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Chasing distortion

I have an Audio Hungary Qualiton A50i.  Recently, one of the power tubes (KT120) red-plated and I purchased a replacement.  When installed, I heard distortion, along with music, coming out of both channels.  The distortion sounds similar to a blown speaker.

Thinking maybe the replacement tube was no good, I swapped the entire quad of power tubes with a set of KT150s.  After installing them I found that the sound was significantly lower, but I could still hear the distortion.

What should I try next?  Start swapping the input tubes (2) or the driver tubes (4)?

Thanks!

128x128Ag insider logo xs@2xmikedc
czarivey
5,946 posts
 

You need to check bias on all tubes. input/driver tubes can also go bad, but my suspicion that bad power tube could "take away" other components so bias checking is essential. 

mikedc's avatar
Ag insider logo xs@2x
mikedc OP
33 posts
 

It has an auto-bias circuit.  I'm thinking maybe that got fried.

oddiofyl's avatar
oddiofyl
3,815 posts
 

Get it serviced.  Probably has a blown resistor in the auto bias circuit.  Will just kill more tubes.

dogearedaudio
119 posts
 

^^^This.  Definitely get it serviced, you're wasting your money right now trying to replace tubes.

katzenjammer27
39 posts
 

My rig can only handle up to KT 120's (4 per each monoblock), That would be neat if I could go to a 150...anyway, how's your voltage? My voltage is all over the place since we live across the river from a nuclear power plant...some times of the day - usually going into evening, it spikes to about 127v, normally during the day time it's at 120v or 118v. I was blowing tubes regularly, then installed variac transormers at each monoblock and problem solved. no more melting tubes.

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