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Vintage threads from the first ten years
amp with shorting noise
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 7/17/2000 4:34 AM |
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| Tristan Money | amp with shorting noise hello, I have this amp (early 60's no name), and when I turn it on, there loud shorting and scratching noises. This continues for about a minute, and stops when the amp warms up. This amp is without a standby switch, but has a tubed rectifier, but maybe this could be part of the problem? or a fried tube? The filter cap is gone, but I thought that only resulted in humming....maybe not? suggestions? thanks, -Tristan |
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| 7/17/2000 5:41 AM |
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JohnC |
I'd start by pulling the output tubes and see if the noise is still there. If it is, then the noise is the output TX arcing. If there is no noise, put the output tubes back in and start taking out the small signal valves starting near the output stage and working bak to the input. Pull a single valve and switch the amp back on. If there is no noise then the problem is further towards the input. Keep doing this one valve at a time until the problem reoccurs. This will isolate the problem to a single gain stage and a handful of components. Replace them one by one until the problem is cured It could be anything but I reckon its a plate resistor. Good Luck John |
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| 7/17/2000 6:10 AM |
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| Tristan Money | Does it not harm the amp by pulling out tubes and then powering up the amp? I have no evidence to say that it would, but I don't wanna ruin the amp. Thanks, -Tristan |
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| 7/17/2000 7:23 AM |
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| JohnC |
Running the amp without tubes will cause the B+ voltage to rise. In a badly designed amp the voltage may rise above the voltage rating on the power supply electrolytic capacitors. Check the voltage ratings on the caps and then measure the actual voltage in the amp. If the B+ volts are within 20v or so of the cap rating then it might not be a good idea to pull out the power tubes (since these draw most of the current and therefore cause most of the voltage drop when the amp is switched on). I find most caps can handle 10% above their voltage rating. If you aren't sure about what you are doing you should take the amp to a technician. John |
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| 7/17/2000 7:58 PM |
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| Tristan | JohnC: the filter cap for this thing is blown anyway, I just have to find a cheap replacement. I just found out last night that one of the two channels works fine (the first one) so I put a newer 12AX7 in the channel that was fine and it seemed to have the same scatching noise as the other one when I turned it on. the thing is, with this amp, it isn't worth taking it to a tech.... the amp isn't worth spending any more money than the price of a new filter cap on 
-Tristan |
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| 7/17/2000 9:43 PM |
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| Brian |
What kind of tubes are you using? I seem to remember something about sovtek 12AX7WXT+ 's having wierd transient problems like this. If you have a bad tube, this can certainly happen. BT |
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| 7/17/2000 10:47 PM |
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| Tristan | They are all different kinda of tubes, sovtek, westinghouse and others... they are all from the mid '60s, the guy I got it from had never replaced them since he got it. guess I will have to go pickup some new 12AX7's.. -Tristan |
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