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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Slightly Sick Amplifier

Graham Woods

Member
2008-08-31 10:08 am
Hi to all: and especially to those who can help me on this one!

I have a push-pull tetrode amp (early 90’s VTL, KT88 output valves). A hum has developed in the left-hand channel, and music in that channel is distorted. Bias voltage is within spec. on the unaffected channel (300mV across 10 Ohm); both output valves on the sick left channel measure around 120mV.

The problem manifested suddenly: the amp was fine when we stopped listening one night, but sick in the left channel the next time we switched on, about a week ago.

Could you suggest a few possible causes? Within the past twelve months or so I’ve replaced all electrolytics except three original ‘computer grade’ electrolytics per channel, two of which are filter caps. I’m suspicious of these oldies, but I might be barking up the wrong tree.

I'd really appreciate it if somebody can point me in the right direction ...

Graham Woods

Melbourne, Australia
 
Check the current draw per tube - across the cathode resistor is best.
Check the current draw across the screen resistors too.
Look for charred or burnt resistors of any sort.

Sounds like you don't have a scope?

But you replaced the caps - so resolder all your cap connections. One of the main sources of failure is faulty (and sometimes looking good) solder connections.

The electrolytics, assuming this is a stereo amp now, will be providing power to both channels so can not be at fault.

(also check with the inputs shorted - if the hum goes away the problem is in the interconnects or the preamp, etc...)
 

Graham Woods

Member
2008-08-31 10:08 am
Thanks Bear, thanks DeathRex. I'll take the steps you suggest. Fingers Crossed. BTW, the amp is two channels on a single chassis, but is dual-mono except for the single power tranny, which has separate windings for each channel. That rules IN the electrolytics again I reckon, but I can check those last anyway.
 

Graham Woods

Member
2008-08-31 10:08 am
Hi again to Bear and DeathRex. I went through the steps you suggested, and kept the check of the filter caps until last. Murphy's Law: it turned out to be the filter caps (one or the other - maybe both) in the sick channel. No harm in checking everything else though: everything seems okay, the amp is up and going again and I have peace of mind with nice new filter caps (AND a couple of other caps to replace the remaining oldies). Thanks for your thoughts guys.

Graham