• 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.

The Midlife Crisis - My 833C Amp Build

In my high Voltage (1000 Volts) plate supplies I use Hammond 193M 10 Henry chokes which I put between polypropylene capacitors on the negative side to ground because these chokes are only pot tested to 800 Volts and I hated putting them inside plastic cages ungrounded. My 10 Volts DC 10 Amps is brute force filtered with 3.5 Farads with a pre-charge system to prevent turn on surge from frying my 50 Amp rated rectifiers and transformer. Your MOSFET regulation system is more sophisticated. I very much like Lundahl transformers, but nobody makes an output transformer as heavy ducty as the Hammond 1642SE pot tested at 3500 Volts and good for 300 ma with the 833A drawing about 125 ma. It has multiple outputs which are perfect for Magnepans with DWM panels added. I wire the two elements of the DWM panels in series at 4 + 4 Ohms to the 8 and 16 Ohm transformer output and keep it in series with the 4 Ohm output for my Magnepan 0.7's (the biggest speakers I can fit in my little Futuro house.

The MOSFETs I mention above are not for regulating the plate supply, they are in the cathode of the 833 and function basically as active cathode resistors, holding the cathode at 240VDC while sinking the idle and grid current.
 
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Magz, I recommend you make the change you are talking about sooner than later. The problem you have stumbled on is a very real problem with switching mosfets operated even briefly in the linear region.

I designed an inrush suppressor about a decade ago for charging bulk capacitance in a motor drive and it took me a while to find a mosfet that actually worked and survived more than one cycle in that application. (I was designing a replacement for an inrush suppressor designed by my predecessor that blew after 2 - 3 cycles at most.. LOL) Part of the complication was that there wasn't much space in the machine (a surgical robot) for anything taking up more than a small number of sq inches. In operation the unit had to handle up to 20A at 48V continuously, and 100A surges for a couple of ms.
 
Magz, I recommend you make the change you are talking about sooner than later. The problem you have stumbled on is a very real problem with switching mosfets operated even briefly in the linear region.

I designed an inrush suppressor about a decade ago for charging bulk capacitance in a motor drive and it took me a while to find a mosfet that actually worked and survived more than one cycle in that application. (I was designing a replacement for an inrush suppressor designed by my predecessor that blew after 2 - 3 cycles at most.. LOL) Part of the complication was that there wasn't much space in the machine (a surgical robot) for anything taking up more than a small number of sq inches. In operation the unit had to handle up to 20A at 48V continuously, and 100A surges for a couple of ms.

Thanks for sharing your experience, Kevin. I ordered two of the IXYS 1000V linear MOSFETs from Mouser; with the chip shortage Digikey was out until at least January of 2022 and Mouser only had 4 in stock so I immediately grabbed two of them. Not cheap, but considering what I spent on the amp so far, a drop in the bucket.
 
I use a 9 Volt wall wart through 100 Ohms to pre-charge the 4.5 Farad capacitors. Leaving it on costs less than a quarter a month in electricity. I have relay switches to cut off the 10 Volts from the cathodes so they don't discharge when I shut off the 10 Volts. This is very simple and the capacitors display Voltage readouts. The Voltage at the 833A cathodes measures not much above 9 Volts which is within published operating limits for that tube and I figure it will cause the tubes to last longer.
 
the unit had to handle up to 20A at 48V continuously, and 100A surges for a couple of ms.

Interestingly, my application has some similarities, handling .16A at 240V continuously with up to .30A or higher surges on loud bass transients. In fact both times the MOSFET has failed in 7 years it was playing the same song with thunderous kettledrum hits at excessive volume.

I just have to get motivated to lug the amps to the basement again, and find a suitably strong helper. ;-)
 
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Our modest 833A SETs would never do that. I use them at low power output to make certain no distortion sets in from pushing them too hard. But they play music through Magnepans more than loud enough to match how loud an orchestra is at a front row seat.

I have always agreed with what those more experienced in audio reviewing than I will ever be that there is no such thing as the best or one and only one perfect amp or speaker or combination. It will be different for everyone and every listening room
 
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Took more than 20 years - more of an evolution. Precipitated by my little audio business going under forcing me to sell the 1.7QR to pay one more month's rent. I ended up with an insanely efficient pair of JBL Rhodes C37 (reflex) with D140 woofers and 075 annular ring horn tweeters. They sang in a way that Maggies never could and on quite low power - that is the beginning of the path to where I am today.. LOL
 
I made a second stereo to use out in my weight training room during exercise. I can do one or two more pull-ups with the right kind of Wagner overture. I made sensitive speakers with DS q8 Pro-X6BM woofers with 0.3 mh low pass filter and Parts Express horns part number 294-2924 candy apple green horns (Blue 294-3342, Red 294-3026, Gold 294-3024, Chrome 294-3022, Orange 2924-2923 for other favorite colors) with Goldwood GT-1188 pezo 800 Hz 270-082 93 dB $7.82 tweeters which have their own high pass capacitors built in. I mount it in a a 1 ft by 1 ft 1/4 inch Plexiglass my favorite color see through to which I bolted the old Magnepan stands after I got aftermarket upgrade Magnepan stands for the house stereo. I drive the weight room speakers with 45 SETs and choke loaded mu follower 6SN7s and a small ebay tube preamp with my laptop music library.
 
Cool. After the filter (in the amp box) I use two parallel Colemen regs for the 10A. So far very reliable and very quiet.
Lrt me know if anybody else builds an 833A SET and let me know how the brick sounds. How quite is it? I would be concerned if someone might turn up the Voltage too high by mistake and destroy the tube.
I have also built a 833A amp, which has been working happily for two years now. My solution for the 833A filament is: I have a transformer with two 15V 5A secondary. The secondaries are bridge rectified separately with a 10000u capacitor each, and fed through a total of four LT1083 constructed 2.5A constant current sources, delivering a total of 10A to the 833A filament. The result is hum only audible when my ear is within a few inches to my 98db speakers. Using CCS also achieves the benefit of a soft start.
 
Ive built some supplies for a client that run of 10VAC to 10VDC with LT4320 saligny boards, and MIC5156 LDO controller, IXTH80N075L2 pass device.

works flawless and doesnt need much in the way of cooling, power loss is less than 20W for DC fillaments. And all in a 52x100mm package.(for813) and 52x160 for 833.

PM if you want me to roll you some tailored to 833.
 

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