ST-70 randomly blows fuse.

Hi

I turned on my ST-70 the other day and the fuse popped. I replaced the fuse and watched the tubes when I turned it back on. Nothing odd happened, none of the tubes are arcing or glowing weird and the amp played fine. That was about a week or so ago. The amp has worked great since then. When I turned it on today, the fuse popped again. I changed it and watched the amp for anything weird and once again, there was nothing and the amp appears to be playing normally. Nothing is getting hot and there isn't anything that I can find burned. Has anyone else experienced this? Is there something specific that I should be looking for that is causing the fuse to pop?

Any help is appreciated.

Chris

Got a spare 5AR4 to try? If not pick one up from www.guitarcenter.com, they have Sovtek 5AR4 for about $13. Are you running a stock/original quad cap? That could be a problem. Have you noticed any increase in hum recently? That might also indicate a leaky quad cap. I am not a fan of the quad cap and have stopped using them. I assume that you have replaced your selenium stack with 1n400x, recentered your bias pots and inserted high voltage forward diodes ahead of rectifier pins 4 and 6? If not these are all easy to do and may improve the reliability of your amp.

This is one of Bob Latino's kits that I built not quite a year ago. There hasn't been any increase in hum or anything along those lines. I unfortunately do not have a spare rectifier to see if that is the problem. I have been meaning to order a set of spare tubes and may have to soon since the amp is giving me trouble. I have noticed every now and then I will get a scratchy noise out of the right channel that would appear to be coming from the amp. I have isolated that noise by disconnecting the equipment attached to the amp. The scratchy noise is random and only shows up in the right channel and has been that way for a couple months now. I never paid it much attention because it goes away once I get music playing through the amp.

Okay, I was assuming that you were running a vintage ST-70. When the fuse blows, does it blow right as you power on, or does it blow 15-30 seconds later when the 5AR4/GZ-34 starts supplying high voltage dc? It would good to know if the scratchy sound moves from the right channel to the left when you swap the left channel output tubes with the right. Might be a cold solder joint or a poorly tensioned tube socket. Have ever run other output tubes in your amp?

Well, I tried moving the output tubes. The noise didn't move and the amplifier still appeared to be functioning normally. All of a sudden before I could try the 12AT7 tubes, I now have a worse problem. Now I don't have much of any audio level coming from the amp. I have to turn the volume up at least halfway before I can get much of any sound out of the amp. Any idea why all of a sudden I have little to no sound coming from the amp? My preamp appears to be functioning properly.

Chris

Do you have one of the wires that goes from a walkman plug to a pair of RCAs? You could try to hook up an MP3 player direct to the amp? Use the volume on the MP3 player to adjust the volume... Dun know.. I was just thinking this to test the amp and only the amp.

Other ideas would be to look at past threads on voltage readings... I think I've seen a few in this forum where the expected voltage readings are listed.. This can tell you quite a bit.

If you have spare tubes you could roll some in and out. Test.

If you have not already done so... you need to get a replacement 5AR4. Back on 5/1/11 I suggested you order a 5ar4 rectifier from guitar center for $13. I would number your three 12at7 with a Sharpie so you can keep track of which of their original positions. Have you already tried swapping the middle one with one of the side ones. If that restores one channel to normal operation you can probably assume a bad 12at7.

Well, I don't have spare tubes. I moved the 12AT7 and did not have any results. I tried directly connecting a MP3 player to the amp and do get some sound out of it but at a diminished level. I'm starting to think that maybe I will give up on the tubes and go solid state.

If your ST70 is one of the new ones then it is just a Rectifier tube as J Bede said... You should just get a spare or two. These go out every 1 year or so I think in a ST70 and I think you are just hitting the 1 year mark. I thought you mentioned in another thread this was a 1 year old amp

Just get 2 of em (GZ34 rectifier that is) and you are done. If this is really getting you upset then get a Webber Copper Cap GZ68 and be done with it. These are soid state are are basically 2 GZ34's

You just have to spend the 12 - 15 bucks a year or so on a recitifer owning the amp. Not bad for sweet sound

Have you measured the voltage on pin#8 of your rectifier?

Your local guitar shop has 5AR4/GZ-34 in stock... they won't be $13 like they are from gc.com. Go buy one and see if that gets you back to one scratchy channel. You need to eliminate the rectifier as the root cause of the reduced output. NOTE: A failed output tube can take the (new) rectifier out with it. You need spare output tubes, preferbly some cheap ones for debug use. I suggested you check the pin tension in your output tube sockets a week ago. Did you do this? You might try connecting your amp to a pair of speakers that are "disposable" then tapping or slightly wiggling each tube to hunt down a bad socket or connection. Please don't try this test with your LS3/5A or Quad57s connected or we will be helping you debug your blown speakers too. Warning: Wiggling tubes when the amp is on is not a recommended repair practice. I only suggest this because you seem hesitant to acquire spare tubes.

FYI: threatening TDTAF with going "solid state" will not help you debug your amp... it may produce offers to buy your ST-70 however

since the problem is still on the same right side after swapping tubes I think you've got a bad solder connection on one of those sockets.
It may not LOOK bad, but it's easy & quick just to reflow solder on each tube socket, and 90% chance that will cure all your ails.
A scratchy sound from the right channel only is not likely related to the 5AR4 rectifier tube.

I'm going to have to pull the amp out and go through it. I hope it is something stupid like a solder joint or loose connection. I do think I am going to replace the rectifier with a copper cap. I'm kind-of in a pickle for parts out here where I live. I can't run down to the local store and get a tube unfortunately, I have to order them which makes it more challenging to try and fix this issue. That's what you get for living in a rural area. I hope this isn't going to get real expensive because if it does, I may just put it up for sale.

tubes4hifi wrote:since the problem is still on the same right side after swapping tubes I think you've got a bad solder connection on one of those sockets.
It may not LOOK bad, but it's easy & quick just to reflow solder on each tube socket, and 90% chance that will cure all your ails.
A scratchy sound from the right channel only is not likely related to the 5AR4 rectifier tube.

Did you read post #6 above? There may be more than one issue with his amp. Getting the amp back working with a scratchy right channel would be progress at this point, as I understand the current situation.