Power Tube Help!

hochpt21

If you had a tube amp with 4 KT120’s with around 200 hours, and one of them broke...what would you do and why?

Replace the broken one?
Replace a pair? (And what position)
Replace the quad?

Thanks,
Paul

pitdogg2

I'd check bias. 200 hours is not a lot of time to be honest.

kharp1

I agree that 200 hrs is nothing time wise, but, a tube can go south for any number of reasons. I prefer to replace a pair and keep the odd one for backup.

michaeljhsda2

Just replace the one bad tube. Re bias all four after it's installed

hochpt21

I should have clarified. Tube was damaged during shipping. Nickle sized circle-ish crack...

hochpt21

michaeljhsda2 wrote: »

Just replace the one bad tube. Re bias all four after it's installed

That would be ideal. So even with different hours, as long as the bias is checked every few months I should be good?

hochpt21

kharp1 wrote: »

I agree that 200 hrs is nothing time wise, but, a tube can go south for any number of reasons. I prefer to replace a pair and keep the odd one for backup.

If I replaced a pair, does it matter what position they go in relative to the other ones?

mhardy6647

Probably. No idea what the amp's topology is, but if it is stereo(?) then there should be two push-pull pairs. The push and pull halves of one channel should be as identical as possible (in the best case). This is critical if the bias cannot be adjusted (as was often the case in many, even very fine, vintage amplifiers.

If it's a monoblock with four tubes, it's probably push pull parallel.

In either case, if the bias can be adjusted for each tube individually, it's less of a factor than it would be in a fixed bias amplifier with no adjustment. Note, "fixed bias" simply means that there's a bias supply circuit (as opposed to, e.g., "cathode bias" in which the tube itself is actively involved in the bias circuit). A "fixed bias" amp can have adjustable bias, and nowadays many do!

pitdogg2

hochpt21 wrote: »

I should have clarified. Tube was damaged during shipping. Nickle sized circle-ish crack...

With this "new" info I'd say you may be replacing more sooner than later if it was rough handling. I'd find a pair.

hochpt21

mhardy6647 wrote: »

Probably. No idea what the amp's topology is, but if it is stereo(?) then there should be two push-pull pairs. The push and pull halves of one channel should be as identical as possible (in the best case). This is critical if the bias cannot be adjusted (as was often the case in many, even very fine, vintage amplifiers.

If it's a monoblock with four tubes, it's probably push pull parallel.

In either case, if the bias can be adjusted for each tube individually, it's less of a factor than it would be in a fixed bias amplifier with no adjustment. Note, "fixed bias" simply means that there's a bias supply circuit (as opposed to, e.g., "cathode bias" in which the tube itself is actively involved in the bias circuit). A "fixed bias" amp can have adjustable bias, and nowadays many do!

Just changed my sig...Rogue Audio Cronus integrated amp. Bias is individual.

michaeljhsda2

hochpt21 wrote: »

michaeljhsda2 wrote: »

Just replace the one bad tube. Re bias all four after it's installed

That would be ideal. So even with different hours, as long as the bias is checked every few months I should be good?

Definitely

hochpt21

Just purchased a pair, and will keep one for back up.

Does it matter what position I install them in?

hochpt21

.

michaeljhsda2

hochpt21 wrote: »

Just purchased a pair, and will keep one for back up.

Does it matter what position I install them in?

No

hochpt21

michaeljhsda2 wrote: »

hochpt21 wrote: »

Just purchased a pair, and will keep one for back up.

Does it matter what position I install them in?

No

Perfect, thank you.

hochpt21

Got her up and running. Changed power tubes in 1 and 4. Bias set.

Sounds beautiful.

jumpindick

If you ever want to roll tubes, the one in the middle (preamp) tube is that one to change. That is the one that has max impact on the overall sound coming out of that amp. I tried a few of them, but the best one, for me at least, was Psvane 12AU7 which is what you got..

nooshinjohn

The folks a Rogue would have replaced the damaged tube for free. I just lost a power tube on one of my amps, and after 7 years of faithful service, I am getting ready to replace all 12 KT120's.

hochpt21

I have a few tubes coming from @heiney9 , but the Psvanes sound pretty exceptional in there right now.