Power tube question *HELP

[Charlie]

So I got a Hot Plate today and hooked it up for the first time to my F-50. I had the amp on channel one with the gain dimed and the volume at 6 oclock for a while and then I smelled something burning. I took a look at the back of the amp and one power tube looked fine but the other one was glowing red hot like a mother&*^#$.

The whole time the amp sounded fine though.

The guy I bought the amp from said he only played it at low volumes in his home and that his playing time on the amp was literally hours. So this should be the first time the tubes were pushed hard.... is there some sort of break in period? Is this normal? I've never seen this with any of my other tube amps.

Oh... and the amp is using the stock Mesa 6l6 tubes.

*EDIT - I did change out all of the preamp tubes to non mesa tubes.

Thanks!

 

[Charlie]

So after going through the Tubes forum I'm guessing this is what I should try:

1: Re-seat the power tubes

2: Change the PI (V4) preamp tube back to the mesa tube I got the amp with.

And if this doesn't fix the problem should I get a new set of power tubes? I was planning on getting the Mesa 5881's anyway just to try them out and see if they suit me better than the 6l6's.

And if that doesn't work I guess I should take the amp to a tech... ?

 

[Charlie]

.... one more thing. I'm guessing I should take the power tubes out and swap their positions when I re-seat them?

If the same tube continues to glow red even after I swap its position then I know that tube is the problem.

If the other tube in the same position as the one before it glows red then I know it's something other than the tube that is the problem.

Is this correct?

Thanks again!

 

[Russ]

I would say you are on the right track. I would get a balanced tube in PI also. Because your hotplate allows you to turn up you need to take notice to things like this. It is a working sign that something isn't optimal.

 

[Charlie]

I would say you are on the right track. I would get a balanced tube in PI also. Because your hotplate allows you to turn up you need to take notice to things like this. It is a working sign that something isn't optimal.

Thanks for the reply. I plan on getting a balanced tube for PI... but would an unbalanced tube really cause this to happen? I've been using tube amps for 15 years and never heard of using a balanced preamp tube for the Phase Inverter until I started visiting this message board. I thought that having a balanced tube in the PI position was more of an audiophile thing... not something that is required for an amp to work properly... ?

 

[Russ]

By Randall Smith logic you don't need a balanced PI. Most every other manufacturer suggests to have your PI tube balanced. Basically it keeps your tubes working together instead of burning one up. That is the way I understand it. That is how I have run my amps since the 80's. Granted my amps were mainly Marshalls until I got my Mark IV but that was how I learned about having a balanced PI. I believe Randall said that the additional distortion created by having an unbalanced PI is pleasing to the ear. They also sell tubes. They don't match many across the dual triode. GT does. If a balanced PI was not an issue then why would so many amps need them? The manufacturer wouldn't necessarily care whose tubes you bought unless by chance they were in the tube business too and didn't grade for matched triodes in 12ax7's for phase inverters. Why would Eurotubes send out a matched PI with their kits if it wasn't important? Why would GT have a matched PI tube in their SAG amp kits if it didn't matter? Sure it is not required per se but it can keep you from having a case where the tube is so imbalanced that it might redplate one and not the other. I never saw this happen before but was told about this years ago. Your amp is the first one that I have actually heard about first hand that seems to be suffering like that.

 

[domct203]

A balanced PI tube keeps both sides of a push-pull (class AB) amp in sync.

A 12AX7 is really 2 tubes in one bottle. With the PI, one side handles the "push" while the other handles the "pull". If the two sides are out of balance, the point where they cross-over (transfer from push to pull) will be out of balance and could actually set up phase cancellations squashing notes at the absolute worst case. Remember this transfer from push to pull happens 60 times a second. A difference in rise time from each side of the PI can really throw things amuck.

The real question is....... Can you hear the difference? Only if the PI is really, really, bad IMO. But for the little bit more $ for a balanced tube it well woth it IMO to keep such a big investment running in tip top shape.

Just my 0.02

Dom

 

[jbird]

6 o'clock on an amp? I don't think 5881's are replacement tubes for 6L6's? And one tube glowing more than another is quite normal!

