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Old 24th November 2014, 10:25 PM   #1
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Join Date: Oct 2010
Location: se england
Default 833a spud amplifier

Hi Thought I'd start a thread about building a very cheap, smallish, practical and safe 833a amplifier. I'm not planning a build in the magz vein, just read all the 'midlife crisis ' thread and was hugely impressed.
I picked up a pair of second hand but seemingly good rca's for 25 and then a 750va machine control transformer configurable for 480v for 35 delivered. a root about delivered two 350ma 10H chokes and half a dozen 900v polypropylene caps plus 1000v 3 A uf diodes etc.
Half hour spent on psud showed a doubler could deliver 300+ma at 1100v.
26 bought 2 x 12v@ 20A smp and a couple of 20A 1ohm ntc makes a usable filiment supply. I'm a big fan of Rods filiment boards, must have 8 in use, but this is meant to be a practical build so heat is my enemy. The smp were warm but not hot after 30mins. The ntc was hot but running @ 2/3 rated Diss so hopefully will be ok.
The achillies heal is obviously the output transformers. Hammond 5k 75w were tempting but at 500 a pair not an option.
Bezdz in China are sending me a pair of 5.7kg '75w' 6-7k se opt for $175 plus a not inconsiderable shipping charge, good for 2500v and 300+ma.
The spud comes in as I really don't need all the power an 833 can deliver. I use a 4p1l preamp amp and my cdp and phonostage produce 5.6v p-p so x 8 ish will deliver 40+ volts into the 'powerdrive' mosfet 'tubelabe C'. Rough calcs that should give 40 or so watts. If I need more I'll fit a 6e5p / dn2540 driver.
I've actually had music from the 833s by wiring them into my 6hv5a breadboard, sounded positive.
The work will be building a chassis that is completely safe. It will be a stereo chassis with everything on board. I'm thinking 30 x 60 by 75 high with the tubes enclosed and no burn or electrical hazards. These are not my first amplifiers and my last two were 1100v + but I am not taking any chances.


I have a few bits left to buy and work out but hope to have a prototype by the end of Christmas holidays.
Any input as to the practicality of fitting chokes either side of the filiments to try and separate the smp and music would be appreciated! cathode will go to 0v via a current meter.


Last edited by enzoastro; 24th November 2014 at 10:27 PM.
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Old 24th November 2014, 11:42 PM   #2
Magz is online now Magz  United States
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Location: New Jersey, USA
Good luck! Should be an interesting build.
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Old 24th November 2014, 11:44 PM   #3
djn is offline djn  United States
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Location: Waterford Michigan
Hi Enzo. When I clicked on the link, I saw your GM70 amp. How did you wire up the half wave rectifiers....They are some thing 22 right? Those are the same I have for my project but have not looked into how to wire them up.
My "Art" website http://home.comcast.net/~djnagle/site/
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Old 25th November 2014, 01:11 AM   #4
multi is offline multi  Australia
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I have imported lots of output and Power transformers from China; I have them shipped by China post surface $US60 for 20 Kilo's, have found it very reliable to Australia takes about 4 weeks.England is a lot further!!! I have built both Se & PP 805's about 45 watts Se similar spec's to the 833. Sound wonderful PP.
Interested how you like Bezdz.
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Old 25th November 2014, 08:26 AM   #5
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Join Date: Oct 2010
Location: se england
Thanks for your replys!
DJN THE GM70 had 6d22s in a hybrid bridge with 3a 1000v uf54xx diodes, two damper diodes per phase to help with the 310ma draw.

I've not used Bezdz before but there finished amplifiers seem to get well reviewed in context. And they are very cheap! Communication seems good enough so far.
Who have you used for transformers multi?
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