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Vibrolux squeals briefly when taken off standby

 
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yowza



Joined: 29 Jan 2011
Posts: 67
Location: Virginia

PostPosted: Sun Jul 24, 2011 4:33 am    Post subject: Vibrolux squeals briefly when taken off standby Reply with quote

I have an old black faced SF Fender Vibrolux and tonight the amp squealed very briefly when I took it off standby and after a second or two the squealing stopped and it seemed to be fine and sounded really good. I flipped standby back on again, waited a few seconds and then turned standby off and it squealed again just like before. It almost sounds like a siren blowing for a second or two then goes away and the amp sounds good. It only happened when coming out of standby but I haven't tried leaving standby off and turning the amp on and letting it come on that way. I had all of the amp volume and tone knobs down and there was no guitar plugged into the input so it wasn't acoustic feedback.

Could the stand by switch have failed or is this likely a tube or other component? It's a little late to mess with it any more tonight but tomorrow I'm going to swap each preamp tube out individually and make sure it's not a tube

Is there any other spot in the circuit to look?

Thanks!
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martin manning



Joined: 06 Jul 2008
Posts: 5190
Location: Queen City

PostPosted: Sun Jul 24, 2011 1:55 pm    Post subject: Reply with quote

Your plan sounds good. Strange that it occured with all the controls down, though. Look here for some more suggestions and a good general toubleshooting guide: http://www.geofex.com/ampdbug/ampdebug.htm

Last edited by martin manning on Sun Jul 24, 2011 1:57 pm; edited 1 time in total
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Firestorm



Joined: 25 Jan 2008
Posts: 2600
Location: Connecticut

PostPosted: Sun Jul 24, 2011 1:56 pm    Post subject: Reply with quote

Pull the PI tube and see if it squeals then. If it does, you don't have to waste time swapping out the preamp tubes. This circuit is supposed to make a solid "pop" when coming off standby. A squeal instead seems like it might be filter caps or output tubes.
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Reeltarded



Joined: 14 Feb 2009
Posts: 6991
Location: GA USA

PostPosted: Sun Jul 24, 2011 6:47 pm    Post subject: Reply with quote

I used to have a girlfriend that did the same thing.
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yowza



Joined: 29 Jan 2011
Posts: 67
Location: Virginia

PostPosted: Sun Jul 24, 2011 7:00 pm    Post subject: Reply with quote

I removed the PI tube and it didn't squeal but there was no sound at all which I wouldn't expect there to be anyway. I tried removing and then swapping out V1 and V2 separately but the amp still squealed however the quality of the squealing has changed to sometimes not being as long or loud as before. I began to think maybe the tube sockets were contributing to the issue so I tightened a few of them up and sprayed Deoxit and inserted a tube several times but the whoop still occurs. When I take out v3 there is no squeal at all so I'm wondering if the reverb circuit is part of the issue.

I also noticed that sometimes after being on standby and turning it on the amp doesn't squeal but makes a hum with a thump at the end and this lasts about as long as the squeal normally did, for just a second. It's that initial rush of current when taken off standby that causes either a hum with the thump or the siren whoop and then the amp sounds normal again.

I can tell this one is likely to be hard to track down and unfortunately I don't have time to troubleshoot further today as I have leave soon and rehearse for a local run of the musical "Hairspray".

Any further suggestions are very much appreciated!

Thanks!


Ed
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selloutrr



Joined: 14 Oct 2007
Posts: 3705
Location: Southern California

PostPosted: Sun Jul 24, 2011 8:34 pm    Post subject: Reply with quote

Reeltarded wrote:
I used to have a girlfriend that did the same thing.


