• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

DHP driver for 845

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After collecting parts for an 813 amp for more than a year, I´m giving up on them and aims for something a little more...doable.
813 requires a little too much of everything, and by everything I mean plate voltage and filament current.

The new plan is to sell my matched pairs of 813´s and buy some Valve Art 845´s instead. According to several posts at AA these babies are happy with everything from 300 to 1300V at the plates.
Seems like a good match for my 9,2k/80mA OPTs and my 700-750VDC PSU. Should deliver close to 10W, which is perfectly fine for my Fostex FE107E´s in BL horns.

I understand that 845 needs a bit of a driver stage.
For the 813 project I bought some russian 6S19 low Rp triodes and a pair of Lundahl IT:s, that should be fine for the 845´s too.
Though, I think it would be cool with a DH driver and after digging through my junkpiles I found a few of these:


I bought them from a friend some time ago for a small DHP SE project that never got finished.
With a resistive load of some 15-17k and B+ of 600-700V it seems to me that they can deliver close to 400V p-p, by far good enough for this project! All that with only some 16V p-p at the input!
This almost calls for a two stage design with input transformer:devilr:



Joined 2003
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Sure is a pretty data-sheet.

If I'm reading the German correctly, the maximum standing plate voltage is 250V? That with the soft knee suggests only about 150V peak signal before the negative plate swings get roundy. And you don't want that, because the output tube also loses gain at that point of the cycle.

Still, that can support a 845 to 700V, even 1,000V.

You would be running the AL1 far below its ability. And it has that funny socket.

What is the grid-resistance limit on 845? Do you really need a driver as low-Z as 15K? Even so, I think parallel 12AU7 could drive 15K to nearly 200V peak. Might need 15V peak drive. I know a ceramic UHF triode that could do over 100V peak with less than 2V drive, but it has no glow and is very costly and hard to find.

Ah, but AL1 is that rare beast: "penthode, direkt geheizt". Filaments went out about the time pentodes came in. Cathodes and pentodes made all the difference between the lame radios of the 1920s and the good radios of the 1930s.
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