Tube failure ?

Just found this informative forum, Very nice gentlemen !  Thank you for the opportunity.
Although I'm new to tube amplification, I'm a long term audio
listener and enthusiast.

I own two ST70s, one completely redone with new everything, she is running just fine, sounds wonderful
and I couldn't ask for more.  The second unit………..not so much.  She was running fairly well till this morning,
low hum that started as nothing then louder and louder, I looked up and the front left tube was glowing
cherry red, no other symptoms whatsoever.  I shut her down quickly and completely and unplugged her.

Here is the problem:  I know squat about whats happened, I know it ain't good and frankly I'm a little
scared to plug it back in and look.  No other outward sounds, rectifier looks ok, no light show at all, just that one left front EL34 and the hum.
Let me add they are older tubes and mismatched for the most part.
Could this be a simple tube failure or something more sinister ?

I did a search and didn't find the exact same problem, If I missed it, sorry.

Thanks in advance for any help.

jim

Edistojim wrote:Just found this informative forum, Very nice gentlemen !  Thank you for the opportunity.
Although I'm new to tube amplification, I'm a long term audio
listener and enthusiast.

I own two ST70s, one completely redone with new everything, she is running just fine, sounds wonderful
and I couldn't ask for more.  The second unit………..not so much.  She was running fairly well till this morning,
low hum that started as nothing then louder and louder, I looked up and the front left tube was glowing
cherry red, no other symptoms whatsoever.  I shut her down quickly and completely and unplugged her.

Here is the problem:  I know squat about whats happened, I know it ain't good and frankly I'm a little
scared to plug it back in and look.  No other outward sounds, rectifier looks ok, no light show at all, just that one left front EL34 and the hum.
Let me add they are older tubes and mismatched for the most part.
Could this be a simple tube failure or something more sinister ?

I did a search and didn't find the exact same problem, If I missed it, sorry.

Thanks in advance for any help.

jim

I'd start with removing the left el34's. Make shure speakers are connected during the whole
procedure.

Then power on and measure on pin 5 on one of the el34 sockets on the left, if more then -30V
and varies with the bias pot the i'll say that bias circuit is ok. Turn the bias pot to get the
most negative voltage and leave it there.

power off, check the cathode resistors ( check that it is 15.6 ohm from pin 8 in any el34 socket to ground.

If all above is ok, install a new matched pair of el34 in the left sockets, power on and measure the
voltage on pin8 of the external socket ( it has a small arrow ), it should be less then 1.5V when
the tubes are warmed up. If it rises above 1.5 V turn off immediatly as this indicates somthing else then a tube failure. ( see page 10 of the manual for detailed instructions). Turn the
biad pot for left channel and make shure that the voltage changes, stop when 1.4V is reached.
Do the same adjustment on the other side.
If voltages are stable and no hum is heard you have done the repair. If not, problems are
still there.

If you feel uncertain of these procedures, don't hesitate to turn it in to a qualified repair shop.

Thanks for the reply. Thats probably more than I'm comfortable with, my knowledge is limited.
Figures with my luck it would be more than a fairly simple tube change. I happen to live in the
middle of nowhere, literally, so any repair shop is miles upon miles away.
Can anyone give me the worst case vs best case on this amp, or, does she need to go in
for a complete overhaul?

I think Peterh gave some very sensible inputs as well clear instructions that you should be able to follow. If you don't have a basic multimeter you can get one for less than $10 at your local Harbor Freight or Radio Shack. That will suffice for now.

As a minimum you need to swap your left and right channel output tubes to see if the red plating moves with the tube.

Time to get comfortable! ;-}

A good start if you're a non-techie type would be to pop open the chassis and take a real close look for any burn marks or obviously damaged components. Also give it the old sniff test ... new damage tends to leave an odor. My first guess would be a bias resistor that went south. You should see one attached to each power socket.

PS ... once a tube "red plates" as yours did, you can pretty much write it off. Good time to pick up spares if you don't already have them. If you can't resist and want to try another tube in that socket, keep your finger on the trigger and shut it down at the first sign of anything unusual. They can survive if you don't let it go too long. I'd still hold off until doing the inspection underneath.

Going to pick up a digital multimeter in the morning, got a new set of EL34 tubes ordered plus a new rectifier tube on the way just to be safe. I'll wait till tomorrow to pull the bottom off the unit.
I didn't smell anything unusual when this happened, I was sitting right in front of it maybe 4 feet away and the hum started and I looked up and the tube was pretty much cherry red.
Thanks for all the info guys.