I think we dated the same girl Cool

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martin manning



Joined: 06 Jul 2008
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Location: Queen City

PostPosted: Sun Jul 24, 2011 9:34 pm    Post subject: Reply with quote

Since removing V3 stops the squeal, did you try replacing it with a known good tube?
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Cliff Schecht



Joined: 30 Dec 2009
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Location: Austin

PostPosted: Sun Jul 24, 2011 10:15 pm    Post subject: Reply with quote

Are the e-caps good? If they are bad the impedance is very high and they don't do a good job of keeping different circuits from interacting negatively.
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yowza



Joined: 29 Jan 2011
Posts: 67
Location: Virginia

PostPosted: Mon Jul 25, 2011 5:38 am    Post subject: Reply with quote

The E caps were changed in 2007, still very new looking Sprauge's so they should be ok though the amp was not played much for a year, stored in a closet.

When I tried it again earlier it didn't squeal at all but the hum was still there. Over the span of a second or so, the hum starts quietly then swells up and stops. Sometimes there is a thump when the hum stops but the amp sounds fine after this. The squeal did the same swelling up and stopping as the hum now does, weird. When I let the amp start up without standby on I don't hear the squeal or hum so it has to do with being in stand by and then turning that off

The hum doesn't appear if the reverb tube tube is out so it could be something in the reverb circuit maybe the reverb cap. It also seems to have something to do with taking it out of standby so I'm tempted to change the standby switch to eliminate that too.
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T Wilcox



Joined: 11 Feb 2011
Posts: 225
Location: Sacramento, CA

PostPosted: Mon Jul 25, 2011 1:14 pm    Post subject: Reply with quote

Its probably just humming because it doesn't know the lyrics Laughing

Sorry couldn't resist

Todd
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selloutrr



Joined: 14 Oct 2007
Posts: 3705
Location: Southern California

PostPosted: Mon Jul 25, 2011 4:18 pm    Post subject: Reply with quote

when was the last time you changed out the 100K preamp resistors?

I'd also look at the ceramic disc caps and opto bug.

has it been altered in anyway to Black Face spec?

you may also have a bad tube that needs to be tested multiple times after being allowed to heat up.

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Firestorm



Joined: 25 Jan 2008
Posts: 2600
Location: Connecticut

PostPosted: Mon Jul 25, 2011 4:57 pm    Post subject: Reply with quote

yowza wrote:
The hum doesn't appear if the reverb tube tube is out so it could be something in the reverb circuit maybe the reverb cap. It also seems to have something to do with taking it out of standby so I'm tempted to change the standby switch to eliminate that too.


If you're speaking of the reverb driver tube, it is powered from the screen supply which is also the first of the filter caps after the standby switch. Have you tried replacing that tube? Some SF Fenders beat the crap out of it.
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yowza



Joined: 29 Jan 2011
Posts: 67
Location: Virginia

PostPosted: Tue Jul 26, 2011 3:27 pm    Post subject: Reply with quote

selloutrr wrote:
when was the last time you changed out the 100K preamp resistors?

I'd also look at the ceramic disc caps and opto bug.

has it been altered in anyway to Black Face spec?

you may also have a bad tube that needs to be tested multiple times after being allowed to heat up.


If you mean the 100K plate load resistors, yes they've all been recently changed and the amp has been Black faced. I recently changed out almost all of the old original carbon comp resistors with carbon film and the amp is a lot quieter now, there was a LOT of the sputtering, frying sound before. I haven't changed the resistors in the vibrato circuit yet (don't use it) but I'll probably do that soon. TBH I'm actually thinking about taking the vibrato circuit out totally as it sucks. Has anyone ever removed this?
All of the orange drop coupling and tone caps were recently replaced with Mojo Dijon caps which sound so much better and I also added a 100 ohm hum balance pot as I had some heater hum I could not get rid of but now I can dial it out with the pot.

After living with the stand by noise for a few days it seems to have settled down and I'm no longer getting the weird siren like squeal when flipping stand by off so that's good. I systematically swapped out tubes several times and tightened the tube sockets for the reverb tubes which I think was causing squeal. I didn't want to take the amp to a gig and have everyone give me weird looks when my amp howls when I turn it on. I haven't swapped out the stand by switch yet but I can live with the brief hum as it is. It just sounds like the amp is coming on so it's not a huge deal and the amp sounds fine after it's on.

I appreciate everyone's help.

Thanks!

Ed
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