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Phil Allison[_4_] Phil Allison[_4_] is offline
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Default Best new EL84 tubes?

Big Bad Bob wrote:
Phil Allison wrote:

Most recent models from PV, Fender and VOX run their EL84s waaayyyy too
hot and burn them out in short order.



keeps costs down, but requires frequent tube replacement.


** Plain bad design IMO - I modify ones that come my way to solve the problem.

Cost SFA to do in the factory.


and so they sell tubes.



** Not likely, there is way too much competition in the tube market for tubes branded Fender or Peavey to dominate.



A good power output tube should last for YEARS, without obvious signs of
failure.


** Well, thousands of hours of normal operation is quite possible.


the GE book tells me that EL84/6BQ5 has a plate dissipation of 12W. It's
a pretty fair bet that class AB operation would give you 12-15W RMS with
properly designed "everything". Squeezing more than that out of it is
going to kill the tubes in a short period of time.

the ref design in the GE book has 300V power supply and 8k p-p
transformer load (AB1) with a 130 ohm cathode bias resistor, for 17W
output [probably heavily distorted 17W]. I'm sure you could work out an
AB2 design with fixed bias to crank it up even higher, but that's not
necessarily good for the tubes...


** FYI, I have a prototype 2x EL84 output stage here that delivers over 50 watts with low THD and no overheating. The trick is to use a split supply of 325V and 650V for screens and plates respectively.

There have been commercial amplifiers that did much the same.



..... Phil