 

[Charlie]

Jbird - the entire plate on the offending tube was red hot and something smelled like it was burning up. 5881 is supposed to be a direct replacement for a 6L6 according to Mesa's website.

Well I have a feeling it's probably a bad tube but I'm still going to get and install a balanced tube in the Phase Inverter position ASAP. The amp and it's power tubes have been shipped several times so I wouldn't be surprised if it messed up the tube.

I did find an interesting article that is linked off of the Groove Tubes website:


The following text is from this link:

http://www.guitaramplifierblueprinting.com/GTlink.html

The most overlooked and misunderstood part of the output section is the 12AX7/ECC83 (Marshall style) or 12AT7 (Fender style in vintage cases) Phase Inverter tube. This is the tube that drives the output tubes. A lot of folks that specialize in making amps sound great don't understand this,
but fix this accidentally. They tend to use very good tubes, such as JAN spec 5751's etc., where the match is closer, and closer matched tubes in the output section. They also use tubes that sound good in the first gain stage positions, rather than the common Sovtek WA tubes which
most manufacturers use (because they are sturdy, not as expensive, and ship well without developing microphonics).

When I scope an amp in the lower frequency region, the vast majority of the time, the upper and lower parts of the sinewave are not even close to equal. This is more disparent than just a slightly mismatched set of output tubes. At this point, I install a matched phase inverter / driver.

The problem with phase inverters, is finding a matched tube. You have to remember that a 12AX7 / 12AT7 etc., is NOT a single function tube as an output tube. It is TWO tubes (two triodes), sharing a single bottle.

VERY FEW TUBE COMPANIES MATCH THE A AND B SIDES OF PREAMP TUBES. They warrant the tubes to work, and warrant them not to be microphonic, but do not say they are matched. This is not any bad commentary on tube suppliers .... to do this matching is time consuming and requires specialized equipment. If you can find somebody that has a Tektronix tube curve tracer, and bring them a bunch of tubes, maybe you will be lucky and find a match. There are a few tube vendors on my feeble little website that do offer matched preamp tubes, mostly those dealing in high end audio applications.

Matched phase inverters and output tubes are one of the reasons some amps "sing" and others are pedestrian compared to their brothers and sisters.

If you seem to have a lot of dead spots, try a new phase inverter tube. This is usually the preamp tube that is the closest to your output tubes. It is a trial and error process, but you may get lucky.

 

[Charlie]

Rabies/Jbird - When I said 6 o'clock I meant 12 o'clock! Doh!

I've always run my Marshalls turned up halfway... sometimes a little more and never had any problems. I really like the sound of having the power tubes distort a bit.

I let the amp cool off as soon as I realized what was going on I'm surprised it didn't blow a fuse. The amp should be able to handle being run at half power without any problems something is wrong with it. I'm just hoping it's a matter of replacing the power tubes or maybe the PI tube.

 

[Charlie]

I already ordered a new set from Mesa. They should be in early next week. For now I'm going to try swapping the power tubes I already have in the amp with each other and see if the same tube continues to have the problem when it is in the other position.

If it still malfunctions when it is in the other position this would mean that, to the best of my knowledge, the problem is the with the tube.

 

[Russ]

It could very well just be the one tube. Sometimes you can get a bad tube even if it is new. That is why getting a graded tube is important. This is why I buy GT or Mesa if possible. When buying other than a graded tube you are really subjecting yourself to a crapshoot though NOS tends to be less risky. Even with NOS though you have to be careful because there are still the odd chances of getting a bad tube.

 

[stokes]

An "unbalanced" PI tube is not going to cause a power tube to glow red.The PI is only going to amplify the signal as far as it is capable,if one side is somewhat weaker it wont cause the tube on that side to red plate.Red plating is caused by the tube drawing more current than it is capable of,either from a bad tube or a bias voltage problem.Dom,switch the position of the tube that is redplating and see if the problem moves with the tube,but be ready to shut it down quickly when you see the tube start to glow,a glowing tube can cause other problems.It would be wiser to just get a new pair of tubes and make sure they are the proper rating for your amp.

