813 homebrew amp with power supply from recycled parts

8/14/2019 813 Homebrew Amp With Power Supply From Recycled Parts

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813 Homebrew Amp with Power Supply

from recycled parts.

This home-brew amp with two 813 tubes for the final was made by an unknown amateurradio operator (ham) some years ago. I am assuming it to be about 35 to ! years old. I

originally bought the pie"e at au"tion be"ause of the ra"k-mount "abinet in whi"h it wasinstalled but be"ame intrigued by what seems to be a relati#ely simple "ir"uit$ a parallel

wired pair of 813 tubes in a grounded-grid "onfiguration. I did not ha#e a high #oltage

power supply for it so had to build one (more info on power supply below). The pie"e hasa filament transformer mounted under the "hassis. %s "an be seen$ the 813 tubes glow

&uite brightly with the filaments powered (1! #olts at 1! amps).


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Caution

I ha#e limited e'perien"e with transmitters and amplifiers so tend to err on the side of"aution. %lso$ lethal high voltage !ver "#$$ volts %C& is present on top of thechassis as well as under when a high #oltage power supply is "onne"ted.*ue "are and

"aution are warranted. %n understanding of the "ir"uit is a must. I ha#e not yet found thee'a"t "ir"uit in any of the ham radio maga+ines. ,ditors and ,ngineers 'A%(!

HA)%*!!+, 1-th edition, by *ill !rr "omes the "losest with a grounded grid "ir"uit

that allows for #arious final tubes in"luding a pair of 813 types. The power input wasdesigned to be about a kilowatt.

(nitial checs and testsI "leaned the unit and found se#eral bad solder oints on the bandswit"h$ its "oil$ and on

one of the filament "onne"tions. %pparently the hea#y "opper wires needed a hottersoldering iron than was a#ailable. The #ariable "ap pre-loading had been loosened to the

point that the "aps s"ar"ely held their settings. I adusted the preloading. The %/ power

"hassis "onne"tor was a ,0%, "onne"tor re&uiring a plug-to-plug %/ "ord$ adangerous situation. 2in"e the unit is homebrew$ I modified the opening and mounted a

"omputer-style 3 wire male "hassis "onne"tor. n powering the unit with filament


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#oltage (shown)$ I measured 1!.! #olts at the tube pins$ the "orre"t #oltage. The "urrent

meter responded to a low */ #oltage test$ but it pro#ed to ha#e a sti"ky meter pointer. I

will repair or repla"e it. The filament "hoke is a ni"e homebrew bi-filar winding on aferrite rod. The e'"iter input "ir"uit is untuned$ ust a !.!!1 mike "ap to the "athode.

%esign of a power supplyI in&uired about using an ele"tri"ally mat"hed pair of unk mi"rowa#e o#en transformers

(se"ondary is about 41!! #olts %/) and re"tifiers for a high #oltage power supply.pinion is di#ided on whether this idea will work. The transformers ha#e one side of the

se"ondary "onne"ted to the frame. The use of a pair will allow for a full-wa#e power

supply while keeping those "onne"tions on the frame at or near ground potential.

ere is a 6* file of an arti"le in 7, that des"ribes su"h a supply.

I lo"ated a set of ten 5!! 0 ele"trolyti"s at a #oltage rating of 31! ea"h. I reformed ea"h

one separately. These "ame with 1!9 ohm ten watt e&uali+ing resistors.

(f you recogni/e this amp or if you have e0perience with similar homebrew amps

andor use of microwave oven transformer power supplies, let me now.

ere is an%ustralian ham "lubs homebrew kilowatt amp using a pair of 813 tubes and a

pair of mi"rowa#e transformers in the power supply.

ere is a link to :;8,< s linear amp proe"t using a pair of 813 tubes. The amp is

under "onstru"tion.

2ore progress reports6i"tures of the finished power supply I built are below. The

magneti" shunts were remo#ed from the mi"rowa#e o#en transformers as per theinstru"tions in the arti"les noted abo#e. I used #ery low #oltage for testing and to

determine the proper phasing of the transformers. The original 3 #olt magnetron filamentwinding was "onne"ted in series with the primary to redu"e the high #oltage a bit. I re-

used the two high #oltage diodes. 0i"rowa#e o#en diodes are typi"ally rated at around a

half ampere at well o#er twi"e the 6I= needed in this "ir"uit.
http://web.archive.org/web/*/http:/www.vaxxine.com/phil/QEX_HV_Power_Supply.pdfhttp://www.qsl.net/vk4wss/projects.htmhttp://www.qsl.net/wb8erj/813amplifier.htmhttp://web.archive.org/web/*/http:/www.vaxxine.com/phil/QEX_HV_Power_Supply.pdfhttp://www.qsl.net/vk4wss/projects.htmhttp://www.qsl.net/wb8erj/813amplifier.htm


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The fan was already in the ni"e re"y"led ra"k mount "abinet. The soft-start "ir"uit is on

the left.

ather than measuring the entire high #oltage */ for the high #oltage meter$ I used a

portion of the bleeder "hain to feed the meter. That keeps high #oltage stress on the series

meter resistors to a minimum. To measure "urrent draw$ I lifted the grounded end of the

high #oltage se"ondary on ea"h transformer and inserted a milliammeter and its shuntbetween those points and "hassis ground.


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The initial "urrent to a "apa"itor "hain is &uite high sin"e the supply sees an initial near-

short-"ir"uit until the "aps are partly "harged. I used a #aria" for early testing to bring the#oltage up slowly to a#oid stressing the diodes. In looking for a soft start "ir"uit$ I noti"ed

that the tiny mi"rowa#e o#en "ontrol relay from the left-o#er parts had a "urrent rating of

1> amperes at 45! =%/. The mi"rowa#e o#en "ir"uit board "ontaining the relay also had

a small transformer$ diodes$ and "ap that supplied a bit of power to a"ti#ate the relay "oil.I remo#ed the e'"ess "ir"uitry in"luding the dis"rete transistors "ontrolling the relay$

bypassed the relays transistor "ontrol "onne"tion and tested the board. ?sing a #aria"$ I

determined that the relay "losed when about >! #olts was supplied to the board. That

"ould be e'tended upwards by adding a resistor in series with the relay boards %/ input.The little "ir"uit board was perfe"t for soft start of the power supply. I used a 45 watt 45

ohm and a 15 watt 14 ohm resistor in series to limit the turn-on surge. The relay "loses tobypass these power resistors after the transformer primaries "limb abo#e >! #olts or other

sele"ted point. The soft-start worked perfe"tly. 6ower supply output is 455! #olts with

the draw of the bleeder resistors.

he voltages in this power supply and in microwave circuits are lethal. Caps retain

charges.(f you are not e0perienced with building and using high voltages sources, or

do not fully understand the circuits and riss, do not build or e0periment with a

supply of this type.


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A Heathit %451$$ was the previous item on the bench.

6o bac to the *A Pi0 Homepage.
http://www.ohio.edu/people/postr/bapix/DX100.htmhttp://www.ohio.edu/people/postr/bapix/index.htmhttp://www.ohio.edu/people/postr/bapix/DX100.htmhttp://www.ohio.edu/people/postr/bapix/index.htm

813 homebrew amp with power supply from recycled parts

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