Amp Trouble - Blown Fuse Directly After Retube

[notjonahbutnoah]

Ok, so I ordered a full retube kit for my Mesa Stiletto Deuce Stage II head from Eurotubes. I've used them a lot in the past and always had good luck. I got the JJ E34L's, they're a little warmer than the EL34B's, and what I had in there before.

 

Yesterday, I installed them and about a minute into playing, when I tried to crank it up, a seriously BRIGHT white light came from the inside for a second or two, and then the fuse blew.

 

They shut off the protection grid!

 

 

 

Details:

-I used contact cleaner like you're supposed to when changing tubes.

-I was using my new 412 for the first time. A 1960A. Switch was in the mono position, and I was using the 4ohm jack, hooked to the 4ohm jack on my head.

-Only had it on stand by for a minute or so... did I push em too far too fast?

-It worked just fine before the retube. Had it for years and never blown a fuse

 

 

So I know, with some certainty, the problem is from the tubes in one regard or another. Could the contact cleaner have still been wet and shorted something? Seems weird because a little puddle of it dries almost instantly. I also know that a fuse blowing means something else is wrong.

 

My plan of action is to go pickup a handful of fuses (4A Slo Blo). Then, I will install a new one, leave the new tubes in there, and see what happens. If that works fine, then alrighty. If not, I will switch back to the old power tubes, and see what's up.

 

I mean, I know how to handle this I believe, but what I wanna know is if this has happened before to any of you.

 

What do you think it could be?

[WRGKMC]

If its the power fuse to the power transformer, You have a serious problem.

I would not just throw fuses in there and keep cranking it up.

 

The first thing may be is the tubes were not biased and its drawing too much or two little current.

Second may be you have some blown grid resistors or possibly a bad power cap. I'd have to look at the

circuit diagram to get more specific. You may be luckey and just have a bad power tube. tubes are fragile and

one of the plates inside may be touching something they shouldnt be. If theres a white coating inside the tube

it means the tube had a crack orair leak.

 

If you do power up the new tubes, you want watch the tubes to see if the plates glow cherry red.

Thats a symptom or a bias problem that needs to be fixed immediately.

If it didnt do that for very long, the tubes may still be good and simply need rebiasing.

If the fuse blows immediately, you have more serious issues. Remove all tubes and see if the

fuse still blows. If it does it needs a trip to the shop. It may be a blown resistor, bad cap or shorted transformer.

[fretmonster]

In reading my Mesa manual, they seem to be anal about using correctly balanced color coded tubes preferably their own. My Heartbreaker can take EL34s, 6L6 and 6V6 but I'm rather apprehensive about changing values esp. with non-Mesas. I'm probably not telling you anything new, I'm actually learning from you guys.

[WRGKMC]

I did a little reading up on that model of Mesa.

Apparently they get around the biasing issue by only selling tubes within

a narrow bias parameter. The circuit cant be biased.

 

This forces you to buy their tubes only as per their manual.

If you use other manufacturers tubes which can require bias settings all over the place,

its simply pot luck on weather they may cause a problem. You rolled the dice and lost.

JJ's allow high voltage bias adjustment for their tubes. I have no idea what stock Mesa tubes might be

without testing them, which isnt all that difficult. Getting something to match is.

 

The tubes you bought were likely too far out of specs for that head.

They recomend their own tubes specifically so you dont have to pay a tech to bias the head.

The result is, you have to buy their tubes only and possibly at a much higher cost for that convenience.

 

The head doesnt self adjust to the tubes. You have to use tubes within a specific

perameter. If you want to use other manufacturers tubes, you'll likely have to modify the circuit

so its either adjustable by adding bias pots, or you replace the resistors specific to the set of tubes

you purchased. Otherwise you have to use their tubes for the head to run optimaly.

[notjonahbutnoah]

Ok, so I got some fuses, this time I didn't crank it, but the two power tubes on the left were glowing like crazy, and the sound was a little off.

 

Turned it off, changed back to the old tubes, and it worked just fine. Still might take it to a tech though.

 

 

 

 

I did a little reading up on that model of Mesa. Apparently they get around the biasing issue by only selling tubes within a narrow bias parameter. The circuit cant be biased. This forces you to buy their tubes only as per their manual. If you use other manufacturers tubes which can require bias settings all over the place, its simply pot luck on weather they may cause a problem. You rolled the dice and lost. JJ's allow high voltage bias adjustment for their tubes. I have no idea what stock Mesa tubes might be without testing them, which isnt all that difficult. Getting something to match is. The tubes you bought were likely too far out of specs for that head. They recomend their own tubes specifically so you dont have to pay a tech to bias the head. The result is, you have to buy their tubes only and possibly at a much higher cost for that convenience. The head doesnt self adjust to the tubes. You have to use tubes within a specific perameter. If you want to use other manufacturers tubes, you'll likely have to modify the circuit so its either adjustable by adding bias pots, or you replace the resistors specific to the set of tubes you purchased. Otherwise you have to use their tubes for the head to run optimaly.

 

 

 

Right, I've had 3 Mesa amps and have only used the Mesa branded tubes once, and they didn't sound great.

 

I've probably retubed with non Mesa tubes 10 times and had great luck/tone so far. They usually know the tolerances and whatnot, they say they do, I just got a bad batch this time.

