Help w Volcano hot EL34 on Music Man 210HD 130

[LBrewski]

Hi, I could use some help/advice here. I'm working on an old Music Man amp for a good friend. This one has had a frustrating issue for years of working fine for about 30 minutes or so then a hum comes out of the speakers and one of the 4 EL34 (6CA7) is fire-red hot. The original tube I pulled out has deformed glass.

After inspection - nothing found - I changed the electrolytic caps, re-tubed and biased; hoping that there was a bad tube causing the problem. No dice, 30 minutes of playing on this very good sounding amp and here we go again. Once it cooled down it was ok again.

I'll remove the chassis again and measure what I can statically but I'm scratching my head. I am wondering if it's an output transformer heating up, or even the power supply. Although it appears to be related to the one tube next to the OT.

Has anyone come across anything like this? Advice on how to proceed?

I have some pics posted in this album:

Music Man 210-HD pictures by ElBrewski - Photobucket

Here is the schematic
http://www.music-man.com/techinfo/ol...&_2275-130.pdf

[Albert Kreuzer]

How did you bias it?
These are class B amps and supposed to be biased to almost zero mA at idle.

If it's always the same socket where the tube glows, maybe a bad or dirty contact.

HTH
Albert

[melvin]

Am I reading this right? It looks like the signal drives cathode sinks (Q1, Q2). If one of those is failing, you would probably have a runaway condition...

[Steve Conner]

Yep, but that would affect two tubes, since each transistor drives two. If it was one of those trannies, I'd expect it to fail catastrophically and make both tubes glow bright red, or even blow a fuse.

I suggest checking R55, R57, R60, and R62 for bad solder joints or loose connections, also check the resistance of them is correct, as well as cleaning and retensioning the power tube sockets.

These amps are a bit feisty with their 700 volt B+, be careful in there!

[melvin]

Whether or not it's the same tube socket each time could be significant, with 700 Volts on the plates I wouldn't be too surprised if (when retubing) you found another one that just couldn't handle it.

[LBrewski]

Am I reading this right? It looks like the signal drives cathode sinks (Q1, Q2). If one of those is failing, you would probably have a runaway condition...

This amp is the older one - the 2nd schema with the 12ax7 PI/driver (I may have confused the chassis #). So no transistors there.

yep - the idea of 725v in the area sure keeps my attention.

How did you bias it?
These are class B amps and supposed to be biased to almost zero mA at idle.

If it's always the same socket where the tube glows, maybe a bad or dirty contact.

HTH
Albert

The bias setting is .5v across the 10 ohm cathode resistor as per this thread:

Music Man Amps Discussion Forum: Setting Bias on Music Man Amps

[Albert Kreuzer]

This amp is the older one - the 2nd schema with the 12ax7 PI/driver (I may have confused the chassis #). So no transistors there.

yep - the idea of 725v in the area sure keeps my attention.



The bias setting is .5v across the 10 ohm cathode resistor as per this thread:

Music Man Amps Discussion Forum: Setting Bias on Music Man Amps

Yes, .5V is what MM recommended. But with 725V and 25mA per tube this is more than 18W at idle. Not all modern tubes will take that.
Maybe try biasing a little colder and watch what happens.

Of course check all the resistors, contacts and solder joints first, like Steve said.

Cheers,
Albert

[Steve Conner]

Oh, if it has a 12AX7 PI, then check the usual stuff: leaky coupling capacitors, bias feed resistors that have gone open, etc. as well as those four stopper resistors. You posted the schematic for the later one with the transistor drive to the cathodes.

Something of an outside shot, though, since again a leaky coupling cap or bad bias feed resistor should make two tubes go "volcano", not just one.

[bluesfreak]

Leaky caps would kill at least two tubes but the "problem" was seen only on one socket so I would focus on this one, maybe it has a bad socket contact or a open/bad stopper resistor. Evaluate them carefully, sometimes they do crack and still measure right when cold but open up when getting warm. When replacing the tubes get tough ones which can handle the voltages seen in these amps.
BTW: How did EL34 get into this amp? Originally this version was shipped with either 6L6 or 6CA7 tubes...

[LBrewski]

Leaky caps would kill at least two tubes but the "problem" was seen only on one socket so I would focus on this one, maybe it has a bad socket contact or a open/bad stopper resistor. Evaluate them carefully, sometimes they do crack and still measure right when cold but open up when getting warm. When replacing the tubes get tough ones which can handle the voltages seen in these amps.
BTW: How did EL34 get into this amp? Originally this version was shipped with either 6L6 or 6CA7 tubes...

It's possible that the other tube in the pair is getting hot as well, but it definitely was not as hot visually - I shut it off very quickly when it happened. So I'll check for issues suggested here that can affect one or both.

The replacement tubes I bought are Electro-Harmonix 6CA7EH, I was under the impression that EL34 and 6CA7 are essentially the same, based on the description: "A direct replacement, with military reliability, for any EL34." :

Electro-Harmonix 6CA7 EH

Hopefully these are tough enough.

Yes, .5V is what MM recommended. But with 725V and 25mA per tube this is more than 18W at idle. Not all modern tubes will take that.
Maybe try biasing a little colder and watch what happens.

That certainly brings some perspective, I'd rather run it much cooler.

Thanks for all the advice, I'm going in...

[bluesfreak]

EH specs the 6CA7 for max. 800V in Class B so be careful. Ensure that the Grid Stoppers are 1k or 1.5k at least. I still suspect one of these failing causing the tube to go nuts...

[Enzo]

It is dissipation, hence current, that will kill the tube, not the voltage.

[melvin]

I imagine that beyond a certain B+, the tube may not be as stable or reliable... Amp designers seem to have been exceeding Vmax ratings for decades, but I have always thought they were stated by the tube manufacturer for a reason.

[Enzo]

They ARE stated for reasons. It is just that when you run the tube way over, it is till how much current flowing through it that will ultimatey burn it up... or not. At 700-800v on the plate, each milliamp increase in current is also 3/4 of a watt.

When RCA says X voltage is the design max, they are telling you, the designer, that they will consider your design proper adn expect long reliable life from their tubes if you stay within them. Mom and Dad did not want to replace the 6BQ5s in their table radio twice a year, but we in guitar amp land consider that normal.

When they tell you 300v on a plate, that doesn;t mean 301 will arc the tube. It means that when Leo Fender gets away with running 6V6s 100v over the book, he can;t go crying to RCA that their tubes don;t hold up. Not that his amps were tube eaters.

[melvin]

Well said. I appreciate your clarity. And the image of Leo Fender crying to RCA is especially nice... I suppose I am always particularly concerned about reliability. I want to keep the chances of an amp going down at a gig low, and I want to know what to expect when pushing the limits. I'd rather the tubes just wear out tone-wise every 6 months, than sound great until catastrophic failure at 12. I was under the impression that higher dissipation corresponded more so with the former, but Vmax always had the portent of the latter...now that I think about it, I'm unsure. By the way, in your experience, how high of a voltage can 6V6's reliably tolerate in guitar amps?