Crate VC5310 power tube failure ,and fuse blown.

[gtrplayr1976]

Hi Folks,

I have a Crate VC5310 I'm working on for a friend. When I first powered it up there was hum,and when I took it off standby there was lots of noise. Popping ,cracking ,hissing. It soon faded ,but when I tried to play through it the volume was low ,and then it would pop really loud ,and the volume would go up. I turned it off ,and found one of the power tubes V7 was smoked. I replaced it with one I had to take voltage readings. The schematic states to take readings with all controls at max except reverb (off) ,and level on the dirty channel at 5. As I was taking readings I hear a kind of hiss ,and then kind of dead sound. Another tube blew. V6 this time ,and the fuse F2 was blown. All of the grid resistors check fine. The caps have not been replaced. On some of the readings I got there were what seems significant differences. I have attached some pictures ,and the TP readings I was able to get. Any thoughts would help. I had replaced all of the tubes in this amp a few years ago. It was seldom used. Any ideas appreciated.

Thank you !

[Enzo]

The white tube is broken - physically. That stuff turns white when exposed to air. Air has gotten into that tube, so it is cracked. If that one cracked, I would not be surprised if others in the set were damaged. A new set of power tubes is needed, not just one.


The only test point readings that matter right now are DC ones. Oh, I guess the AC ripple on power supplies too. All the signal level stuff can wait until the amp is stable.

[gtrplayr1976]

The white tube is broken - physically. That stuff turns white when exposed to air. Air has gotten into that tube, so it is cracked. If that one cracked, I would not be surprised if others in the set were damaged. A new set of power tubes is needed, not just one.


The only test point readings that matter right now are DC ones. Oh, I guess the AC ripple on power supplies too. All the signal level stuff can wait until the amp is stable.

Thanks Enzo. I knew the tube was broken when I saw it. When I first got this set two of the four were cracked ,and they had to send me two more. This amp wasn't used much after the tubes were replaced ,and then this happened. I'll get a new quad ,and go from there. Do you think the caps should be replaced since it is fairly old ?

[MarkusBass]

I wouldn't change anything as long the amp is blowing fuses. The caps look quite good. I see that you have bias voltage in TP17. I would check whether the same voltage is available on the tube socket. This is of course to be tested without tubes. Have you checked that the grid resistors are OK (R44-R47)?

Mark

[gtrplayr1976]

I wouldn't change anything as long the amp is blowing fuses. The caps look quite good. I see that you have bias voltage in TP17. I would check whether the same voltage is available on the tube socket. This is of course to be tested without tubes. Have you checked that the grid resistors are OK (R44-R47)?

Mark

Thanks Mark.
I powered it last night with only the preamp tubes ,and took some readings on tube sockets. I attached those. Looks like I'm getting the -16 vdc on pin 2. It didn't blow a fuse without power tubes. It was on for probably 30 minutes.

[MarkusBass]

I don't see anything unusual when I look at voltages on V4-V9 tubes. I assume you have 0V on pin #3 of V5-V8. Just to be sure that preamp tubes are not a source of problems, you can either remove them, or plug in a dummy Jack into the Effects Loop (and short hot wire to the ground). It seems to me that this may be related to either one of power tubes not contacting (or even failed), or maybe the power amp is oscillating. I would put two power tubes in and power up the amp with a bulb limiter (with Effects Loop input shorted). A then measure voltages on power tubes. I assume that the fuse that failed was of correct value and type (2A, SLO-BLO), was it?
BTW, when you turn on a power amp like this one for the first time, I think that all controls should be turned down.

Mark

[gtrplayr1976]

I don't see anything unusual when I look at voltages on V4-V9 tubes. I assume you have 0V on pin #3 of V5-V8. Just to be sure that preamp tubes are not a source of problems, you can either remove them, or plug in a dummy Jack into the Effects Loop (and short hot wire to the ground). It seems to me that this may be related to either one of power tubes not contacting (or even failed), or maybe the power amp is oscillating. I would put two power tubes in and power up the amp with a bulb limiter (with Effects Loop input shorted). A then measure voltages on power tubes. I assume that the fuse that failed was of correct value and type (2A, SLO-BLO), was it?
BTW, when you turn on a power amp like this one for the first time, I think that all controls should be turned down.

Mark

Yes Mark 0 volts on pin #3. What does plugging a dummy jack in do ? What positions should I put the tubes in ? Either the 2 inside or the 2 outside ? The fuse that failed was the .25 amp fuse. The schematic shows a .5 amp ,but the board shows .25 I always have the controls at zero when I first power them on.

[MarkusBass]

What does plugging a dummy jack in do?

