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Old 22nd November 2012, 04:14 PM   #1
Mnvizb is offline Mnvizb  Lithuania
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Default Problem with tube amplifier

Hello,

The cable to the speaker got accidentaly disconnected from my tube amp and it stopped working normally. At first, I thought that either tubes or output transformer died. Well, when i turned it off it started playing as good as previously. The problem is - it only plays for very short amount of time (2-3 seconds) until the tubes are still lit.

Does anyone experienced such thing? How could I fix it?
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Old 22nd November 2012, 09:17 PM   #2
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One gets a volt ohmmeter or a digital volt ohmmeter, and one checks the DC voltages at the points shown on the print versus the reality. Warning, high voltage is dangerous, use one hand at a time. No jewelry. Don't work alone. Use a clip lead to attach the negative of your meter to the chassis so that you only have to use one hand. Use a lot of light. The numbers on the bottom of the tube sockets are pin numbers on the print. don't touch anything under the chassis until you have verified that it is less than 25 VDC to the chassis. Devices higher in voltage should be discharged using a tool made in accordance with the high voltage for newbies thread above.
Once one finds a DC discrepancy, particularly one that occurs when the sound quits, then one can replace components starting at the cheapest and most likely first, working up to the least likely. The most likely are electrolytic capacitors over twenty years old, followed by plate and grid capacitors stressed at >70 volts. Then carbon comp resistors (these can be ohm checked with the power off) then tubes. Then disk and mica capacitors, then transformers. In your case, the output transformer is a bit likely because you stressed it by not having a speaker hooked to the output stage. Or a 1kohm 3 watt resistor, which I solder across the output winding of the output transformer of all my tube amps.
If DC voltages are correct, then one injects AC voltage into the amplifier using a transistor radio or wave generator and looks for the proper AC voltages at the appropriate points. Warning, VOM will respond to DC voltages unless an RF probe is used or a blocking capacitor is put between the meter probe and the AC source. Warning, DVM's average over 2-4 seconds and won't show beats of music or whatever the way a VOM will.
Or one can buy an oscilloscope, and two probes. Warning, "PC-scope" cards are not isolated ground the way a professional scope is, and are not safe to use with tube or any high voltage circuit.
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Last edited by indianajo; 22nd November 2012 at 09:24 PM.
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