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Patrick Turner Patrick Turner is offline
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Default 211 and 845 operating Ea/Ia bias points.



Chris Hornbeck wrote:

On Wed, 08 Aug 2007 12:32:18 GMT, Patrick Turner
wrote:

The 845 amps I have been strugling to find time to build
are approaching a stage where I must decide
on the actual working points that would be most suitable for
both 211 and 845 including class A2 operation,
by means of cathode follower buffer choke loaded to the
fixed bias supply.

So far having Ea at 1,000V and Ea at 80mA with RL = 12.5k
could seem OK but it means Pda = 80 watts,
and only KR845 or KR211 look able to take this idling condition,
one reason being that the KR heaters are 10V at 1A instead
of the RCA GE use of 10V at 3.3A.

RCA data gives Pda = 67 watts, and if Pd-filament = 33 watts,
the total tube Pd = 100 watts


Oh, I forgot to say that the RCA and GE type 211's and 845's
will run forever at 100 watts plate dissipation. It's not a
problem. Other than the very first 1920's versions, that's
their CCS rating.


I am slightly shy about running tubes at their maximum Pda ratings.

In 9 times out of 10, most amp makers of the past were using
serious amps tubes mainly in PP designs where the biasing
was at maybe 1/4 Pda, and there was never a 100% duty cycle,
but tube makers still had to give a figure for design, so they did.

The 13E1 has data specified Pda max rating = 90 watts, and that's how I
set up the
first one I found in SE, a mode of operation rarely if ever used by
engineers.
After 1/2 an how the anode began turning red, and sound went to mud.

I found 75 watts was maximum constant Pda, and this tube needs a total
of 33 watts of heating
power to its two cathodes.
I settled on 72 watts only, so actual total heat from the tube = 72 + 33
= 105 watts.
The owner of the amps with 13EI has done many thousands of hours trouble
free and music rich
for 5 years.

KR Audio 845 are seen at
http://www.kraudioproducts.com/Kr/Pr...13&ProductID=6

These have 100W ratings, but have only 10 watts of heating power,
so they should last.

Compared side by side to an RCA 845, the KR looks a darn sight more able
to handle the heat.

So I will be quite happy NOT tempting fate by running at 100W, and 65W
will be fine,
as it is the case with running 300B at 32 watts max, and KT88 at 32
watts max, EL34 at 22 watts max etc.

Patrick Turner.

Thanks, as always,

Chris Hornbeck
"It's just this little Chromium Switch.
You people are SO superstitious."

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