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Post subject: Blew up my Bassman
Posted: Sat Jun 08, 2013 10:02 pm
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Brought my '67 Bassman to rehearsal, but left the speaker cabinet at home because I had to rely on public transit. Used the in-house Marshall 4x12 cabinet. Plugged in to the 1x4 ohm output, with toggle switch set to Mono (300 watts). Half way through rehearsal, the Bassman shut off and would not turn back on. Feel like a total moron!

Is there some fundamental law for using an amplifier that I overlooked? Or did I just buy a bad unit?


This Bassman has been somewhat cursed. I brought it to a repair shop in January and did not get it back until May. The mechanic, who has 20+ years of experience as an amp tech, said he had never seen an amplifier with as significant and strange problems as my amplifier. There was a great deal of corrosion. The power section in particular had a lot of problems, and would cut out intermittently. He put in over 10 hours of work on the amp before he could get it back to good health (yet charged me a very reasonable rate in the end).

But despite this history I can't help but feel I made a big mistake by using the Marshall cab.

Plan to take the amp to a technician next week. Any help is greatly appreciated; the right advice may cut down repair costs. Thanks!


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Post subject: Re: Blew up my Bassman
Posted: Sat Jun 08, 2013 10:41 pm
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Demaria-Daniel, seems you did nothing wrong. The 4 ohm input on the Marshall 4X12 is correct, as long as the cab is working correctly. Corrosion and heat can be big-time problems for the older amps. But a thorough check of the amp will explain the fault. It doesn't necessarily mean a costly repair!!! As long as the transformers operate correctly, the rest is just a matter of diagnosis and repair. Just a few questions: Was the amp working fine at home, with the regular speaker cab? Did the fuse blow? Art

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Post subject: Re: Blew up my Bassman
Posted: Sat Jun 08, 2013 11:03 pm
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Yes a simple fuse is the first place to start, check it first before bringing it in for service.

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Post subject: Re: Blew up my Bassman
Posted: Sun Jun 09, 2013 1:45 am
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I'm relatively certain that use of the Marshall cab was not a factor in this incident. Likely, some tired component within the amp finally gave up the ghost.

Arjay

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Post subject: Re: Blew up my Bassman
Posted: Mon Jun 10, 2013 4:58 pm
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Yep it was just the fuse!

It's always great when you all you need to fix a problem is to spend $1.25 at a good electronics store :lol:

Thanks guys, this is my first tube amp so I'm still new to a lot of this.


I LOVE this amp. I've played around with different tubes and speakers a little, I'll post what I learned in my original post about this amp some time soon.

Thanks again!


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Post subject: Re: Blew up my Bassman
Posted: Mon Jun 10, 2013 5:54 pm
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Daniel,

It's good to keep a few sapre fuses taped to the inside of the amp's cab. Just for this reason. May want to clean the contacts in the fuse holder with a little contact cleaner. If the amp has that much corrosion... you may have intermittent contact issues that can lead to sudden changes in current draw.

I assume that the previous tech cleaned and sprayed all the usual suspects (sockets, switches, jacks --- esp the speaker jack(s), etc.).

If not, you should do this. De-Oxit GN-5.

http://www.amazon.com/DeoxIT%C2%AE-GOLD ... Deoxit+GN5

Good luck!


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Post subject: Re: Blew up my Bassman
Posted: Mon Jun 10, 2013 6:16 pm
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BMW2002Ti wrote:
May want to clean the contacts in the fuse holder with a little contact cleaner. If the amp has that much corrosion... you may have intermittent contact issues that can lead to sudden changes in current draw.


As well, check the fuse post's internal spring to ensure there is sufficient tension to hold the fuse securely in place. Age, heat, and constant pressure can all cause a steel spring to weaken, thus causing potential intermittent-contact issues.

Arjay

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Post subject: Re: Blew up my Bassman
Posted: Mon Jun 10, 2013 10:11 pm
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Are we looking at a happy ending?

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Post subject: Re: Blew up my Bassman
Posted: Mon Jun 10, 2013 10:46 pm
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demaria.daniel wrote:
Yep it was just the fuse!


But what caused the fuse to blow in the first place? Blown fuses should not occur, normally.


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Post subject: Re: Blew up my Bassman
Posted: Tue Jun 11, 2013 7:18 am
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Fuses pop occasionally on amps that work fine. Due to line surge in outlets. Or brownouts causing excessive current compensation.

If the amp continues to blow fuses, prolly needs a REAL good looking over. Since the amp popped the fuse, after some time --- could be induced instabilities in the circuit. Or eddy currents in a that output tranny. Esp if the lammies have lost some of the insulation, due to weathering.


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Post subject: Re: Blew up my Bassman
Posted: Tue Jun 11, 2013 9:14 am
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BMW2002Ti wrote:
Fuses pop occasionally on amps that work fine. Due to line surge in outlets. Or brownouts causing excessive current compensation.

If the amp continues to blow fuses, prolly needs a REAL good looking over. Since the amp popped the fuse, after some time --- could be induced instabilities in the circuit. Or eddy currents in a that output tranny. Esp if the lammies have lost some of the insulation, due to weathering.


+1

A power conditioner can help where varying line-voltages are suspect.

However, continuously blown fuses are an indication of other more serious amp issues and are ignored at the owner's peril.

Arjay

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Post subject: Re: Blew up my Bassman
Posted: Tue Jun 11, 2013 4:02 pm
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Bad news. The replacement fuse blew.

I'll take the amp apart and take pictures tonight, and post everything I know based on what the tech told me. He did do a thorough look over and extensive repairs, but had a great deal of difficulty. Much of this stems from modifications made by the previous owner. I have a performance scheduled for Monday night -- my first in four years!! -- and any help getting this girl back in good health by then is greatly appreciated.

I gave my tech a call this evening and I'll bring the amp in to a shop tomorrow, with any advice I can collect from you guys.


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Post subject: Re: Blew up my Bassman
Posted: Tue Jun 11, 2013 4:18 pm
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Securing a back-rig might be a wise-and-prudent contingency plan.

Arjay

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Post subject: Re: Blew up my Bassman
Posted: Tue Jun 11, 2013 4:45 pm
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Yes I am definitely not placing any bets on having this thing ready by Monday, though I've gotta try


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Post subject: Re: Blew up my Bassman
Posted: Tue Jun 11, 2013 5:45 pm
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Ok. So I took the amp apart and immediately noticed this:

Burn Spot on Power Tube Socket
http://www.flickr.com/photos/97332706@N05/9019542451/

Burn Spot on Power Tube between pins 2&3
http://www.flickr.com/photos/97332706@N05/9019542545/


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