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Old 13th January 2013, 07:12 PM   #11
cotdt is offline cotdt  United States
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I can't think of any technical reason why low voltage (ie. 700V) would be bad for the 845. Go ahead and try it. The GM70 would be an economical alternative to the 845.

I would bias the crap out of those things if using lower voltage. Looking at the graphs, if anything the tube would run more linear at such operating points.
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Old 16th January 2013, 01:10 AM   #12
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by cotdt View Post
I can't think of any technical reason why low voltage (ie. 700V) would be bad for the 845. Go ahead and try it. The GM70 would be an economical alternative to the 845.

I would bias the crap out of those things if using lower voltage. Looking at the graphs, if anything the tube would run more linear at such operating points.
I have the same idea with you.

With Vak ~ 750V, 70-50mA and opt 5-6K, we will have near 14-15W which is enough to dirve almost speaker in market. In this operation point, 845 has very very longlife.

I've heard some try AC heater for 845 with small hum noise. But I never try it.

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Old 16th January 2013, 09:39 AM   #13
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Quote:
Originally Posted by tcqanh View Post
I have the same idea with you.
With Vak ~ 750V, 70-50mA and opt 5-6K, we will have near 14-15W which is enough to dirve almost speaker in market. In this operation point, 845 has very very longlife.
I didnt't check it myself, but a very experienced guy said that the plates are covered with titanium dioxide which has a gettering effect only if it is heated at more than 450C, that corresponds to 90W dissipation.

So the 845s may live longer if they are run hotter!
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Old 16th January 2013, 09:52 AM   #14
multi is offline multi  Australia
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I have built several SE 845 amps running at 750 volts 3.5k out put transformer {Sowter}.
I drive mine with a Screen driven 5881 and{ peter millets} HI mu Pentode.
advantage of the screen drive 5881 it can handle 750 volts; I use a tube voltage regulator for the Pentode. Amp is very dynamic.
Phil
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Old 16th January 2013, 02:37 PM   #15
palmas is offline palmas  Portugal
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I would go for a GM70 , 750V 150mA , bias would be round -50V
It is quite linear in that area
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Old 16th January 2013, 02:53 PM   #16
Mush is offline Mush  United States
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Hi Multi,
Do you have a schematic of your 5881 driver circuit?
Thanks
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Old 20th January 2013, 02:31 AM   #17
multi is offline multi  Australia
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Hi Mush
I will try and find and send; i got it from Audioxpress i think.
Phil
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