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Buhl 20th March 2005 08:15 PM

Help wanted with fixing french DRG valve preamp....
Hello !

Hope that some of you can help me with fixing an french valve preamp named DRG - from the 80's I guess.
It have (with help from a poweramp) killed a pair of JBL 12" drivers - when turned on, it made a very loud buzzing sound, and the drivers were dead. It belongs to a friend of mine, who have bought it from a guy that have modified it. A first my friend used it with an McIntosch poweramp, and the were no problems, mabye because the Mc have som DC protection in the input - but when he used another amp, he killed the JBL's

My initial thought is that a cap in the output is bypassed, and thus the amp spills DC on the output - on turn on I measure 180 volts dc (!!!) on the output, slowly dropping to 6 volts, when a remove the voltmeter, and then measure again, it reads about 50 volts dc, then slovly dropping to 6 volts. just before the output sockets, a 1uF 600volts jensen cap was inserted - as a tweak - and I dont know the DC could pass this ?? before the jensen cap I measure 236 volts DC. Judging from the tweaks, the previous owner found a new place to tap the output signal, and the did it, just using the jensen cap between this point and the output phono socket.

I measure 305 and 16 volts coming out of the seperate PSU.

Can it be true that a valve preamp should have 236 volts standing on the output ? is it a faulty valve, wrong tweak, to small capacitor or ????
Any ideas would be most welcome ....

Cheers !

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