Just a question: Any reason why its just that left front tube, or could it have been any one of them?

I had this happen once with a KT88. The internal tube shorted - big hum and cherry red plate. In my case it took out the cathode resistor on that tube socket. Be sure and check the resistor(s) when you pop the case off.

Edistojim wrote:Thanks for the reply.  Thats probably more than I'm comfortable with, my knowledge is limited.
Figures with my luck it would be more than a fairly simple tube change.  I happen to live in the
middle of nowhere, literally, so any repair shop is miles upon miles away.
Can anyone give me the worst case vs best case on this amp, or, does she need to go in
for a complete overhaul?

The worst case is that you die during repair. The amp contains lethal voltages.
The best case is 2 new el34 and an bias adjustment procedure.

If you are uncertain, do find a repair shop and bring it there! It could save your life!

The worst case is that you die during repair.

(Way to enable, eh.)

Didn't they track down the Great Northeast Blackout of 1965 to a Dynaco repair up in Canada?  

Good to remember though ... DC voltage isn't particular about who it bites and when. Good rule of thumb ... the old adage "Keep one hand in your pocket" is a good reminder not to ground yourself when poking around.

PS ... DC can jump too if you get close enough ... I've got the scars to prove it!

I'm getting more confident by the minute, thanks.

Just a side note:  I'm a ships captain by trade, think very large boat………anyway, I deal with complicated electronics every day, navigation and communication.  If problems arise, they often do, we have spares and most systems are redundant times two.  One hard and fast rule is under no circumstance, absolutely none, are you to take the back off anything and even look at it, not to mention sticking some probe, piece of metal, screwdriver or anything else in a piece of electrical equipment.  Never.  The complicated stuff is better left to the ones that understand it, better known as professionals, which I am not.  Just by my general nature and my job I am very cautious.  
I think we'll refer this repair to a pro.  I enjoy my tunes and love the vintage equipment and its pure sound.  Kudos to you guys that have the knowledge, self taught or otherwise, to pull that complicated piece down and troubleshoot it.
I have some apprehension about blowing up my very nice equipment, causing a fire and electrocuting myself in the process.  Although I'm older, I'd like to work a few more years.
Thanks again, I believe I'll hang around and learn something about what I enjoy.

Edistojim wrote:I'm getting more confident by the minute, thanks.

Just a side note:  I'm a ships captain by trade, think very large boat………anyway, I deal with complicated electronics every day, navigation and communication.  If problems arise, they often do, we have spares and most systems are redundant times two.  One hard and fast rule is under no circumstance, absolutely none, are you to take the back off anything and even look at it, not to mention sticking some probe, piece of metal, screwdriver or anything else in a piece of electrical equipment.  Never.  The complicated stuff is better left to the ones that understand it, better known as professionals, which I am not.  Just by my general nature and my job I am very cautious.  
I think we'll refer this repair to a pro.  I enjoy my tunes and love the vintage equipment and its pure sound.  Kudos to you guys that have the knowledge, self taught or otherwise, to pull that complicated piece down and troubleshoot it.
I have some apprehension about blowing up my very nice equipment, causing a fire and electrocuting myself in the process.  Although I'm older, I'd like to work a few more years.
Thanks again, I believe I'll hang around and learn something about what I enjoy.

If you don't have access to a local repair shop, there are a couple of guys on this board (tmadden, audiobill, and montanaway) who are quite adept at fixing or even building our stuff.

What about me? I can fix stuff ... I can fix it real good!

So, sKIzo, this is how you "butter" your solder connections, eh?

When people say "it's the amps that kill you" I always thought they were talking about amperes. Turns out it is the amps... tube amps. No joke.

Edisto, you here in South Carolina?? Do this; go back to the sticky on READ ME SAFETY and then let us lead you step-by-step, day-by-day. Always works for me. They be a wealth of help here, You just have to put up with all the in-betweens!! Welcome, Dale

Indeed, its Edisto Island SC. I'm familiar with Loris near Myrtle Beach.
mall world, haven't seen many southerners floating around here. And thanks.

That burning house really built my confidence.

OK, Its at the repair shop recommended by a friend. The guy seemed to think it was a coupling
capacitor, does that sound remotely correct?

Might be time to just upgrade that amp to a new board like the VTA. If one cap is bad, it may not be long before others fail since this i the original amp I understand. I'd go with the octal this time if it were me, though my older VTA board sounds just fine to me. And recap the power supply too!

Jason