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Post subject: reverb screeching on vibrolux
Posted: Wed Oct 22, 2014 5:30 pm
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Hi all. Hope we can sort this out without going to a tech ($$$$)
first off, the amp is a a 2012 custom vibrolux reverb.
the problem is not the typical hiss associated with these amps.
the problem occurred after transporting from a gig.
when i played the amp, it was making a terrible hiss-screech, when the reverb is rolled on. goes away with the reverb off, kinda.
the rca cable going to the return the amp was damaged, the pin came out.
So i replaced the cable with a new one, didn't fix it.

Ive done a bunch of tests, it is not the tank, it is not the cables, and is not the tubes.

If I leave the cables off, and turn up the reverb knob, it hisses loudly. if i plug the cables and tank back in, it hisses and screechs, but the reverb is working.

If i pull the cables and tank, and pull the v3 tube, it still hisses when i turn up the reverb.
I tried a different tube, same problem.
Its possible its a resitor or cap, just not sure where to start looking.
any help would be appreciated


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Post subject: Re: reverb screeching on vibrolux
Posted: Wed Oct 22, 2014 11:38 pm
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Hi willydee,

There is one tube for the reverb driver, V3, which should be a 12AT7, and another tube for reverb recovery, V4, which should be a 12AX7. Did you try both tubes, and are both the correct tubes? I would make sure that whichever tubes you try for substitutes are known good tubes, it is possible that even a "new" tube can be microphonic.

Welcome to the forum.

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Post subject: Re: reverb screeching on vibrolux
Posted: Thu Oct 23, 2014 2:37 am
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shimmilou wrote:
Hi willydee,

There is one tube for the reverb driver, V3, which should be a 12AT7, and another tube for reverb recovery, V4, which should be a 12AX7. Did you try both tubes, and are both the correct tubes? I would make sure that whichever tubes you try for substitutes are known good tubes, it is possible that even a "new" tube can be microphonic.

Welcome to the forum.



+1 You are right he must try 2 new tubes.

Tubes are very fragile when hot , best is wait before moving a amp


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Post subject: Re: reverb screeching on vibrolux
Posted: Thu Oct 23, 2014 1:57 pm
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Thanks for your replies.
Yes Ive tried different tubes in the v4 as well. No Change.

My amp doesn't work with the tube removed from v4. is this normalÉ


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Post subject: Re: reverb screeching on vibrolux
Posted: Thu Oct 23, 2014 3:25 pm
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willydee wrote:
Thanks for your replies.
Yes Ive tried different tubes in the v4 as well. No Change.

My amp doesn't work with the tube removed from v4. is this normalÉ



Yes


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Post subject: Re: reverb screeching on vibrolux
Posted: Thu Oct 23, 2014 3:26 pm
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willydee wrote:
H

Ive done a bunch of tests, it is not the tank, d


How do you test the tank ?


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Post subject: Re: reverb screeching on vibrolux
Posted: Thu Oct 23, 2014 6:28 pm
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it makes the noises without the tank or cables plugged in


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Post subject: Re: reverb screeching on vibrolux
Posted: Thu Oct 23, 2014 6:35 pm
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willydee wrote:
...the problem occurred after transporting from a gig...the rca cable going to the return the amp was damaged, the pin came out.
So i replaced the cable with a new one, didn't fix it...


I would have a close look at that jack. Anything that caused damage to the RCA cable could have also damaged the jack. Did you get all of the pin out of the jack? Possibly a missing ground on the jack, or damaged jack. :?:

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Post subject: Re: reverb screeching on vibrolux
Posted: Thu Oct 23, 2014 6:50 pm
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that's what i figured too. but i don't see anything wrong at all visually. the pin is completely out, the connections look fine, ive jiggled just about every wire and shocked myself a few times. nothing has helped or hurt. I haven't taken the board out of its chassis yet to look underneath, but the top of the board appears fine.

I wonder if the pin grounded out at some point, would this have damaged somethingÉ

or could plugging the the reverb cables in backwards have damaged somethingÉ

all the problems seem to occour because of the return rca jack on the amp, the one on the left when looking at the back of the amp. this is the same plug that had the pin stuck in it.

and why cant i make a question markÉÉé


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Post subject: Re: reverb screeching on vibrolux
Posted: Thu Oct 23, 2014 6:52 pm
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ok got the question mark back. ?


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Post subject: Re: reverb screeching on vibrolux
Posted: Fri Oct 24, 2014 2:08 am
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stratele52 wrote:
willydee wrote:
H

Ive done a bunch of tests, it is not the tank, d


How do you test the tank ?



Answer ?

We don't know how you are qualified to said that , may be you are ?


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Post subject: Re: reverb screeching on vibrolux
Posted: Fri Oct 24, 2014 12:54 pm
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forget the tank, and the cables. it ,makes the noise without them even plugged in


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Post subject: Re: reverb screeching on vibrolux
Posted: Fri Oct 24, 2014 4:41 pm
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That would seem to narrow the problem down, starting with the return jack forward. :idea:

Of course, removing V4 you will lose all sound, as that tube has two halves used for two different circuits; half reverb return and half gain stage in the signal path.

Have you checked the circuit around V4, namely the Plate resistor R29, the bypass cap C15, and less likely but possible coupling cap C16? Be sure to thoroughly check that return jack.

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Post subject: Re: reverb screeching on vibrolux
Posted: Sat Oct 25, 2014 1:28 am
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Check about wiring dress , all free wires ( non shielded ) must be run close to the chassis .

I see some pictures of this amp and if it is the right amp , wiring need some light work .

Any picture inside your amp ?


I write that if you are sure about tubes you try . some new one could be noisy


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Post subject: Re: reverb screeching on vibrolux
Posted: Sun Oct 26, 2014 2:49 pm
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shimmilou wrote:

Have you checked the circuit around V4, namely the Plate resistor R29, the bypass cap C15, and less likely but possible coupling cap C16? Be sure to thoroughly check that return jack.


ive had a look at these items, all appear normal, how can i test them with a multimeter?
Ive disconnected the shield wire and ground going to the rca jack, no change, so the jack is fine.

the hot wire on the return, as soon as i touch this wire, either with finger or plugging into the tank, the amp screeches at high volume, whether the reverb is turned up or not, or even if the volume is down on the amp


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