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My 833-A amplifier
My 833-A amplifier
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Old 31st August 2019, 02:18 PM   #1
drbarney1 is offline drbarney1  United States
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Default My 833-A amplifier

I have completed and perfected my magnum opus, an 833-A single ended triode amplifier which drives a pair of Magnepan 0.7's. It is one and a half years in the making. I am interested in guiding anyone who would like to use this wonderful triode which I consider the Stradivari of vacuum tubes for music. It works best at 1000 Volts with zero grid bias where on the chart it is just as linear as it is at any other voltages, something I verified by using batteries to test both positive and negative bias applied to the grid which had the effect of symmetrical changes in plate current with respect to grid voltage, +1.5 Volts raised the plate current by 8 ma and -1.5 Volts lowered it by 8 ma. It sounds best at zero grid bias which causes the tube to draw 110 ma.

It costs well under $2000 to build a pair of these and it uses Hammond 1642SE output transformers rated to carry 300ma pot tested to 4kV. I drive it with Lundahl LL2765 transformers to step down from about 5k to 500 Ohms on the grids. The 1000Volt plate supply is straight forward with proper caging for safety and fuses on the power transformers and the 833-A cathode requires a very well filtered 10 Volts DC at 10 Amps for which I use 4.5 Farad filter capacitors which pre-charge before turning it on.
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Old 1st September 2019, 02:22 PM   #2
ratbagp is offline ratbagp  United States
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Could you provide a schematic? And what is the red material?

Anyhow, congratulations on your amp. It must have been quite a project.

ray
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Old 1st September 2019, 03:31 PM   #3
stevenrotterdam is offline stevenrotterdam  Netherlands
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That's the way to test!
I checked my 6v6 preamp with 1,5 volt + and- and the output change was totally symmetrical. With +/- 9 V there was a slight asymmetry, but which fool puts 9 V input in a pre-amp?
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Old 1st September 2019, 04:01 PM   #4
drbarney1 is offline drbarney1  United States
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The sides of my final stage amplification chassis are covered with red velvet over foam rubber with sunken buttons and the edges secured with upholstery tacks from the hardware store. I made it in the style of steampunk out of a Jules Verne novel. I encourage builders to build their amplifiers in their own favorite style. Here is a schematic.
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Old 1st September 2019, 06:22 PM   #5
jazbo8 is offline jazbo8
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Please learn how to upload images properly: How to attach images to your posts.
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Old 1st September 2019, 08:31 PM   #6
drbarney1 is offline drbarney1  United States
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I wondered how to make it work. Here they are:
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File Type: jpg 833-A Amplifier Circuit.jpg (477.7 KB, 239 views)
File Type: jpg 1000Volt power supply.jpg (207.9 KB, 238 views)
File Type: jpg Cathode heater for 833-A.jpg (213.6 KB, 222 views)
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Old 1st September 2019, 10:02 PM   #7
Mark Tillotson is offline Mark Tillotson
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No voltage equalizing resistors in those series capacitor chains?
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Old 1st September 2019, 10:53 PM   #8
drbarney1 is offline drbarney1  United States
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The 10 Amp 10 Volt supply has 0.1 Ohms which puts the voltage after the 4.5 Farad capacitor just under 10 Volts which is ideal. The 1000 Volt supply uses chokes which have to be isolated in plastic cages because they are not pot tested as high as 1000 Volts.
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Old 2nd September 2019, 06:05 AM   #9
PRR is offline PRR  United States
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> No voltage equalizing resistors

If electrolytic, all from the same lot, it will be OK, for a while. One gets higher voltage, which increases its leakage, which reduces its voltage.

I'd be more interested in the 1Meg 1/2 Watt "bleeder". Few 1/2W resistors are rated over 350V long-term. (Also it does not bleed much... 90 seconds to reach 50V; _IF_ the resistor hasn't failed from voltage stress.)
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Old 2nd September 2019, 10:11 AM   #10
drbarney1 is offline drbarney1  United States
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It has been over a year and a half. I just put enough of them in series to have them overrated. So far, no signs of trouble. www.mouser.com and www.digikey.com have resistors which are rated at the voltages needed. I wish I could find capacitors rated so I did not have to put them in series because, if nothing else, it would be more convenient.
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