Eico ST40 problems

[mdc701]

Hello again. So after thoroughly cleaning everything, the amp has been working properly and sounding good, except for one problem. The sound just “trips off” after about one hour or so of listening. I felt the transformer and it was very hot. Could this be the cause? Because when the amp cools down, it starts to play beautifully again. When it’s working properly, the sound is very nice, outputs on both channels are pretty much equal. I have it hooked up to a pair of vintage KLH Model 33s and the combo sounds amazing. Looking forward to your replies and thanks in advance as well.

 

[Dandy]

I don't think it is a faulty power transformer, but a faulty connection that disconnects when it gets hot. If you are able to measure the voltages when the amp is powered up, you can check whether there is rectified B+ (DC voltage) present at pin 8 of the 5AR4.

 

[mdc701]

I don't think it is a faulty power transformer, but a faulty connection that disconnects when it gets hot. If you are able to measure the voltages when the amp is powered up, you can check whether there is rectified B+ (DC voltage) present at pin 8 of the 5AR4.

Thank you for this. I will tell my technician to look into that. Cheers!

 

[Dandy]

In the long run, it would be worth your getting a multimeter and learning to do voltage checks yourself. It's not very difficult to learn to do this safely. For this check, with the power off and the amp unplugged, attach the black probe to the chassis anywhere and the red one to pin 8 of the 5AR4 rectifier tube. It might be easiest to get probes with clip connections for this.

Set the meter to measure DC volts and the range to over 500V and then plug it in and switch it on. Observe what the the DC voltage settles down to. You should measure about 400VDC. Detach the leads and operate the amp till the sound 'trips off', then switch off and unplug. Then repeat the measurement process while the amp is still warm. Whether or not you get the same voltage reading will tell you whether your power transformer is working okay.

It's usual for the power transformer of a tube amp to get quite hot after running for an hour or so and doesn't of itself indicate a fault.

 

[crispycircuit]

It could be output tubes red plating after a long warm up. Play the amp and get it hot. then look at all the output tubes and see if they're glowing red? That would indicate they are drawing a large amount of current and heating up the power transformer. Make sure you have the correct fuse in place. These are cathode biased amps. Which normally run warm. I'd also monitor the B+ voltage and see what it is when the sound " trips off". It could indicate your filter caps are shorting out...

 

[cademan]

Channel dropouts are usually dirty buttons and switches. When it acts up again, just wiggle all buttons and switches. the power transformers in those things naturally run hot but it could run cooler if the main filter capacitor(s) have been replaced. It looks like it has the original can capacitors which need to be replaced.

Show us a pic of your smokey tube. I'm betting that it's okay as long as there is no red plating or white getter flash.

 

[mdc701]

Thanks to everyone who replied! The amp is playing ok now, after a second trip to,the tech for a thorough cleaning. So far, so good. Now I’m on the hunt for some extra knobs to replace some of the ones in my st 40 as they’ve developed some cracks. Thanks again guys!

 

[Bob King]

Thanks to everyone who replied! The amp is playing ok now, after a second trip to,the tech for a thorough cleaning. So far, so good. Now I’m on the hunt for some extra knobs to replace some of the ones in my st 40 as they’ve developed some cracks. Thanks again guys!

There are some knobs on eBay now.....

 

[mdc701]

There are some knobs on eBay now.....

Thanks!