Conrad Johnson LP66s Problems

Hi All,

I am looking for advice before sending my amp to Conrad Johnson for repair.

I am at least the second owner of the amp and it is all original as far as I know. I have not opened it to look inside.

Recently the amp has been behaving funny:

1. when I go to bias a new set of tubes, a red warning light will flash but not for the tube I am biasing. Used to be only the red light for the specific tube would flash. This can happen on any of the four positions. I keep adjusting each tube and eventually this stops. Is this normal or a sign of a problem? 

2. I start it playing. Occasionally, one channel will go out and then come back on with a thump. A while later, it will do this again. It does this with tubes from three different manufacturers - 6550's and KT88's. I have started trying to rotate tubes and it seems I can move this problem from the right to left channel, but because I only have four tubes from each manufacturer, I cannot figure out which of the two tubes it is. Can 6550 tubes from two manufacturers be intermingled to figure this out?

3. It does not make sense to me that all three sets of tubes would start to exhibit this problem at the same time. All three sets of tubes are used...but I am thinking there is wrong with the amp?....

Advice and suggestions, please. I have tried to contact CJ but get routed through a help desk and then I get a call back a couple of days later from a customer rep and a not a tech. Not making much progress....

Thanks for listening, 



Sounds like it may be more than just a bad tube, ESPECIALLY when you say three different sets of tubes exhibit the same problematic behavior.

I had an Image 65i (with 6550 outputs) a few years back in which, when one tube went bad, took out the bias resistor in the circuit. When I took the unit apart, you could tell the resistor had been fried.  I had to hunt down a replacement and do a bit of soldering to replace the burnt one.  That fixed the problem and everything ran fine afterwards.

Also, on a push-pull tube amp, there are two tubes per channel. Adjusting the bias of one can affect the bias of its mate, so it often takes two or three passes to get both to the right bias level.

The bias light may flash as you bias the other tubes. Just bias them once a s if needed adjust.


dsper, I spoke with the factory today. Jeff asked that you use the website "contact" and send a message and he will get back to you.