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Old 14th March 2010, 10:44 PM   #1
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Default microwave oven transformer..

hello,
I'm new to this site and from the threads that I read, this is a great forum!
so far i did two projects: se kt88 amp & JE lab phono/line preamp.

I started to search for a third project, and I had in mind the 845 output tube witch i want to operate at something around 1KV.

is there any way of making 1KV DC out of MOT (microwave oven transformer)?
I thought of using seven 0D3 voltage regulator tubes in sires to create 1050v dc but I don't really know how to do this..
how do i control the current throe them?

someone have an idea?
thanks!
ziv.
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Old 14th March 2010, 11:12 PM   #2
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Ziv,

Stay away from transmitting types, with their DANGEROUSLY high valued B+ rails, until you get lots more experience. A mistake in handling a 1 KV. B+ rail gets you very dead.
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Old 15th March 2010, 04:16 AM   #3
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Eli Duttman View Post
Ziv,

Stay away from transmitting types, with their DANGEROUSLY high valued B+ rails, until you get lots more experience. A mistake in handling a 1 KV. B+ rail gets you very dead.
I have to agree with Eli, questions like this indicate a serious lack of experience and knowledge that can (and will) get you killed.

I'm working on my first >1kV amp and very cautiously at that, I'm an EE and have been designing and building tube gear for 25yrs now. Slow down, learn a lot, and live to tell about it!

I've serviced and modded a number of SE amps with >1kV supplies and I am extremely uncomfortable with working at these voltage levels, most commonly available meters and probes are not safe, and no standard scope or probe should ever be used at these voltages. HV arcs are all too easily struck, and a careless moment could be your last.

FWIW I can't imagine a much worse power source than a microwave power transformer for a tube amplifier. Reasons: Mechanical noise, lots of heat, and excessively high output voltages being just a few.
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Last edited by kevinkr; 15th March 2010 at 04:21 AM.
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Old 15th March 2010, 04:48 AM   #4
djn is offline djn  United States
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I agree with all that has been said. However, you can build a big A$$ transmitter tube amp with less than 1Kv. here is an amp that has 850v B+. I am still trying to find a link to a GM70 amp that starts out with 450v B+ and then you can increase it from there but it is late and I am not sure where I have the link.

GM- 70 amplifier mono
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Old 15th March 2010, 05:09 AM   #5
kevinkr is offline kevinkr  United States
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Originally Posted by djn View Post
I agree with all that has been said. However, you can build a big A$$ transmitter tube amp with less than 1Kv. here is an amp that has 850v B+. I am still trying to find a link to a GM70 amp that starts out with 450v B+ and then you can increase it from there but it is late and I am not sure where I have the link.

GM- 70 amplifier mono
Even this voltage level is extremely hazardous to the inexperienced - from a safety perspective there is not much difference between 850V and 1kV, and the comments I made about test equipment, safety, and arcing issues at these voltage levels still applies. I would not recommend anything over 450V to most constructors I don't personally know are capable of working safely with higher voltages.
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Old 15th March 2010, 05:29 AM   #6
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Quote:
Originally Posted by zivhackman View Post
hello,
I'm new to this site ... I thought of using seven 0D3 voltage regulator tubes in sires to create 1050v dc but I don't really know how to do this..
how do i control the current throe them?

someone have an idea?
thanks!
ziv.
Someone does - it ain't you!

PLEASE do not do this. If you want high output power, look at a (comparatively) low voltage screen deflection tube project.

Once you are over 300V, its not a tickle anymore. In excess of 500V and its a short bet that you won't breath later. And at 700V plus - it doesn't wait for you to find it - it will come looking for you over surprising (once only...) distances. You risk your life and the life of anyone around the equipment you make.
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Old 15th March 2010, 06:50 AM   #7
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thank you all
i think you probably right!!
i should keep my eyes on a more simple project.

the problem is that i don't have extra efficiency speakers and i Really want to try a DHT apm..

but i think all i gut left is to try the PP world.

thanks, you really got me into proportions!
(I'm sorry for the bad English..)
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Old 15th March 2010, 08:31 AM   #8
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My first build was a 900V SEPP amp. It scared the hell out of me, and as is the case with big projects it needed far to much tinkering under the hood. I decommissioned it at the earliest oportunity.
Coupled to this safety issue will be the very expensive output Iron and you have the makings of a stupid, expensive and deadly project.

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Old 15th March 2010, 01:23 PM   #9
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There are several reasons why the thread starter shouldn't try to build a 1KV+ amplifier, most of them noted. One more, the microwave oven kills more appliance repair techs than all other kitchen appliances combined. Now, lets cover the tech:

The original microwave oven from the late 1960's and early 70's (Amana Radar range, and Litton) used a BIG conventional power transformer and power supply to generate about 2800 volts for the magnetron tube. These power supplies provided a relatively constant voltage under varying loads.It's easy to identify these ovens, you can't pick them up! My mom's Amana weighed 70 pounds and cost $650 in 1970. These transformers are useful for powering a tube device that requires this much voltage, typically a ham radio transmitter.

The power supply in a modern microwave oven is an entirely different animal and pretty useless for tube amplifiers without modification. The early ovens were heavy and expensive due to the large transformer. They also would blow up if ran with no load (empty oven). The solution was the ferro resonant power supply. The power supply in a modern oven operates as a voltage doubler. The magnetron tube itself is the load and second diode in the doubler. The capacitor and the inductance in the power transformer secondary form a resonant circuit at 60 Hz. There are magnetic shunts in the transformer that will cause transformer saturation if the load current goes too high, which then changes the inductance and unbalances the resonant circuit reducing the load voltage. So this type of power supply operates in a quasi constant current mode, and the voltage it generates is half wave pulsating DC varying from 1800 to 3000 volts depending on load. This is what allows you to operate the oven empty without blowing it up.

I have experimented on several oven transformers (MOT's) and found them mostly useless. The magnetic shunts can be removed on some transformers, but one end of the secondary is usually riveted directly to the core, forcing the use of half wave rectification. On some transformers this can be undone, but the insulation may not be capable of supporting a floating secondary. I decided that these didn't make good power transformers for any "normal" power supply. I tried one as a SE OPT too. SUCKS big time. The bandwidth was measured in Hz and losses were huge. They might be useful as a choke, but I decided that it wasn't worth the effort so I gave mine to a metal scrapper.

That said, I have seen an amplifier on the internet and on this forum that used an 833A and claimed to use a MOT for both power and OPT duty. YMMV
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Old 15th March 2010, 01:57 PM   #10
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"" Once you are over 300V, its not a tickle anymore. In excess of 500V and its a short bet that you won't breath later. And at 700V plus - it doesn't wait for you to find it - it will come looking for you over surprising (once only...) distances. You risk your life and the life of anyone around the equipment you make. ""

There is more truth to that statement than meets the eye. While I am not an experienced tube builder (All my builds <500V), I do deal with these voltages every day in my work. Around 680V the world of electricity changes drastically for the worse. On any given day air can ionize and allow a spark to occur (humidity plays a big role). It can and does literally jump some amazing distances, almost like it is looking for you. All it takes is something to pass within the spark gap distance and a conductive arc is established. It doesn't stop arcing and carrying current until the object is pulled many times the spark gap distance. Start getting into the kV range and now one must start to take precautions with corona.

Last Wednesday we had a flashover in our substation. Now it is a few more volts than 680V (35kV to be exact) but we had an absolutley beautiful 35 ft long ribbon arc initiated before the mains were dropped. 35 ft mind you. Looked like the feature event in Back to the Future or a Frankenstein movie.
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