Amp Troubleshooting

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Amp Troubleshooting

Postby cobascis » Mon Feb 04, 2008 10:09 pm

I scraped up a 1970's (I think) Gibson G-40 amp for free from my local music store. It is a 40w solid state amp thats pretty beat up, apparently it was a project amp and the owner dumped it off at the store. I messed around with it (reconnected speaker cable, reconnected and slightly modified spring reverb, put one new pot in, and cleaned it) and it worked for a short time. I could turn it to full volume and no other noises besides a loud buzz, but could play my guitar through it.
The amp no longer works, when I turn it on it's fine, I turn the volume switch to anything past 1 1/2 this horrible noise comes out. The sound is best described as some sort of windshield wiper backed by a horrible screeching noise :wha: .This happens when a guitar is plugged in or not.
I would greatly appreciate any help and insight into this issue.
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Re: Amp Troubleshooting

Postby ricochet » Mon Feb 04, 2008 11:22 pm

I'm not much help with solid state stuff, my electronics education ended in the tube era, but the most likely cause of a loud buzz or hum is a failed filter capacitor in the power supply, and the noise you describe is obviously from an oscillation in the amp, which can come from a number of causes. (Actually, it sounds like both a low frequency oscillation causing the "windshield wiper" sound, and a high frequency oscillation causing the screeching.) If you're not pretty facile with electronics, hie thee to a technician.
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Re: Amp Troubleshooting

Postby cobascis » Mon Feb 04, 2008 11:40 pm

Meh. I was hoping for a fix-it-yourself kinda solution, and at the moment I can't take it in as I just blew my futile savings on a Valve Junior.
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Re: Amp Troubleshooting

Postby thebluesbox » Mon Feb 04, 2008 11:47 pm

throw it to the road and play the epi :-) quick fix!
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Re: Amp Troubleshooting

Postby cobascis » Tue Feb 05, 2008 12:06 am

^Haha

My main line of reasoning of keeping the beast is that it is simply large. Larger then my valve junior at any rate, and I'm not particularly rich (being 12 years of age) and my band has a small gig coming up and I was thinking if I fixed it I could mic the VJ and run it through it?
Thoughts?

EDIT: Besides it's free!
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Re: Amp Troubleshooting

Postby ricochet » Tue Feb 05, 2008 12:10 am

Well, even if it's dead you can always wire it up to use as an auxiliary speaker cab for the VJ. That's an easy DIY project, and it might already be set up with the speaker on a cable plugged into a jack in the back like the VJ. If not, that's what you'd need to rig up.
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Re: Amp Troubleshooting

Postby cobascis » Tue Feb 05, 2008 12:21 am

ricochet wrote:Well, even if it's dead you can always wire it up to use as an auxiliary speaker cab for the VJ. That's an easy DIY project, and it might already be set up with the speaker on a cable plugged into a jack in the back like the VJ. If not, that's what you'd need to rig up.



I thought of this before, so in essence you would; snip the wire that leads into the speaker from the chassis and then solder on another wire?
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Re: Amp Troubleshooting

Postby rustyslide » Tue Feb 05, 2008 12:36 am

Most (recent?) speakers have their wires attached by little clips, so (hopefully) you should be able to just detach them. Regardless, I'd then remove the chassis (the actual amplification bits) and set them aside for when your electrical skills have met your ambition. You can then just attach a speaker cable with your own little clips, and plug it into the appropriate jack on the back of the Valve Jr.
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Re: Amp Troubleshooting

Postby cobascis » Tue Feb 05, 2008 12:54 am

If I set it up as a cab will sound still come out of the VJ or just out of the Gibby?
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Re: Amp Troubleshooting

Postby ricochet » Wed Feb 06, 2008 12:38 am

Just out of the Gibby. You'd unplug the VJ's speaker cable from the jack, and plug in the cable you've attached to the speaker in the Gibby as discussed above.

Nice thing about the VJ combo is that it's a VJ head with its own speaker built in! You can plug it into an external cab anytime you want to, though. Although it comes (or at least my early production VJ combo came) set up for a 4 ohm speaker, you can't hurt it running it through an 8 ohm cab. I've done it with mine. If you want to be proper about it, the 8 ohm lead from the OT is left loose inside the chassis, and it would be a simple DIY job to hook up a second 8 ohm jack to plug into (after unplugging the 4 ohm internal speaker), or to wire both leads and the original jack to an impedance selector switch.

I think the VJ head got fancy with 4, 8 and 16 ohm selections. I don't have one and haven't had my hands on one, though.
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Re: Amp Troubleshooting

Postby cobascis » Wed Feb 06, 2008 2:28 am

^^
I got the new model so, it has 4, 8, and 16 ohm selections, my question is could the Gibby' be in the 8 ohm and the Epiphone (or in my case Eminence) speaker be in the 16 ohm slot (so the head is powering to speakers) or vice versa? Would that blow anything out?
Thanks for all the help
-Garrett
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Re: Amp Troubleshooting

Postby ricochet » Wed Feb 06, 2008 2:41 am

If you have a choice, match the impedance of the speaker and the outlet on the amp.
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Re: Amp Troubleshooting

Postby rustyslide » Wed Feb 06, 2008 5:06 am

He wants to run both the combo's internal Eminence speaker (which is plugged into the 16Ω jack), and the Gibbo-cum-speaker cab into the 8 (or the 4).

I have the VJ head, but it came with the same manual as the old VJ combos, and so doesn't talk about this, despite the amp having 3 output jacks.
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Re: Amp Troubleshooting

Postby ricochet » Wed Feb 06, 2008 7:51 pm

Most amps are set up expecting use of only one output jack at a time. In reality I don't think any combination of speakers you could plug into the VJ could hurt it, but if anything happens they won't honor your warranty if they know you did it, and I sure didn't tell you to! Paralleling speakers lowers the effective impedance of the primary winding of the output transformer. That increases the AC plate current peaks a bit (or reach them quicker) without affecting the DC idle current. You'll get more distortion, because what I just said implies that you're shifting the average level with signal. It'll heat the tube a little more, and might shorten the tube life. But hey, that's why they plug 'em into sockets, right?
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