What to do with a 1350 Volt PT

the thermionic watercooler

What to do with a 1350 Volt PT

Postby bsmif » Wed Jun 06, 2007 4:33 pm

I grabbed a old Tek 585 'scope from a local junk shop for peanuts. Am fascinated by the HUGE ( 6 x 6 x 8 inch) Power transformer, that I hope to harvest from it for an amp project. It powered more than 50 small tubes and a CRT and has multiple taps of 120 130, 150 205 and 210 volts (at least 4 of each), plus 12-15 (!) 6.3 volt taps The Tek guys used the taps to create a 550V, 350V, 225V, 100 and -150v power . Here are some pix

http://www.flickr.com/photos/bsmif/sets ... 320750558/

Seems like something this massive deserves some big tubes. A PPP array of 6550's? Or be counter-intuitive and create the most oversupplied ST35 EVER.

What would YOU do?
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Postby mesherm » Wed Jun 06, 2007 4:47 pm

That tranny would power a quad of 811As just fine.
I used that exact same model scope in electronics lab class back in that late 60's. Did you harvest the tubes from that scope? IIRC there were a lot of low noise 12AX7s and the like inside.
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Tubes galore

Postby bsmif » Wed Jun 06, 2007 5:01 pm

There is definitely one Mullard ( GE but MAde in GT. BRITAIN") 12ax7 a pair of 12au7's, 6au6's and 20 odd 6922's. Not sure what shape they are in. Havent taken them out yet. SOme have lost their paint. I was thinking it would be kind of fun to harvest as many as possible from the scope to make the driver board. The Transformer is indeed massive. If I dont use all the taps will it still suck lots of amps?
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Postby mesherm » Wed Jun 06, 2007 5:28 pm

The power used by the tranny is the sum of the power demanded at the secondaries plus the power lost as heat which pretty much comes from the current flow through the windings and magnetic hysteresis of the core. The power tranny reflects the load the same as an OT does which means as you draw power from the secondaries it lowers the impedance of the primary allowing more current flow. With nothing connected it will use a minimum amount of power. That power used will increase in proportion to what you draw from each tap. You have the transformer equivalent of a big bore V8 engine. Barring the gas lost in just keeping the engine running, your milage will vary with your power demanded.
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Postby erichayes » Wed Jun 06, 2007 8:25 pm

Hi All,

To follow up on what Mike's said, if you power up the transformer with nothing connected to the secondaries and it gets warm after a half hour or so, there's a good possibility that one or more of the windings has a shorted turn. I mention this only because Tektronix PTs of this vintage have been known to fail . . . ask the man who owns one.
Eric in the Jefferson State
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Postby bsmif » Wed Jun 06, 2007 10:46 pm

Havent tested all the leads but it seems to power up pretty well. All the tubes that are in place light up even the CRT though thats not working. Will get the multitester out soon.
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Postby EWBrown » Thu Jun 07, 2007 6:23 am

That trannie could power up a pretty massive amp. The voltages don't "add up" to 1350VAC unless you were to series connect all the windings :o Yellow_Light_Colorz_PDT_11

Those ceramic terminal strips are beyond excellent for amp usage, they are silver-soldered and thusly require a fair amount of heat (and patience) to de-solder and salvage the components and clean up the strips (leave the remaining solder in the "pockets" and definitely don't remove it with a solder sucker or wick).

Note that the component leads and wires are not wrapped, but simply laid straight into the solder points. This makes component salvaging a LOT easier than having to deal with wrapped and twisted leads.

A few years ago I was given the power supply from one of those huge Tek 575 (?) "two piece" scopes, and there are two big trannies inside, one is mostly plate voltages, they used relatively low secondary voltages (200 to 300VAC, with relatively high current capability) and some of those were voltage-doubled as needed. I was also offered the scope for free, but someone elas had previously stripped it of all the tubes and some other good parts, so I left it behind.

The other trannie has a lot of high-current 6.3V and 12V filament windings and a couple of 100-120 VAC secondaries. I have removed the trannies, but haven't yet tried salvaging the terminal strips and usable components, that's a long term "winter" project... This PSU also had a lot of good tubes, mostly honkin' BIG 6080s, several assorted dual triodes, some 6AU6s and 5651 voltage reference tubes. If the electrolytics are still good, that would be an added "bonus"...

Have fun!

/ed B in NH
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Postby wiredbecker » Thu Jun 07, 2007 9:55 am

What's the best way to test 'found' electrolytics? I've got scads of 'em.
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Postby EWBrown » Thu Jun 07, 2007 11:33 am

If you have access to a variable voltage power supply, (I use a Heathkit IP-17, goes up to 400VDC, 100 mA). I use a 10K 10 wat resistor in series with the plus side, connect the electrolytic and set the PS to about 80% of the voltage rating on the cap, and observe the current. This will slowly re-form the cap, and it can be checked / tested by measuring across teh cap, and measuring across the 10K resistor will give an indication of any leakage current that may be present.

