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Post subject: My '69 Twin Just Blew Up.
Posted: Thu May 02, 2013 7:07 pm
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I THINK it did, anyway.

See, Ill be playing, and it'll start making these distorted sounds, which becomes this sort of fuzzy 60 cycle hum. Then the two power tubes on the right will turn bright blue, and the other two will become almost entirely lifeless. It only happens after I play the guitar for a couple seconds. Please help, as this is the family amplifier, and it was given to me by my uncle as a Christmas present two years ago!

Notes:

    1. I am running a 2X10 external cab along with it, but the ohm ratings are only a couple off, and I've been running it for at least a year with no problems, so I highly doubt that it the cause.

    2. It has been running with all 12AX7s for the preamp, but it's been doing that since the eighties, so I don't see a problem with that.

    3. It has been converted (correctly) to a 3 prong cord

This is very, very bad, and I need to know what could possibly we wrong with it.

Any and all help is appreciated!


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Thu May 02, 2013 7:18 pm
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ianjgordon wrote:

See, Ill be playing, and it'll start making these distorted sounds, which becomes this sort of fuzzy 60 cycle hum. Then the two power tubes on the right will turn bright blue, and the other two will become almost entirely lifeless. It only happens after I play the guitar for a couple seconds. Please help, as this is the family amplifier, and it was given to me by my uncle as a Christmas present two years ago!

Notes:

[list]1. I am running a 2X10 external cab along with it, but the ohm ratings are only a couple off, and I've been running it for at least a year with no problems, so I highly doubt that it the cause.

2. It has been running with all 12AX7s for the preamp, but it's been doing that since the eighties, so I don't see a problem with that.



Sounds like you are due for some new tubes. How old are the tubes in your amp?

Start by swapping out the tubes. If you don't have a spare set , get some. This is a must for any tube amp owner anyway.


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Thu May 02, 2013 7:42 pm
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When was the last time this amp was checked by an amp tech? Sounds like it is ready for a check up and at a minimum new Power Tubes and probably a cap job.

T2

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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Thu May 02, 2013 8:08 pm
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+1 on new tubes. However, I think it's time for a checkup. you don't want to risk damaging that amp. I wouldn't call that blowing up. The stock 69' has a bias balance circuit. New tubes and a rebias should do the trick, but there are wiser folks lingering in the shadows. BTW, That's a nice Christmas present. Reminds me of the time a friends brother has a 71 strat and 65 twin is his closet. He wanted $400 for both. Still have the strat and still kick myself for selling the twin.


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Thu May 02, 2013 8:19 pm
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djwhitjr wrote:
ianjgordon wrote:

See, Ill be playing, and it'll start making these distorted sounds, which becomes this sort of fuzzy 60 cycle hum. Then the two power tubes on the right will turn bright blue, and the other two will become almost entirely lifeless. It only happens after I play the guitar for a couple seconds. Please help, as this is the family amplifier, and it was given to me by my uncle as a Christmas present two years ago!

Notes:

[list]1. I am running a 2X10 external cab along with it, but the ohm ratings are only a couple off, and I've been running it for at least a year with no problems, so I highly doubt that it the cause.

2. It has been running with all 12AX7s for the preamp, but it's been doing that since the eighties, so I don't see a problem with that.



Sounds like you are due for some new tubes. How old are the tubes in your amp?

Start by swapping out the tubes. If you don't have a spare set , get some. This is a must for any tube amp owner anyway.

Had the tubes in it for about two years. They are Groove Tubes and they are just barely starting to darken.


T2Stratman wrote:
When was the last time this amp was checked by an amp tech? Sounds like it is ready for a check up and at a minimum new Power Tubes and probably a cap job.

T2

What I meant was that it's been running this tube arrangement since the eighties. I don't think it has ever had a cap job or has been serviced in any way.

Krankster wrote:
+1 on new tubes. However, I think it's time for a checkup. you don't want to risk damaging that amp. I wouldn't call that blowing up. The stock 69' has a bias balance circuit. New tubes and a rebias should do the trick, but there are wiser folks lingering in the shadows. BTW, That's a nice Christmas present. Reminds me of the time a friends brother has a 71 strat and 65 twin is his closet. He wanted $400 for both. Still have the strat and still kick myself for selling the twin.
When I got the new tubes a couple years ago, I re-biased it. According to more knowledgeable amp tech people, I did it properly. However, the Reverb circuit has some problems, and aren't the power tubes for specific channels? I've had to use the normal circuit because of the weirdness of the Reverb circuit, and if the power tubes control different channels, couldn't this affect it?


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Fri May 03, 2013 3:13 am
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Sounds like the filter caps are going South. BTW... I believe that your amp has a hum (or balance) adjustment pot. And not a bias adjustment pot. So, it's not that easy to adjust the bias.

By the way you describe how two tubes go Blue and two go cold --- my bet is that balance pot is out-of-adjustment. Of maybe failing.

Looks like the amp need a good looking over by a good tech.

Good luck! This is a worthy amp to restore to standard operating conditions.

:D


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Sat May 04, 2013 7:38 pm
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So I got inside of it tonight, and it looks like what It really needs is a cap job. Everything looks okay except for the big red film capacitors. A lot of them look slightly deformed and some have darkened or burned spots on the top. Also, a couple of the ceramic caps look swollen. All of the filter caps have little bulges on the ends where the electrolytes are crystallizing. I guess now I need to ask you guys if any of you know some good amp repair places where they wont screw it up or try and modify it. Anybody know of any?


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Sat May 04, 2013 8:08 pm
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Bulging caps = bad. No use the amp until someone fixes it.

Where do you live?


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Sun May 05, 2013 8:00 am
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Morehead, KY. Inside Rowan county. I'm considering doing this myself, as it looks fairly simple. Would this be a bad Idea? I've built and modded pedals before, so this looks pretty easy compared to that.


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Sun May 05, 2013 9:05 am
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ianjgordon,

If you feel confortable reading schematics and working with 450-500VDC, you are prolly ok doing a filter cap change. You already have a good DVM and soldering iron?

BTW... you ever do a tube pedal DIY?


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Sun May 05, 2013 5:18 pm
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I feel entirely comfortable. I have never dealt with tube effects before, though.

P.S: What does DVM stand for?


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Sun May 05, 2013 5:30 pm
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DVm = Digital Volt-Ohm Meter. You need a reliable one.


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Sun May 05, 2013 7:29 pm
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Oh, yes! I'm dumb. Yes, I have a couple.


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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Sun May 05, 2013 9:01 pm
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You need to determine which configuration your amp is. It could be one of these two schematics:

http://ampwares.com/schematics/twin_reverb_aa769.pdf

http://ampwares.com/schematics/twin_reverb_aa270.pdf

What is the impedance of the external cab you are running? If the cab is rated at 16 ohms, plugging that into the amp will give you a total load of 3.2 ohms which would be safe. If the cab is rated at 8 ohms, your total load is 2.7 ohms which may still be OK but puts more strain on things. if the cab is 4 ohms, you are putting a total load of 2 ohms on the amp which again may be OK, but after a year of hard playing, things could be stressed to the breaking point.

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Post subject: Re: My '69 Twin Just Blew Up.
Posted: Sun May 05, 2013 10:06 pm
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Ian, I concur with Bill. Before ordering any components, you'll have to know which circuit you're working with. May also be an AB568. The tube chart can help. But it's best to look at the cathode arrangement on the power tubes, comparing it to all three schemos. Art

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