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DC in Pre-amp output
DC in Pre-amp output
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Old 8th September 2001, 07:52 AM   #11
john is offline john  Philippines
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Join Date: Aug 2001
Location: pasay city
Default DC in Tube preamp...

Tube preamps does have DC in their output upon power on. Just wait for about a minute of 2 before you operate them. I huv used lots of tube preamps and they huv muting circuits incorporated...

Try putting a delay circuit at the output stage of the preamp and add a muting circuit. This will enable the tubes to stabilize the filament or heater.

Use capacitor with very low DC leakage.

TUBE PREAMP - SOLID STATE AMP combination... has this problem of triggering the protection circuitof your amp. And if your amp has no protection circuitry, It will burn your loudspeaker voice coils.

All tube set up( tube preamp - tube ampcombination), will not encounter problem wid DC bcoz of the output trans4mer blocking the DC voltage and also those siganal capacitors at the predriver n driver stage of the tube amp.

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Old 5th August 2004, 10:14 PM   #12
v-man is offline v-man  United Kingdom
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Join Date: Jul 2004
Location: wales
I agree with one of the previous posts,the worrying thing is that the dc is fluctuating from .1mv to 150mv. this points to some instability in the circuit, could be down to the layout. try putting a shorted jack into the input of the preamp, and then take a meter reading on the output after the cap, and see if the small amount of dc voltage is still there.
i have yet to find a perfect capacitor that dosent have any dc leakage. the best capacitor is no capacitor. but with valve amp circuits you dont often have a choice.
also, try testing each stage of the preamp to see if you can find which part the instability originates from. this will only help if the over-all feedback loop, [if it has one] is disconnected.
good luck, v-man.
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