motorbike guy wrote: Wed Jun 09, 2021 10:44 am There is no hard and fast rule that says you have to replace all your power tubes at once. If the amp is dead because of a dead tube, just replace that tube. It might not sound as good as if you replaced all of them, but it will work. Should be cheaper. Tube amps are like cars - shit wears out and they need servicing periodically.

No, but there's a semi-hard and somewhat rapid rule. The problem is that modern tubes are built with a lot less quality control than the vintage stuff. It shouldnt come as a shock that a 6L6GC made en masse by General Electric in the 60s is gonna be of superior quality than one made now in MUCH smaller quantities. It doesnt mean that your average JJ is a shit tube, but just that there are variances in their operating specs. In a push-pull amp you want both sides (the push and the pull) to be operating relatively the same. Not a problem with old tubes where any two are gonna be pretty dang close in spec, but its an issue with modern ones, thus why modern tubes are sold in matched pairs.

You're right in that an amp will work with mismatched tubes, but it might not work for long

“Semi-hard and somewhat rapid” is the name of my next record.

Dr Tony Balls wrote: Wed Jun 09, 2021 1:10 pm

motorbike guy wrote: Wed Jun 09, 2021 10:44 am There is no hard and fast rule that says you have to replace all your power tubes at once. If the amp is dead because of a dead tube, just replace that tube. It might not sound as good as if you replaced all of them, but it will work. Should be cheaper. Tube amps are like cars - shit wears out and they need servicing periodically.

No, but there's a semi-hard and somewhat rapid rule. The problem is that modern tubes are built with a lot less quality control than the vintage stuff. It shouldnt come as a shock that a 6L6GC made en masse by General Electric in the 60s is gonna be of superior quality than one made now in MUCH smaller quantities. It doesnt mean that your average JJ is a shit tube, but just that there are variances in their operating specs. In a push-pull amp you want both sides (the push and the pull) to be operating relatively the same. Not a problem with old tubes where any two are gonna be pretty dang close in spec, but its an issue with modern ones, thus why modern tubes are sold in matched pairs.

You're right in that an amp will work with mismatched tubes, but it might not work for long

are the variations from tube to tube THAT extreme? You would know.

My point was the dude was freaked due to the cost of a set of 4 tubes for his amp. Maybe he could get by with 2 new tubes then?

Wouldn't proper set up and biasing ensure that each tube was operating within an acceptable window, so as not to harm the rest of the circuit?

motorbike guy wrote: Thu Jun 10, 2021 9:27 am
are the variations from tube to tube THAT extreme? You would know.

My point was the dude was freaked due to the cost of a set of 4 tubes for his amp. Maybe he could get by with 2 new tubes then?

Wouldn't proper set up and biasing ensure that each tube was operating within an acceptable window, so as not to harm the rest of the circuit?

Yeah they can be. That wont always be the case, but i've seen some so mismatched that amp just blows fuses.

You're right though, he could always get by with 2 new tubes, or if only one tube has gone south he could run it with two of the tubes that are still good.

Intentional tube mismatching is gonna be the new x pattern 4x12 speakers, or overdrive stacking, isn't it?

tallchris wrote: Wed Jun 09, 2021 12:59 pm What did the tech say is the problem with the amp? Blown transformer? Any other info?

I didn't bring it. Maybe I'm gonna just sell it as it is. I don't know what I was thinking when I bought it.

motorbike guy wrote: Wed Jun 09, 2021 10:44 am There is no hard and fast rule that says you have to replace all your power tubes at once. If the amp is dead because of a dead tube, just replace that tube. It might not sound as good as if you replaced all of them, but it will work.

Thank you. I'll take this solution into consideration but I'm not even sure it's the tubes. On another forum someone said that if there was no further damage, the signal would have passed anyways, even with a dead power tube.
BTW, what are the possible risks of running the amp with mismatched tubes?

motorbike guy wrote: Wed Jun 09, 2021 10:44 am Tube amps are like cars - shit wears out and they need servicing periodically.

That's why I don't own a car.

oZZma wrote: Thu Jun 10, 2021 11:35 am
BTW, what are the possible risks of running the amp with mismatched tubes?

see Dr. Balls post above. Apparently it could cause damage and/or sound shitty.

You should take it to someone and at least get a diagnosis and estimate for fixing it.

if it is a $75 fix, you are going to kick yourself for selling it, since you will not get good money for a non-working amp, and some lucky buyer is going to get a sweet JCM 800 for a nice low price.

Seriously, you owe it to yourself to at least get it looked at, otherwise you can spin this out to the point that it will become a real bummer, perhaps needlessly. If you did own a car, and you got a flat tire, you wouldn't just sell the car or run it into a ditch and walk away. You spent about a grand on this amp, right? Get an estimate!

I have opened the amp just to be 100% sure about the "not glowing tube" thing. I had confirmation that one of the four power tubes is dead. Currently there are 4 Sovteks inside. The 3rd is not glowing at all.
I'm evaluating if I can make an attempt to replace the tubes and bias the amp myself, I have read a lot of stuff... But I don't feel confident at all. Questions:

- If I understand correctly, it's almost sure the fuse is blown, because the amp is silent. Is that right? Unfortunately on this amp the fuse is not accessibile from the outside. And there are TWO values printed on the back panel for the fuse(s?). Not even sure where to find it (them?). Is it easy to replace? Are there other fuses that I could mistake for the "right" fuse?
Anything I could fuck up in the process?



- I am more scared by removing the amp from the chassis than working inside of it. I mean, how should I grab that heavy thing without touching any capacitor of death? Sorry for the stupid question but I don't even know for sure where is it safe to touch.

- Do I really need to discharge capacitors? What's the point, if the amp needs to be powered and switched on for the biasing?

- Do I need to calculate the bias value? I have found a video for cold and hot biasing on my amp, but I don't know if the voltage values are dependent from the tubes you are putting inside, or they are always the same for the same amp.

Not sure I'm gonna go this route but I have found the tubes for 165€ so I'm not keen to spend 300€ if I can do this myself *. Thanks for your help.

- Another question: some say you can safely run the amp at 50watts by removing two tubes and divide by two the impedance to connect your cab. Is that TRULY safe? I have read different opinions on that.

*ETA: Are Electro Harmonics KT88's decent? I have found they are a lot cheaper than other brands so I dunno.

ETA: Another question: I know my amp has "bias terminals" which should make biasing a little easier. I haven't found photos for my specific amp but on another Marshall amp, I saw this "terminal" has three pins. But the bias trim pots are only two. How do I take my measurement exactly? Someone can explain how it works?

If any tubes are glowing your mains fuse is intact; there would be no power to the tube heaters (or anything) if it was blown.