For good measure, stick a tamper-proof drive machine screw in there so dumb dumbs don't try anything.

Electricians use fine threaded sheet metal grounding screws all the time for grounds in commercial buildings, solder may be a more permanent solution...but its got plenty of drawbacks too.

Like with speaker connections: I prefer fastons to solder, because a good crimp joint is easy and doesn't run the risk of making the wire brittle.

Hot take, I know.
 
Stickman393 said:
Electricians use fine threaded sheet metal grounding screws all the time for grounds in commercial buildings
Click to expand...
I won’t deny that people do this, but it’s definitely not correct.

Code requires machine screws into threaded holes. Tap tool usually costs less than failing an inspection.
 
I'm referring to the single and dual gang junction boxes they often use: they're like, 16-12 gauge steel sheet. Still sheet metal, still not a ton of material for the threads to bite onto.

Not the folks that use coarse-threaded self tappers. That's hack shit.
 
Mr Psionic suggests that it's the weight of the transformer which helps to loosen bolts over time. I would imagine that a well torqued bolt, nut and lock washer would be ok for ground connections if that was all that they did - and Lyle at Psionic has said as much himself. In amps with an aluminium chassis that is what I do for ground connections.

If your amp sits at home never moving then it's probably not at risk of PT connected ground eyelets becoming loose. But Lyle fixes amps which get moved from gig to gig and has found an issue and I respect his opinions. It does make sense to me - transformers can be very heavy and are attached to fairly thin sheet metal most of the time. I've seen the sheet metal distort on quite a few amps over the years, and that can't be good for the connections. Anyway, I figure that it's worth fixing the most important ground connections in the amp in the safest way possible!

Friends of mine play out frequently with amps I have built and I would be distraught if one of them suffered catastrophic failure because of poor construction. Once you have the correct soldering iron it's not difficult to solder the grounds to the chassis so that's what I have started to do. IME the main issues with these kinds of connections failing is lack of heat when made or remade with an inadequate iron.
 
You must log in or register to reply here.
Back
Top