It disconnects the preamp from the power amp. Removing preamp tubes is even easier way to acomplish the same result.

What positions should I put the tubes in ? Either the 2 inside or the 2 outside ?

It doesn't matter. You have 2 pairs of tubes and they are connected in parallel in each pair. So you may put in any of V5, V6 tubes and any of V7, V8.

The fuse that failed was the .25 amp fuse. The schematic shows a .5 amp ,but the board shows .25

My mistake - I thought that it was the fuse on primary side of the transformer. And was it SLOW, or FAST type?

Mark

[gtrplayr1976]

It disconnects the preamp from the power amp. Removing preamp tubes is even easier way to acomplish the same result.
It doesn't matter. You have 2 pairs of tubes and they are connected in parallel in each pair. So you may put in any of V5, V6 tubes and any of V7, V8.
My mistake - I thought that it was the fuse on primary side of the transformer. And was it SLOW, or FAST type?

Mark

The fuse was slow.

[gtrplayr1976]

I don't see anything unusual when I look at voltages on V4-V9 tubes. I assume you have 0V on pin #3 of V5-V8. Just to be sure that preamp tubes are not a source of problems, you can either remove them, or plug in a dummy Jack into the Effects Loop (and short hot wire to the ground). It seems to me that this may be related to either one of power tubes not contacting (or even failed), or maybe the power amp is oscillating. I would put two power tubes in and power up the amp with a bulb limiter (with Effects Loop input shorted). A then measure voltages on power tubes. I assume that the fuse that failed was of correct value and type (2A, SLO-BLO), was it?
BTW, when you turn on a power amp like this one for the first time, I think that all controls should be turned down.

Mark

Mark I did what you said. Powered it up with the two power tubes ,and no preamp tubes. The voltages are attached. It ran for about an hour before I shut it down. All voltages on power tube ,and preamp tube pins were about the same. The only thing I noticed different was that I would get a pop ,and hum on the plates of V4 when I touched the leads to the pins. The other power tube that blew was indeed cracked. It didn't show until later when it finally leaked enough to turn white at the top.

[MarkusBass]

The only thing I noticed different was that I would get a pop ,and hum on the plates of V4 when I touched the leads to the pins.

I think that this is correct; plates of V4 are connected to grid of the output tubes and hum when you touch the grid is quite normal.

The other power tube that blew was indeed cracked. It didn't show until later when it finally leaked enough to turn white at the top.

I'm not sure whether I understand this. Does it mean that there were two tubes cracked? This would mean that you just need new tubes and there is nothing wrong with the amp (?).
But before you put two more tubes into the amp, I would measure the current flowing through the tubes. On the schematic there is a jumper from cathode of the power tubes to the ground. You can disconnect it and replace with a meter set to DC current (range 200 mA). And report what current you get. This will tell you whether the bias is set correctly.

Mark

[gtrplayr1976]

I think that this is correct; plates of V4 are connected to grid of the output tubes and hum when you touch the grid is quite normal.
I'm not sure whether I understand this. Does it mean that there were two tubes cracked? This would mean that you just need new tubes and there is nothing wrong with the amp (?).
But before you put two more tubes into the amp, I would measure the current flowing through the tubes. On the schematic there is a jumper from cathode of the power tubes to the ground. You can disconnect it and replace with a meter set to DC current (range 200 mA). And report what current you get. This will tell you whether the bias is set correctly.

Mark

At first there was only the one failed tube. The other three were working. I added one of mine ,and moved what would be the next failed tube to another socket. I may have cracked it when I moved it ,but as I said I had gotten two cracked tubes when I first got these ,and they sent replacements. I have already ordered a new matched quad, but I'm going to try what you suggest. You're speaking of JO? 3 ? The one that comes from the center of the power tubes off of pin #3 ?

[MarkusBass]

You're speaking of JO? 3 ? The one that comes from the center of the power tubes off of pin #3 ?

Yes, pin #3 is a grid of output tubes. Can it be easily be removed? A photo could help.

Mark

[gtrplayr1976]

Yes, pin #3 is a grid of output tubes. Can it be easily be removed? A photo could help.

Mark

See attached. I can take the board out again ,and unsolder it. It doesn't take long. Actually I could probably clip the wire ,and put the meter between it. Then replace it after I test it.

[MarkusBass]

If the current is correct, this will be a waste of time. But if it is not correct, you could save some money (needed for new tubes). That's why I would do it. And it will help you to be on the safe side (I mean having the knowledge what is the current).
Can you also make a photo of the cap in the middle (from a different angle)? It looks a little bit suspicious.

Mark