Biasing the MT 15

[Befuddled]

So I replaced the power tubes and know that they need to be biased. I see a left and right check point on the back and one adjustment screw. Does one adjustment screw set both tubes? I'm just not clear on the process. Can you set me straight.

 

[BrianC]

You should know what you are doing with this before you start. Watch a friend do it a few times, then do it and be safe.

 

[AP515]

So I replaced the power tubes and know that they need to be biased. I see a left and right check point on the back and one adjustment screw. Does one adjustment screw set both tubes? I'm just not clear on the process. Can you set me straight.

Yes, the bias is set for both tubes together, not individually. You can measure them individually but the voltage goes across them both so you should find the average value for both. One slightly hotter and one slightly colder. It also changes as they warm up so let them get to operating temp before you do a fine adjustment.

 

[Befuddled]

You should know what you are doing with this before you start. Watch a friend do it a few times, then do it and be safe.

Probably not a bad idea but I don't have a friend that does that. I'm quite capable of doing the task once shown how to.

 

[Befuddled]

Yes, the bias is set for both tubes together, not individually. You can measure them individually but the voltage goes across them both so you should find the average value for both. One slightly hotter and one slightly colder. It also changes as they warm up so let them get to operating temp before you do a fine adjustment.

Thank you for the reply. That is what I was looking for.

 

[László]

This video was made by PRS several years ago showing, among other things, how they set the bias at the factory during assembly. With PRS’ external bias jacks, it’s a very fast, safe and simple procedure, since you don’t have to open the chassis. The amp is biased at around 2 minutes in.

 

[Befuddled]

This video was made by PRS several years ago showing, among other things, how they set the bias at the factory during assembly. With PRS’ external bias jacks, it’s a very fast, safe and simple procedure, since you don’t have to open the chassis. The amp is biased at around 2 minutes in.

Thank you! Your efforts are much appreciated.

 

[Tremontinator]

Can I piggyback this question?

I'm new to tube amps. How do I know when my MT15 needs re-biased?

 

[László]

Can I piggyback this question?

I'm new to tube amps. How do I know when my MT15 needs re-biased?

The bias can drift as tubes wear. Sometimes you notice a change in the tone of the amp. It can get less responsive, or sound dull. But those can also be signs of worn out power tubes. However checking the bias takes about a minute, and then you’ll know the source of the problem.

Then again, it never hurts to get an inexpensive multimeter and simply check the bias periodically. Takes two minutes or less.

Power tube life varies. I get years out of mine, but folks who do long gigs need to change them every six months or so. AC/DC were so hard on their amps that they changed them every show!

Preamp tubes can last ten times longer, though noisy preamp tubes are common, and tubes have been steadily declining in quality of manufacture for many years. I only use NOS tubes made in the 60s or 70s now.

However, there’s hope. A company in the Czech Republic has started making extremely high quality, handmade tubes for the hi fi industry. I think right now they’re making only very expensive KT88s, but I’d bet they’ll start branching out to other models at some point. Unfortunately, their tubes are made slowly in such small batches that a pair runs close to $900. And yeah, I’d spend that much on a pair of NOS Mullards, so...I’m kinda glad that my amps aren’t KT88, or I’d be tempted!

 

[Tremontinator]

The bias can drift as tubes wear. Sometimes you notice a change in the tone of the amp. It can get less responsive, or sound dull. But those can also be signs of worn out power tubes. However checking the bias takes about a minute, and then you’ll know the source of the problem.

Then again, it never hurts to get an inexpensive multimeter and simply check the bias periodically. Takes two minutes or less.

Power tube life varies. I get years out of mine, but folks who do long gigs need to change them every six months or so. AC/DC were so hard on their amps that they changed them every show!

Preamp tubes can last ten times longer, though noisy preamp tubes are common, and tubes have been steadily declining in quality of manufacture for many years. I only use NOS tubes made in the 60s or 70s now.

However, there’s hope. A company in the Czech Republic has started making extremely high quality, handmade tubes for the hi fi industry. I think right now they’re making only very expensive KT88s, but I’d bet they’ll start branching out to other models at some point. Unfortunately, their tubes are made slowly in such small batches that a pair runs close to $900. And yeah, I’d spend that much on a pair of NOS Mullards, so...I’m kinda glad that my amps aren’t KT88, or I’d be tempted!

That's obscene!



Why doesn't PRS make tubes? Think about this: Everything PRSh does turns to fucking gold. Have you seen a budget guitar that's better than an SE? No. Mid-range better than an S2? No. Have you seen anything better than a core? No. Better than PS? Hell no. Better amps? Cabs? Doubt it. PRS uses the best materials, figures out the best manufacturing processes, and produces the highest quality gear.



Why doesn't PRS manufacture tubes?

 

[László]

That's obscene!

Yeah, it’s a lot, but voodoo and unicorn horn are very costly ingredients!

 

[Tremontinator]

Hmm