 

Part of me wants to sell my Stiletto and Heartbreaker in favor of a less complicated one channel amp.

 

Who knows.

[Wyatt]

One of the new tubes may have shorted internally. Or you may have arced across two pins of the socket. I would contact Eurotubes and see how they want to check them for warranty; modern tubes are inconsistent, it's not unusual to have one die of infanticide. Do you always replace all the tubes? Then chances are you have 9 sets or preamp tubes sitting around that have been used a fraction of their lifespan, everything except the maybe the PI should be able to outlast s few power tubes.

 

Did you let the contact cleaner evaporate entirely? It shouldn't short anything, but I have had issues with wet DeoxIT inhibiting conduction.

 

As for mesa and their fixed bias. I believe the "hottest" biasing color is White, which is still only 60% or maybe less, I wouldn't even bother with anything that isn't White or Blue.

[lowbrow]

 

Part of me wants to sell my Stiletto and Heartbreaker in favor of a less complicated one channel amp.

 

 

I'm a fan of simplicity. Still debating what my amp of choice will be when we play out for the first time with the new band but am thinking it'll come down to my Pignose combo or my Bassman head. I've never played the Bassman out, but it sounds glorious at home.

[WRGKMC]

The Glowing like crazy is the indication the tubes had a bias issue.

Its unusual for only one to glow like that. If it was a matched pair

they both should have glowed if the bias was off.

 

Since its only one, it makes me suspect you had an unmatched set.

(a very unmatched set)

 

If you requested a matched set, you can complain they sent you an unbalanced set.

If you got cheap and bought singles, then the fault is on you.

Even with a new balanced set, this can occur again.

 

If you knew what the recomended current draw was, you could buy tubes with the

a close rating. Groove tubes used to have the ratings marked on them and you could

choose between them. They are the only tubes I've seen marked that way and since they were

bought out by Fender, I dont know if they do that any more.

 

The only other option is to use a tube tester which are really hard to find these days.

maybe a mom and pop repair shop might have one, but you still need the tubes to find

a set that are in the range of the mesa.

 

best thing you can do is have the head modded so the tubes can be biased.

That way, you can use any tubes from any manufacturer. You would have to learn how to bias them though.

Its really not that hard to do using a volt meter over a resistor. The voltage reading corresponds to the plate current

for that amp.

[wagdog]

Chances are one of the new tubes had a short.

 

Your amp needs to be checked out by a tech. Sorry, all the internet knowledge in the world won't suffice; someone who knows what they're doing needs to look at your amp and make sure something isn't burned, or half burned.

[WRGKMC]

^^^ No, A short wouldnt let him run the amp. He was able to run the amp, even though the tube glowed red.

Thats a clear sign it was not a shorted element, and also clear sign of a high voltage bias imbalance.

 

Anyone whose done tube biasing knows you can over bias tubes and make them run too hot.

When you hear the term, biasing tubes too hot, the tubes are actually running too hot and glow.

Its not just a sound thing as many seem to think. If he had let it run that way, the tubes could eventually short.

A short would blow a fuse instantly, and not allow you to run the amp at all. Shorting the fuse or putting in a

higher valuse ould blow out components.

 

When I used to repair tube amps for a living, most major damage is done to amps were by the owners.

People dont trust the fuse is doing its job and telling them they have a problem so they jam a higher amp fuse

in there and melt the thing down. I commonly found Nails, Screws, and tin foil in the fuse holders too.

Instead of a $25 fix they wound up spending $250 or more.

 

You can read the last paragraph here. http://www.aikenamps.com/Biasing.html

You'll find the same description of tube glow in most bias adjustment writeups

if they were written by experienced techs who know.

[wagdog]

^^^^

I do understand biasing, I've built amps.

 

And I do appreciate what you're saying.

 

However, when a fuse blows it's because there was too much current being drawn, for whatever reason. It is quite possible there was no damage to his amp. However, it's also possible there is now an almost baked screen resistor. I'm all for saving money, but with an amp like a Mesa, I would have someone who knows what they're doing look at the innards. The bench charge would be well spent on peace of mind knowing that everything is OK.

 

I mean, if it were YOURS, would you pop the chassis and have a look?

[ugameus]

Mesa's are wicked crazy on the inside. Some techs won't even work on them. I got rid of mine in favor of simple single channel amps.

[notjonahbutnoah]

 

Mesa's are wicked crazy on the inside. Some techs won't even work on them. I got rid of mine in favor of simple single channel amps.

 

 

Yeah, they're amazing pieces of equipment, but that drawback is the complication. They're like Volvos, realiable, but when there's trouble, it's expensive and difficult.

[lowbrow]

 

...Those new Marshall VM 2266 heads have pooped up on my radar. I wonder how they are.....

 

 

I don't follow new stuff like I probably should, but I have pondered selling everything and just buying a JCM800 for about a year now. I probably should just do it. Or build one.

[notjonahbutnoah]

 

I don't follow new stuff like I probably should, but I have pondered selling everything and just buying a JCM800 for about a year now. I probably should just do it. Or build one.

 

 

Yeah, there's a white 900 on ebay right now that is givin me a chub. It would match my white 1960... sexual.