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Old 16th October 2003, 10:34 PM   #1
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Default What can i do with 805 triode valves( NOS)

Hi ! Dear DIY friends , i can get ten 805 triode valves NOS at €10 each . I have the power tranformer needed . Who can get me a SE schematic for this valve ? Please !

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Old 16th October 2003, 11:49 PM   #2
Colt45 is offline Colt45  Serbia
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http://tdsl.duncanamps.com/pdf/805.pdf

pretty powerful, but in SE you will need to run it with a positive bias on the grid.. hence the grid will draw current.. a fair bit too.
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Old 17th October 2003, 12:02 AM   #3
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SHould be a big amp, power should be more than a 211 / 845 as it is an A2 tube. From the picture looks like a 845 with a top cap. Should probably go with an input stage coupled to cathode follower/or driver with transformer coupled to the 805 so that its grid requirements are fulfill.
If you willing to spare some, i don't mind to take them and try something out with these. I interested in transmitter class tubes now days.
I haven't look at the data sheet for gain requirements but i think
i would go with
input 6gk5
driver el34 (cathode follower ) cap coupled to the input
805 direct coupled from the driver
Well thats what i plan to do with my GM70, except i use a pentode in the input
There always something about transmitter class that thrills me. When they light up it is a sight to be seen.
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Old 17th October 2003, 12:18 PM   #4
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Default WAVAC 805

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File Type: jpg wavac 805.jpg (20.1 KB, 1381 views)
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Old 17th October 2003, 05:29 PM   #5
PRR is offline PRR  United States
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> i can get ten 805 triode valves NOS at €10 each

Expensive at half the price.

Look at the graph:
Click the image to open in full size.

In "good audio", we normally do not want to cross the grid zero-volt line (in red). At negative voltage, the grid is an infinite impedance; at positive voltage the grid is a diode with about 1KΩ resistance.

If you try to draw a load-line that does not cross the grid-zero line, about the best you can do is 1,500V B+, 20mA bias, 40K load, 800V swing, 8 watts out at large (10%-15%) distortion. That's an awful high B+ and load impedance and filament power for 8 dirty watts. With stock output transformers the power will be more like 2 watts, and even more distortion.

If we cross the zero-grid line, that transition from infinite to low-Z will cause the driver to distort. Because 1K is a low load for tubes, distortion is large. It can be managable with a step-down driver transformer. But usually the distortion curve rises quickly at low-mid power, then drops as we get far into Class 2 operation, before finally rising. That up-down-up distortion curve is un-natural.

Also note that the Plate Resistance of th 805 tube is MUCH higher than any of the Triodes normally used for audio. Rp is 10K or more, while all practical loadlines for this tube are under 10K. It will have low damping and high distortion compared to the triodes born and bred for "good" audio amplifiers. Yes, the datasheet shows an audio application, but it is a "monster power" commercial application, where quantity is more important than system cost or quality.

Many good A2 amplifiers have been built. But it is MUCH harder than building an A1 amplifier. By the time you over-build the driver to give ample drive power and grid current, you probably could have used that expense to buy a tube that is easier to drive and will run well in A1 mode.

If you must use this tube as class A2: look at 600V B+, 100mA bias, 5K load. That works out to about 25 watts RMS. The grid bias is about +20V, the grid swing will need to be about 50V peak, with about 50mA on positive peaks (zero on negative peaks). Up to about 5 watts, it will stay in Class A2 mode, constantly sucking grid current, so the driver load is low but fairly stable. A choke-coupled 6L6 as Triode could drive it. You could direct-couple if you mess with a negative supply (maybe -50V 50mA) pull-down. Note that the driver is dissipating over 30 watts, none of which gets to the speaker. Brett's transformer-coupled driver wastes less power, but needs a special transformer.

You could double the B+ and load impedance for 50 Watts "SET" Class A2, but that will be a very expensive custom transformer.
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Old 17th October 2003, 06:12 PM   #6
RS is offline RS  Germany
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Default 805 design (and 838)

There is an approved 805 amp concept from Nobu Shishido, this is for sure a class A2 (positive grid) design.
I have actually also a 805 Project in a planning stage, using a similar-to-WE102 tube in the line, a 46 as a driver, interstage coupled to the 805. Operating point of the 805 is 800V Ua at 125mA Idc and +25V Ug. Not very surprising, it is very close to the Shishido design. With a voltage swing of +/-35V you can get approx. 25W Po. Output transformer has 5k impedance (but you can go down to 3k to 3.5k).

BTW the 805 ist closest to the 838, both are variable µ triodes.
A friend of mine from Italy had build a 838 class A2 amp, using for line/driver a 6EA7 and a Lundahl IT to couple to the 838.

Regards

Ralf
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Old 17th October 2003, 08:12 PM   #7
Fuling is offline Fuling  Sweden
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Quote:
But it is MUCH harder than building an A1 amplifier
Tell me ´bout it, I finished the last of my two 811A SE monoblocks two hours ago. Pure A2 driven by EL86 cathode followers direct-coupled to the grids. It´s a bloody mess, but I would´ve been happy if it wasn´t for the mechanical buzz from the amps.
Seems to come from the rectifiers for the driver stage (PY 82), which is weird...
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Old 17th October 2003, 09:29 PM   #8
DrS is offline DrS  South Africa
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Quote:
In "good audio", we normally do not want to cross the grid zero-volt line (in red).
If that were true then then the Ongaku and several other creations of Audio Note would have to be "bad audio" or abnormal...
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Old 17th October 2003, 10:06 PM   #9
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Hi,

Quote:
If that were true then then the Ongaku and several other creations of Audio Note would have to be "bad audio" or abnormal...
Really?
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Old 18th October 2003, 12:08 PM   #10
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It seems hard work is needed . I think i will look for the 211 or
845 instead . Thank you guys for the feedback .

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