 

[Russ]

What happens when you don't have the hotplate in your rig? Someone told me a couple days ago that the use of an attenuator voids your warranty and that came stragiht from the mouth of Mesa. I am selling my PowerBrake and this is why he backed out of the deal.

I tried to explain to him that attenuators will not damage your amp any more than playing them alone would. Usually an attenuator will allow you to turn your amp up and make it run the way it was designed. Unfortunately things like old components are not always ready for them. In this case people blame the attenuator even though in time the weak link would rear its face anyway.

 

[Charlie]

What happens when you don't have the hotplate in your rig? Someone told me a couple days ago that the use of an attenuator voids your warranty and that came stragiht from the mouth of Mesa. I am selling my PowerBrake and this is why he backed out of the deal.

I tried to explain to him that attenuators will not damage your amp any more than playing them alone would. Usually an attenuator will allow you to turn your amp up and make it run the way it was designed. Unfortunately things like old components are not always ready for them. In this case people blame the attenuator even though in time the weak link would rear its face anyway.

Crap. I didn't know that using an attenuator would void the warranty. Oh well... I'm not even sure if it's still under warranty... transferable or what. I've read from many different sources that an attenuator is no different than running the amp hard w/ speakers alone otherwise I probably wouldn't use one. I live in an apartment so I haven't been able to run the amp that hard without the attenuator.

 

[Russ]

If it is something that isn't just a tube I wouldn't say anything about having a hotplate when you make your warranty claim :wink:

 

[Russ]

Oh yeah, and the attenuator doesn't actually do anything to your amp that your amp wouldn't do by itself. As you said, using an attenuator is much like using your amp without it just at a higher level. Basically, it just cuts db's. Some can act as a load box or add features like a line out or headphone jack. Your amp is still working as hard as it was supposed to be at whatever levels you had its knobs at. For example, if you had a volume setting of say 2 o' clock for months then your amp would not know any difference with or without your attenuator only your ears would. The thing that happens to those that are not conscious of their gear is that they will prematurely wear out their tubes from heavy use and then take out internal parts when a tube fails under heavy load. It is not uncommon. Then like I said before people blame the attenuator because it is easier than blaming themselves for not paying enough attention to their gear. In some cases people will blow a tranny. This is not the attenuator's fault. The tranny was probably on its way out and they had a tube failure. Though the tranny is supposed to be protected by fuse sometimes it is still damaged. You'll see things like grid resistors and stuff get wasted by a pushed tube failing. If you run an attenuator just pay close attention to your amp and its maintenance. If you have a Mesa amp that is still under it's original warranty "mum" is the word.

 

[Charlie]

Well I got my new power tubes today. Put them in the amp which I ran through the Hot Plate again but this time I was running the Hot Plate as a load box and using it's line out into my Toneport. I'm using the 8ohm speaker output from the amp into an 8 ohm Hot Plate and I triple checked to make sure the Plate was set to load.

Everything was going great but after about a half an hour the volume started to drop out just a little. Nothing major but definitely noticeable. I wasn't running the amp hard - the master volume was at 9 o'clock. I turned the amp off for a while and then it ran fine again for about another half hour and then the same thing happened again. I'm hoping it's not the amp and that it's something else in the signal chain (maybe it's the Hot Plate?) but I'm not sure what to check. Anybody have any ideas?

I'm loving the sound of this amp but it really bums me out that I'm having these problems with it. I've never had any problems with any of my amps before.

 

[Jak0lantern01]

Perhaps try it without the hotplate? It sounds to me that the amp ran fine until you added it.

 

[Charlie]

Perhaps try it without the hotplate? It sounds to me that the amp ran fine until you added it.

I'm going to try that but a lot of people use the Hot Plate without any problems. It provides the circuit with the proper ohm load so I don't see why it would damage the amp unless there is something wrong with it.

I think the problem started when I turned the amp up for the first time and one of my power tubes began to red plate. Before that I had only used the amp with the master volume set very low. It's possible that the heat from the bad tube damaged a solder joint.. a resistor... who knows?

I'm going to try my setup with a different set of cables, different guitar, no hotplate and isolate the problem hopefully it's not the amp.

Unforunately if the problem continues it looks like it's time to take it in to a tech.