I have a ST70 that has gone through an evolution over the years.
I bought it in original condition.
I rebuilt the power supply and put in a fresh CE 4 section cap.
The amp played fine.
Then I did a total rebuild using an octal board with NOS 6SN7s, oversized replacement power transformer.
Stayed with the 4 section cap and 5AR4 rectifier.
New input jacks and binding posts on the output.
The only things original is the chassis, tube sockets (clean and tight) and output transformers (later plastic lead version)
It sounded wonderful and
I used it for several months with no problems. I had at least a couple hundred hours on it.
Then I leant it to a friend who used it for several months with no problems. Then all of a sudden he started having problems with some hum and distortion.
When I checked it out B+ was low the first section of the filter cap had failed. I replaced it with a board I had made with individual high quality Nichicon snap in caps. B+ and other voltages were back to normal it seemed to work well for a while.
But then after it had warmed up well there would be a loud pop and the volume would drop for a second or so. And then it would be good for several minutes again. I thought maybe the rectifier was failing so I tried another. But the same thing was happening. I then noticed that pop always came just from the left channel. And it was just that channel that the volume dropped in.
So I swapped the output tubes. Bias was correct and stable. I can't see any signs of carbon arc traces on the socket plastic.
Tried swapping driver tubes, swapped in another input tube but the problem remained in the left channel. All the solder joints look good throughout the amp.
About the only thing I can think of that's left are the coupling caps to the output grids which of course were new with the octal board and the output transformer which would be difficult to swap to try and find out.
Of course the coupling caps would be easy enough to try but the amp got mothballed before I got around to trying that.
I'm thinking of getting back to it sometime soon and thought I'd throw this out there and see if anyone can think of something I'm missing?
I bought it in original condition.
I rebuilt the power supply and put in a fresh CE 4 section cap.
The amp played fine.
Then I did a total rebuild using an octal board with NOS 6SN7s, oversized replacement power transformer.
Stayed with the 4 section cap and 5AR4 rectifier.
New input jacks and binding posts on the output.
The only things original is the chassis, tube sockets (clean and tight) and output transformers (later plastic lead version)
It sounded wonderful and
I used it for several months with no problems. I had at least a couple hundred hours on it.
Then I leant it to a friend who used it for several months with no problems. Then all of a sudden he started having problems with some hum and distortion.
When I checked it out B+ was low the first section of the filter cap had failed. I replaced it with a board I had made with individual high quality Nichicon snap in caps. B+ and other voltages were back to normal it seemed to work well for a while.
But then after it had warmed up well there would be a loud pop and the volume would drop for a second or so. And then it would be good for several minutes again. I thought maybe the rectifier was failing so I tried another. But the same thing was happening. I then noticed that pop always came just from the left channel. And it was just that channel that the volume dropped in.
So I swapped the output tubes. Bias was correct and stable. I can't see any signs of carbon arc traces on the socket plastic.
Tried swapping driver tubes, swapped in another input tube but the problem remained in the left channel. All the solder joints look good throughout the amp.
About the only thing I can think of that's left are the coupling caps to the output grids which of course were new with the octal board and the output transformer which would be difficult to swap to try and find out.
Of course the coupling caps would be easy enough to try but the amp got mothballed before I got around to trying that.
I'm thinking of getting back to it sometime soon and thought I'd throw this out there and see if anyone can think of something I'm missing?
