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Old 11th July 2011, 02:46 AM   #1
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Default 845 Operating Point

Guys,
I'm about to embark on an 845 Single Ended HiFi Amp. I have some good OT's I got from Naz.

On the weekend I started looking at a power supply for this amp.

I have a power tranny 1050-0-1050 at 300mA (with taps at 750V) which will be more than adequate for the job (2 channels).

PSUDII tells me that I will get 1100V DC for choke input supply or up to 1450V using a C-L-C with as little as 5uF for the first cap.

"Popular Wisdom" (if thats not an oximoron) says that something special starts to happen when then 845 is operated at 1250V and above BUT that "wisdom" never states if that is anode to cathode or if it includes a cathode bias allowance (which would be as much as 140 to 150 Volts).

I am going to be fixed biasing. Can anyone offer guidance as to wether I should stick to pure choke input for an 1100V supply or go for a C-L-C and aim for 1250 or more volts.

ASIDE:
Not fussed about caps - I have a bag of about 20 off EPCOS 470uF/450V which I can series parallel for whatever I need.

Thanks,
Ian
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Old 11th July 2011, 03:05 AM   #2
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Quote:
Originally Posted by gingertube View Post
"Popular Wisdom" (if thats not an oximoron) says that something special starts to happen when then 845 is operated at 1250V and above...
More like Audiophool Nonsense. Operating the 845 at higher voltages allows for higher load impedance, and lower Q-Point currents. Both help sonic performance. Triodes always produce less distortion the more lightly they're loaded.

If you get the plate current down, you lessen the problem of DC core magnetization, don't need as wide a gap to prevent saturation, and that helps get the Xlpri up to improve low frequency performance, and make those reactive load ellipses more closely approach loadlines.

Quote:
I am going to be fixed biasing. Can anyone offer guidance as to wether I should stick to pure choke input for an 1100V supply or go for a C-L-C and aim for 1250 or more volts.
Use the 1100V.
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Old 11th July 2011, 08:34 AM   #3
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Miles,
That makes sense.
Thanks,
Ian
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