Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on April 02, 2011, 04:11:12 am
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The new built 5E3 tweed deluxe starts crackling after ~1h playing.
I thought first that it only occurs with bass notes but then I noticed that a tap on the chassis or cabinet
can start it as well. It's also very difficult to troubleshoot because it exists only if the chassis is warm.
E.g. if I open the cabinet from backside it disappears quite soon after that. Any ideas? Could that be related
to the filter caps if they warm too much?
/Leevi
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Could be lots of things... When it's cool, can you get the noise if you bang on the chassis? That would make it a little easier to find. First, isolate the noise when it happens by pulling tubes. Pull the PI tube and then you'll know if it's in the power tubes or preamp. If it stops, put the PI tube back and pull the next preamp tube.
The problem could be a Tube, a socket, a component going bad, a solder connection. etc.
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I can get the crackling on when I move the high voltage secondary wires.
The wires are in same bunch and the first filter cap is placed quite close to it.
I have twisted all the secondary wires.
/Leevi
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Delayed crackling aye? A couple of things spring to mind.
1) A dodgy tube (swapping them will show that up) - always the first thing to suspect.
2) Heat-related partial failure of a coupling cap, causing bias to be thrown out of whack at the next stage. Difficult to detect because you have to switch the amp off to measure for leaky coupling caps. You could try switching the amp off and spraying the coupling caps one at a time with a non-conductive aerosol of some sort that cools the parts down to see if it goes away.
3) Heat-related failure of a bypass cap, causing partial shorting of cap, causing bias to be randomly affected - test the bypass caps for shorts (disconnect them one at a time)
3) Heat-related failure of a pre-amp filter cap, causing partial shorting, randomly affecting the B+ rail and thereby possibly inducing 'random signal noise' into the signal path from there on. - Test the filter caps for shorts
4) Unusual heat related failure of high-voltage wire insulation, causing voltage leakage, randomly affecting everything. - separate the B+ wires from other wires you may have them entwined/tangled-up with.
5) Maybe a bad solder joint? - reflow the joints
6) a loose socket pin that loosens further with heat/vibration. Re-tension the pins
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4) Unusual heat related failure of high-voltage wire insulation, causing voltage leakage, randomly affecting everything. - separate the B+ wires from other wires you may have them entwined/tangled-up with.
SOLVED, the reason was the PT primary and secondary wires that were placed too close each other.
Thanks tubeswell
/Leevi