Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jukelemon on January 23, 2014, 10:26:12 am
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Hi all.
Buddy of mine has a 59 Super clone.
Like a switch, the amp stopped working beautifully.
Here are the odd symptoms..
IF the amp is cold i.e. not played in 4 hours or so AND IF you do not increase the volume past 5, the amp sounds wonderful and you can play for hours and hours.
IF the amp is cold OR hot (played) AND you increase the volume (either channel) past 5, the signal is extremely distorted and the power drops off significantly. And once that happens, the amp will not go back to being able to be played Pre 5 on the volume and get a clean/normal tone i.e. at that point ALL Volume settings = a crappy, low power distorted tone.
What is odd to me is that it is not time related i.e. you could play it for 2 minutes below 5 on Volume and then crank it and you get the same result as same playing it for 2 hours below 5 and then crank it.
I checked voltages and bias and changed out ALL tubes and there is no change.
Cleaned out the pots. No change.
I would have a hard time believing both Volume pots went at the same time.
Thoughts?
Thanks all.
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Check the speaker ,
You check the voltage in cold ( working condition ) or in hot condition , amp not working ?
You must check voltage in hot condition .
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I think it's parasitic oscillation when the volume is above "5". The oscillation can definitely create distortion and also totally kill volume. I've had that happen with a bad power tube before. Very frustrating. New tube completely resolved the issue.
Could be a bad tube or bad connection somewhere. My first effort would be to check voltages, then try new tubes, then chopstick for a bad connection.
I think whether the amp is cold or hot is not the primary issue (as my best guess).
With respect, Tubenit
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Bad solder connection?
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Buddy of mine has a 59 Super clone.
Like a switch, the amp stopped working beautifully.
Here are the odd symptoms..
...
Thoughts?
Signal generator and a scope (or listening amp).
You can waste you time making guesses and chasing your tail... Or you can apply a signal, set the controls to make it crap out, and see/hear where the signal is being disrupted. If there's an oscillation, you'll see it on the scope trace. If the volume pots are to blame, you'll see/hear that as the point the signal gets weird.
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Hi all.
Thanks and sorry the for the late reply. I have been playing shows in Memphis this week.
I swapped all tubes with known good tubes so I don't think it is a tube issue.
I checked the voltages with the amp hot i.e. after ~30 minutes of playing. Voltages look fine/in spec.
I do not have a scope but I recall there is a DIY on the listening device. I will try to build one and hopefully that will assist.
Thanks.
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Oh my goodness---Memphis!!! I was coming home from a trip to NC on I-40 and hit Memphis at 6:00 oclock PM on Friday afternoon---I thought I was going to die!!! I've never been in traffic so horrendous. I managed to escape off the interstate to a motel without injury--snuck out of town the next morning at a little slower pace back to my Louisiana red hills. That's no way for a soul to live! :l2:
On your amp, I would recommend a live chop stick probe test to see if you can cause a change to the exisiting. Platefire
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The Memphis trip was for the 2014 IBC.
I have already chop sticked the heck out of it and reflowed all of the joints.
I am going to make that Listening device in a little box and run it into my Bassman. That should be clean enough I would think.
Thanks all.
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Sure sounds like an oscillation issue to me.
PLEASE post what you find out. IF you've already tried new tubes, voltages are good and chopsticked everything ................ I am not sure what it would be? So I will be curious to find out?
With respect, Tubenit
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What I don't get is this..and sorry if I missed the explanation..
Why would it start and then not be able to stop i.e. once you go past 5 on the dial, it does not matter if you turn down. It is like a switch is flipped and the amp just goes to its knees.
But if you don't go past 5, it is fine???
Odd to me.
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I went and re-read your original explanation of symptoms and things you have tried and didn't see these:
1-Clean tube pin & sockets with contact cleaner. Spray cleaner on tube pins and work them up and down in each socket. Also check tube socket pin grippers for proper tension and adjust any loose ones.
2-Do a dark test playing through a live amp observing the circuit at the problem vol settings and see if you see any arcing in the circuit.
3-Rework the lead dress to give grid, DC power and cathode wiring breathing room or separation from each other where possible.
4-Sometimes on stubborn problems, I do continuity test down the grid path like input jack to grid resistor, grid resistor to tube pin input of gain stage, plate output pin to coupling cap---ect. Also check all grounding leads for continuity to chassis.
5-Someone said check the speaker and I didn't see where you did? but if you haven't, I would hook up to another known good speaker.
You may have already tried all this. These are just things I try when a problem is playing hide and seek like this one. Platefire
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Why would it start and then not be able to stop i.e. once you go past 5 on the dial, it does not matter if you turn down. It is like a switch is flipped and the amp just goes to its knees.
But if you don't go past 5, it is fine??? Odd to me.
Both amps I had that oscillated this this very thing. Once it started oscillating ............ only turning off the amp and then turning it back on would resolve it (as long as I did not crank the volume past a certain point).
One time it was simply moving a wire over about 1/4-1/2" that resolved it. Other time was a bad power tube.
with respect, Tubenit
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Thanks all.
I will try all of those and build that listening device.