If you don't have the variable power supply, a simple set up can be made with a 300VAC output transformer, a rectifier diode (1N4007 is good enough) and a cheap variac in order to control the voltage and bring it up slowly.

If you don't haev a variac, then wire in a ceramic light socket in series with the primary, the bulb will act as a current limiter, start with a low wattage bulb, 25W or less, then sstep up through 40W, 60W, 75W, etc as required. FOr lower current limiting, use a "candelabra" type socket and the smaller 7W "christmas tree light" bulbs can be used to start out with. If the bulb satays bright, and doesn't dim down then the cap is bad (shorted or very leaky). THis is pretty much "Fred Flinstone" technology, but it works... Just search for "chicken stick" on this site, I'm sure I've written this up in more detail Yellow_Light_Colorz_PDT_02

Here, I'll save you the trouble....

Years ago, I made up a simple but vital piece of test equipment, forever to be known as the "chicken stick". Basically a 6 foot long 2X4 piece of wood, with an outlet box on one end, and a regular wall switch box on the other, and a ceramic light bulb socket next to, and wired in series with the outlet box, and, of course, a line cord. The device under test was plugged on, a lightbulb (or a properly rated screw base fuse) was inserted into the lamp socket, and then, the device io be tested (or sometimes detonated) was turned on with the switch on the far (safe) end of the stick. This ensured an extra margin of safety, by having to be at least six feet away when the switch was flipped.

(Yes, I had one too many catastrophic capacitor explosions in my past).

Actually I remember seeing a similar arrangement on a test and repair bench when I was doing techie work in the army way back long ago (1969-1972). Sometimes the term "fire it up" was just a bit too literal...
Later on (1993-1996) when I was doing product safety testing, which involved intentionally creating malfunctions in power supplies, and making sure that they failed in a safe manner (no flames, gushing hot fluids or shock hazard) , this same technique was used, as sometimes the test resulted in a rather spectacular explosion, showers of sparks and hot electrolytes, and shooting flames.


/ed B in NH
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Silver Solder

Postby bsmif » Thu Jun 07, 2007 1:57 pm

Indeed it has a warning printed inside that ONLY silver solder should be used on it. ITs amazing the number of trimpots embedded in every control. These must have been areal bear to keep running. Those higher voltages are indeed reached by adding the lower voltages in series. As you can tell from the photos the tranny has lots of options. Also nicely you can hook it up for either 117, 124, 220 or 240 primary voltages depending on how you hook up the primary taps. Nicely thought out.

Any favored PPP schematics out there? There are some Dynaco Mark 6 OPTS on ebay. I bet this thing could power 8 6550's

the 811a's are triodes right?
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Postby wiredbecker » Fri Jun 08, 2007 12:50 am

thanks ed that was a cool read
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Re: Silver Solder

Postby mesherm » Fri Jun 08, 2007 1:30 am

bsmif wrote:the 811a's are triodes right?


The 811a's are big directly heated triodes with Thoriated filaments.
They are a bit tricky to use however in that they are 0 bias tubes and run in AB2 (positive grid current flows). They either need to be interstage transformer driven or driven with a small power tube. Something that can supply 50 ma or so when the grid current goes positive.
A PPP of 6550s would work but the cost involved in the 120+ watt output transformers makes it expensive to build. Fun to imagine building something with that big power tranny but rather impractical.

http://www.dvq.com/hifi/images/1570a.pdf
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Postby sorenj07 » Thu Jun 14, 2007 6:12 pm

why not 813's? 1350V would be fine for them and they're a lot easier to drive, in SE a single 6SN7 should do the trick..
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813's

Postby bsmif » Tue Jun 19, 2007 7:03 pm

Pete Millett's 813 project looks pretty complicated. Well the power supplyu does. IS that neccesary?

http://www.pmillett.com/813_se_triode_amps.htm
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Postby sorenj07 » Tue Jun 19, 2007 8:17 pm

it DEFINITELY does not have to be that complicated. He's got everything wired to display units and probably has automatic adjustment. He has the right idea since such high voltage is dangerous, but there's danger in making something that complicated as well. A simple choke input power supply and a couple of motor run caps in series should be fine. 370VAC caps by Aerovox, according to their staff, is good for over 1KV. Your PSU would be around 1.2KV. Not much capacitance is needed at such high voltages, maybe 20uF should be OK.

Some info:
http://home.zonnet.nl/horneman/813.htm

In particular:
http://homepages.ihug.co.nz/~rwillis/alex/

Compactness will probably be an issue, normally you'd want a separate supply for the driver tube, but since your PT has so many taps, you could probably just use a 300V tap into a little 6CA4 or something for your secondary B+ supply.
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