From Newsgroup: rec.audio.tubes

On 2019-04-02 18:50, 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. and so they
sell tubes. Sorta like the way HP sells printer ink.

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

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...
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From Newsgroup: rec.audio.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
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From Newsgroup: rec.audio.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.

probably works ok for hi-fi but I wonder how it would handle output tube overload distortion for a guitar amp...

I guess you could run clipped sine waves through it to find out. I used
to test power amplifiers under conditions like that by dunking a somewhat-high-wattage 8 ohm resistor (tied to speaker outputs) in a
glass of water. It's funny to watch it boil the water just a bit...

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From Newsgroup: rec.audio.tubes

Big Bad Bob wrote:

** 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.

probably works ok for hi-fi but I wonder how it would handle output tube overload distortion for a guitar amp...

** Just fine, the European made amps that used a similar set up were instrument amps. Clipping the output wave causes low plate dissipation but high screen - keeping the screen voltage down helps a lot.

I guess you could run clipped sine waves through it to find out. I used
to test power amplifiers under conditions like that by dunking a somewhat-high-wattage 8 ohm resistor (tied to speaker outputs) in a
glass of water. It's funny to watch it boil the water just a bit...

** I use a bucket of water with a pair of 100W, tubular ceramic, 8 ohm WW resistors. A simple switch box allows 4, 8 and 16 ohm loads. Copes with 2kW fine.

https://www.ebay.com/itm/8-ohm-100-Watts-Wirewound-Ceramic-Tube-Resistor-100W-/201153517823


.... Phil
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From Newsgroup: rec.audio.tubes

The EL84s I like the best are the new old stock Soviet tubes from the 70s and 80s.They are starting to get a little scarcer on ebay,but they are still not that expensive.
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