Hey ECC83! gotta question ............

Lt. Bob

Lt. Bob

Spread the Daf!
I just got a little Ampeg GVT-5H ................ tested it at the house ...... worked great.

Went to do a rehearsal and plugged in the little 10" cab I use with ALL my little 5 watt amps and got no sound ...... so I'm holding my git and reached around back to push the speaker plug and make sure it was all the way in and got the crap shocked outta me! I mean fairly highish voltage too ...... woke me right up! :D

So anyway ...... this cab, which I have used for years, has a hardwired cord with a 1/4 plug on the end. At the time I put the cord on I was in a hurry and only had a TRS plug handy so I just used that and left the ring disconnected. Never been a problem but apparently it was a problem with this amp. Once I got home and changed the plug to a standard 1/4" it worked fine.

For the life of me I can't figure out why using a 1/4" TRS would cause a high voltage potential between the input ground and the ground of the speaker jack if the ring on the TRS is not connected to anything.

I opened the amp up and nothing is burnt and it seems to work fine and didn't blow a fuse so it wasn't damaged apparently.
The speaker jacks in the amp look like they have 3 conductors although, of course, they must only use two for the output and they are isolated. And they are 'floating' output jacks isolated from the chassis.

But some of my other 5 watters have isolated jacks too and it's never been a problem 'till this one.
Any ideas?




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The output transformer has failed primary to secondary short.
This happens very rarely, I only know of one other instance, but since CAN happen it is vital that one side of the secondary be bonded to an earthed chassis then you would just keep blowing HT fuses. I cannot imagine what the designers of that amp were thinking by not so earthing it.

Dave.
 
I love this kind of thread. It's like "Hey Bob, did you know there's a PM feature on this forum?" Except I'm curious about this one.

There doesn't seem to be a schematic available on this model, but the block diagram in the manual shows that the speaker jacks should be referenced to the chassis ground with everything else. Is it possible that those jacks use some strange switching such that the speaker return is on the sleeve connection of the jack while actual chassis ground is on the ring? Inserting a TS plug would connect the two. With a TRS jack in there, the shield would kind of be floating, and when you touched the plug, the full power coming through the speaker was trying to complete its circuit through you. Sounds crazy, but it works now with the correct plug, which seems to contradict the shorted transformer.

And let that be a lesson to you! That cable is not shielded is it?
 
ashcat_lt said:
I love this kind of thread. It's like "Hey Bob, did you know there's a PM feature on this forum?" Except I'm curious about this one.
Click to expand...
That's why I made it a thread ..... it seemed like a subject that might be interesting.


ashcat_lt said:
That cable is not shielded is it?
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No ..... it's just standard zip type speaker cable.

And I agree ..... it works great with the right plug in there so it doesn't seem have a failure in the tranny.

Wow .... that was only 5 watts I felt? Surprisingly strong shock ..... more than I would have thought.
The jack does feel like there's some switching going on when you insert the plug and it occurred to me that the sleeve of a TS plug might be making a connection but I couldn't think of where the juice was coming from.
So a TS plug makes the shunt to ground and without that I became the pathway. That's pretty damned interesting.
 
Well, that's about the only thing I can think of that fits all of your facts... We'd need a schematic or a careful look at the insides to say for sure.

I mean, it is completely possible that they just used those TRS jacks because they had a bunch of them and didn't feel like spec'ing any extra parts. That's not particularly uncommon. It could well have been that the shock you got was just some fluke and it would never have happened again.

Honestly, I can't figure out why they fuck they'd have wired it the way I suggested.
 
ashcat_lt said:
Well, that's about the only thing I can think of that fits all of your facts... We'd need a schematic or a careful look at the insides to say for sure.

I mean, it is completely possible that they just used those TRS jacks because they had a bunch of them and didn't feel like spec'ing any extra parts. That's not particularly uncommon. It could well have been that the shock you got was just some fluke and it would never have happened again.

Honestly, I can't figure out why they fuck they'd have wired it the way I suggested.
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yeah ..... that's what I thought too ...... what would be the point?

Do you think really they'd use 'em just cause they had a bunch? :D

I imagine it's Chinese so maybe ....
I had it open last night and it's a well built little thing ...... very neat wiring ..... good sized solid parts really laid out nicely ..... much more substantial than some of my other 5 watters.

I'll have to figure this out ..... it'll drive me crazy ...... I'm gonna tear it down next week and look closer.
 
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