Power Valves for Mesa

Lodious

So,  I've read a lot of conflicting opinions on using non-Mesa outputs valves in their fixed bias amps. What's the truth? Should I buy Mesa, buy some third party graded tubes or just buy anything and stop worrying? If I buy any old valves is it easy to check the bias current to see how they are performing?

ecc83

Don't know about Mesas (amps or their valves) But...

"We" found many modern valves had high grid current which means they could be prone to "runaway" in a fixed biased circuit. High GC does in any event indicate poor quality.

However, it was found that GC dropped by several 100% after about 2 hours "burning in" and even faster if the amp was thrashed on a soak test bench.

So my take would be, fit decent quality bottles and set the bias but check it every 15 minutes over the course of a couple of hours or so.

NB. It is useful to monitor mains or HT volts the while else you can be chasing your tail fiddling with the bias pot when HT wanders!

Dave.

ICBM

What Dave said.

I've never had any issues fitting any decent quality valves in any Mesa. The bias is set fairly conservatively on them so you're unlikely to find a set that's outside the usable range, but if the vendor offers a 'tested for Mesa' option it's best to go with that to be certain.

jpfamps

ecc83 said:

Don't know about Mesas (amps or their valves) But...

"We" found many modern valves had high grid current which means they could be prone to "runaway" in a fixed biased circuit. High GC does in any event indicate poor quality.

However, it was found that GC dropped by several 100% after about 2 hours "burning in" and even faster if the amp was thrashed on a soak test bench.

So my take would be, fit decent quality bottles and set the bias but check it every 15 minutes over the course of a couple of hours or so.

NB. It is useful to monitor mains or HT volts the while else you can be chasing your tail fiddling with the bias pot when HT wanders!

Dave.

That's perfectly normal behaviour as the getter needs to be hot to scrub the vacuum in the valve.

Valves should be burnt in before testing for this very reason.

All good vendours should burn in their valves and test them for grid current. 

The RDH4, written in the heyday of valve production, and from a an era where conventional wisdom is that valves where made to better quality than current production, recommends 56 hours of burn in prior to testing.

Re fitting valves to MESA I have some customers who insist they want MESA valves, for which they pay a premium, however most customers are happy to go with non MESA valves and I've never really had any bias problems using non MESA valves.

ecc83

jpfamps said:

ecc83 said:

Don't know about Mesas (amps or their valves) But...

"We" found many modern valves had high grid current which means they could be prone to "runaway" in a fixed biased circuit. High GC does in any event indicate poor quality.

However, it was found that GC dropped by several 100% after about 2 hours "burning in" and even faster if the amp was thrashed on a soak test bench.

So my take would be, fit decent quality bottles and set the bias but check it every 15 minutes over the course of a couple of hours or so.

NB. It is useful to monitor mains or HT volts the while else you can be chasing your tail fiddling with the bias pot when HT wanders!

Dave.

That's perfectly normal behaviour as the getter needs to be hot to scrub the vacuum in the valve.

Valves should be burnt in before testing for this very reason.

All good vendours should burn in their valves and test them for grid current. 

The RDH4, written in the heyday of valve production, and from a an era where conventional wisdom is that valves where made to better quality than current production, recommends 56 hours of burn in prior to testing.

Re fitting valves to MESA I have some customers who insist they want MESA valves, for which they pay a premium, however most customers are happy to go with non MESA valves and I've never really had any bias problems using non MESA valves.

Ah! The information was imparted to me by the Great Man, I being but a lowly asstnt lab tech and knew my place! I seem to recall an unusually large number of KT88 "Purple Deaths" and they were searching for reasons. Poor QC in respect of valves and the reduction of collateral damage is one reason why most of the designs feature lower than normal grid leak values and low Z (ecc82) drivers in some cases.

I also had some EL84s that used to crackle for the first few hours of used.

Dave.

jpfamps

ecc83 said:

I also had some EL84s that used to crackle for the first few hours of used.

Dave.

The cathode coating needs activating after manufacture; again this requires that the cathode be heated for a period of time.

Again this should have been done at the factory / by the vendour.

Lodious

Thanks guys....non Mesa it is!