Single Ended puzzle

[sepulchre]

I've got a single ended amp on my bench. It worked fine for two years then made some static noise and went quiet. I tried replacing the power tube, a KT88 but that didn't help. Strange thing is, when it first started screwing up it would come on for a bit then the power would cut down drastically then altogether silent except for an occasional thump. Now it's just the occasional thump.

Looking at voltages showed that, for one thing, the bias had dropped from 87 to 53mA, and the plate voltage had jumped to 388V. Also, one of the screen grid resistors, the 5K, is getting hot, Very hot. I don't have it on but for long enough to take some voltages - don't want to cook anything. The schematic shows a bias of around 30V, there's 10V there now.

Here's the power section:

[img568]http://i269.photobucket.com/albums/jj55 ... 16baeb.jpg[/img]

I tried replacing the 100uF cap - no results. And then I lifted a leg on the 10uF (which is there to smooth the tone a bit), again the problem remains.

I'm probably missing something obvious (more than usual). I've been doing this after work when I usually sleep. So, any ideas?

TIA

[Jana]

Measure the voltage drop across the 5K and the 1K resistors and calculate the current flow through each one. All the symptoms you describe sound like the 10uf is shorting. I know you checked that, but explore further.

[sepulchre]

The drop across the 5K is 259V. I didn't measure the 1K. Lifting a leg of the 10uF didn't seem to help, the 5K still got hot. I'll check the 1K.

[sepulchre]

The drop across the 1K is 50V. The 10uF is not connected.

[JazzGuitarGimp]

Maybe the tube has a screen - cathode short?

[sepulchre]

Swapping the tube was the first thing I tried. I put an EL34 in it with the intention of rebiasing, etc. but it never got that far. It was still not working. I could try a 6550. I don't have another KT88.

Because the socket is setup to change to different tube types, like the EL34, it has a jumper connecting pin 1 to pin 8. That shouldn't hurt anything with the KT88 should it?

[thejaf]

Shorted OT primary?

[sepulchre]

Haven't checked. The voltage is awfully high though so could be.

(checking)

And we have a Winner! ... or a loser, actually. I'm using the 2.5K tap and it reads 55 ohms. Hmm, not too good. Ah well, at least I know what it was.

Thanks jaf! I think Chris said he's send me one of these 20 watt SE OTs if I needed it. Time to take him up on it.

Thanks again, everybody!

[Firestorm]

Haven't checked. The voltage is awfully high though so could be.

(checking)

And we have a Winner! ... or a loser, actually. I'm using the 2.5K tap and it reads 55 ohms. Hmm, not too good. Ah well, at least I know what it was.

Thanks jaf! I think Chris said he's send me one of these 20 watt SE OTs if I needed it. Time to take him up on it.

Thanks again, everybody!

Not so fast. There's nothing inherently wrong with the 2K5 tap reading 55 ohms resistance. Whats troubling is the fact that you've got over 50 millimps flowing in the screen circuit. I'd look for wiring or carbon track shorts on the tube socket.

[sepulchre]

Well, I have checked and rechecked all the wiring. But I'm not familiar with carbon track shorts. Are they easy to see / detect?

[vibratoking]

Firestorm is right. 50mA screen current is way out of line. I would expect a few milliamps. That current is going somewhere. Seems likely to be at the tube socket or the tube itself since you removed lifted the 10uF cap.

[sepulchre]

Should I try a 6550? I've got a new one.

[JazzGuitarGimp]

Does the 5K resistor still get hot with the output tube removed?

[sepulchre]

No it does not. And there's practically no voltage on it.

[thejaf]

I would try the 6550 once you are sure there are no shorts at the socket.

Something caused your B+ to rise, whihc led me to suspect the plate is no longer drawing current. Either bad tube, shorted OT, socket short, etc.

It should not be dropping all voltage at the screen; and if it only does this when tube is installed, then either a bad tube or shorted socket connection.