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Post subject: Volume changing randomly on Twin Reverb
Posted: Wed Feb 09, 2011 1:35 pm
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Hi. I have a Model 65 Twin Reverb. Just recently, it started changing the volume randomly. I'll turn it on, and play for like 20 minutes, and then the volume will drop about 25%, and stay that way for maybe 20 more minutes, and then come back up to 100% again. It does this randomly, as I'm just playing... not touching anything on the amp or on my guitar or stomp box.

It does this on all 4 of the input jacks.

It's difficult to troubleshoot, because I can't force it to happen. So to test out another jack, for example, I have to play a long time, and see if it happens. It *seems* like it stops doing this if I've been playing for over an hour. I can't be sure.

Any ideas on what might be causing this?

Thanks.
Tom


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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Wed Feb 09, 2011 1:52 pm
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Cracks in the PCB circuit traces. Probably around the volume control or tubes. As tempature in the amp changes the cracks expand and contract. If you don't know your way around the insides of a tube amp, take it to a qualified amp tech.

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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Wed Feb 09, 2011 3:28 pm
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Volume surges could also be indicative of a bad power tube.

How old is your amp and how old is the glass?

Arjay

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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Wed Feb 09, 2011 7:50 pm
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It's about 10 years old, and I've never done anything with the tubes.


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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Wed Feb 09, 2011 8:12 pm
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Always warm up a tube amp on standby before playing. Also remember to let it sit awhile on standby after playing before switching it off. Everyone already knows this, but I still have to remind myself to do this, then it becomes a good habit. Also never stick your finger inside a tube amp, as this could be fatal.


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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Wed Feb 09, 2011 8:21 pm
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tommy610 wrote:
It's about 10 years old, and I've never done anything with the tubes.


Likely, you're ready for new bottles. Be sure to check the power tube bias and adjust if necessary.

Arjay

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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Fri Feb 11, 2011 10:59 am
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Retroverbial wrote:
Volume surges could also be indicative of a bad power tube.

How old is your amp and how old is the glass?

Arjay


+1 For sure.

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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Fri Feb 11, 2011 11:39 am
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It's in the shop now. Since the answers are either cracked circuit or bad tube - I'm not dealing with either of those, what with the soldering and bias adjust, etc.

I'll definitely respond back with the results from the shop.

Thanks for the input.


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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Fri Feb 11, 2011 1:26 pm
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Sound like you need new power tubes


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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Fri Feb 11, 2011 1:39 pm
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Yes, I'm leaning toward tubes instead of a circuit problem. I've noticed this is random... it goes up, then down, then up. If it were a heating up circuit connection, I'd think once it's warmed up, it would stay engaged and consistent.


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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Fri Feb 11, 2011 10:32 pm
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Actually, unless for some odd reason ALL power tubes are varying output... it maybe kinda hard to hear just one or even two power tubes fluctuating in an amp that has four of them. But, if the phase inverter or first gain stage tube is starting to go bad... you'll definitely hear it. Since this is happening with all four inputs, maybe it's the 12AT7 PI ???

I have had all power tubes vary output in some amps. And new tubes sometimes didn't help. It was a heater line issue or worse, an instability issue--- like oscillations or motorboating.

I hope the new tubes solve your problem. Please post your answer. Interesting topic, for sure.

http://buildthemusic.com/content/amps/F ... manual.pdf


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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Fri Feb 11, 2011 10:39 pm
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BMW2002Ti wrote:
Actually, unless for some odd reason ALL power tubes are varying output... it maybe kinda hard to hear just one or even two power tubes fluctuating in an amp that has four of them.


No it's not. My UL TR exhibited those very same symptoms about ten years ago and upon examination, was found to have a nearly-dead 6L6. Thankfully it happened at rehearsal and I had three days to re-tube and re-bias it.

Arjay

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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Sat Feb 12, 2011 2:04 pm
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I'm an idiot. While the amp was in the shop, I was using another amp, and the same thing happened. So, then I noticed that there was a little red dot lighting up on my GT8 effect box when I press really hard on the volume pedal. Doing that toggles the expression pedal effect. So, it was the pedal all along!

Well, I just let the shop know to not worry about that problem, but it had to go in to fix the reverb anyway. I'm no happy it was something simple like that.

I kept thinking I should do some process of elimination and make sure it was the amp, but I couldn't imagine it being anything else.

Guess I need to read the GT8 manual.


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Post subject: Re: Volume changing randomly on Twin Reverb
Posted: Sat Feb 12, 2011 5:03 pm
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tommy610 wrote:
I'm an idiot. While the amp was in the shop, I was using another amp, and the same thing happened. So, then I noticed that there was a little red dot lighting up on my GT8 effect box when I press really hard on the volume pedal. Doing that toggles the expression pedal effect. So, it was the pedal all along!


GO TO YOUR ROOM!

:mrgreen:

Arjay

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