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Tektronix 120-086 transformer

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Tesla88

Tesla88

Member
Joined 2011
#1
Hello guys,
i'm not sure if tube section is the right place to start the thread about a Tektronix transformer i just bought for 40,00 €

Pic or it didn't happen ,right ? 🙂

The Tek part num. is 120-086 , the guy who sold me the thing told me it was salvaged from a 541 o-scope and service manual show that is true.

So i have all the voltages from the servie manual but no current ratings, except heater windings.

I could use the voltage drop on the series 10 ohm resistors stated on the service manual, but i think this monster is capable of more than this. Dimension and weight tell me this Iron is around 500VA and more...

I found on PMillet some datas but nothing for the 120-086 or 541 scope.
Also found this thread :
http://www.diyaudio.com/forums/tubes-valves/79021-power-transformer-tek545-oscope-2.html

Anyone can give me advices ?

Also would you rather use it for a tube amp or to make a bench supply ? 🙂

Thanks and sorry if my English isn't that good !

An externally hosted image should be here but it was not working when we last tested it.
 
#3
Ratings of most Tek transformers should be similar. But you can measure the winding resistance and estimate the current rating from the copper loss (I^2*R). For instance, a 10 Ohm winding will dissipate 1.6W at 0.4A There may be 2 or 3 Ohm HV windings, good for more like 0.7A. Easily 500VA if all windings are loaded evenly.

The big heater windings are about 6A each (for 1W copper loss), and there are several, and a smaller one for the CRT with high voltage insulation.
 
#4
I went through the laborious task of rating a 120-117 from the schematic so that I could use it in a monster guitar amp. The schematics I had didn't utilise current sense resistors, or have heater current levels :-(

I suggest that TEK would significantly overate the PT spec above the heaviest application load - but that needs a few 'measurements' to firm up. Comparisons with other trafos are obviously needed - the easiest being core volume and winding resistances. I haven't opened mine up, so can't say if winding wire gauge can be measured for certain windings. Temp rise could be tried at your designed load levels, but preferably that needs a sensor on the core and a few hours of spare time and a guesstimate of what rise is allowable. Perhaps if you were to load up the PT for temp rise, then you may be able to gauge regulation levels based on AC loading of secondaries and then make some guesstimates.
 
#5
I'd like to chime in,

the 120-086 is found on a number of tektronix instruments for instance the 543 but also the 541 like you noted.

The A and B versions of the same model number carry different transformers, this is due to the fact that these O scopes tend to use transistors in the horizontal amplifier stages. whereas the older models used a array of 6dk6 or ECC88 for the distributed amplifier

I've checked my scopes for the tek numbers on the transformers.

543/541 120-086
543A 120-0100
543B 120-344-00
545A/ 545B 120-344
585A 120-0141

The Hv windings on the 120-086 core size indicates approximately 600-700VA

8-15 141V .5-8A
7-14 115V .5-8A
5-10 117V .5-8A

20-21 186V .3A
6-11 204V .2A


Heaters (from the manual)

9-16 6.3V 3.3A
22-23 6.3V 4A

27-28 6.3V 7.4A
12-13 6.3V 7.6A
18-19 6.3V 7.1A


Anyway I would keep the current consumption below 500mA to be safe.

I would suggest building the -150V reference part for any amp. this will give you a very steady bias supply. which can also be used to include some 6au6 pentodes as current sources for LTP phase splitters. The Valve Wizard



And on a last note, Please don't scrap these scopes! If you want one of these PT's get two, fix the nicest up and use the iron from the other for a project.
 
#6
Today i managed to take some measurements.

The primary windings which are approximately 115 volts each both have approx 1 ohms winding resistance. And they will carry approximately 2.5A AC under maximum load.

8-15 141V 2.9 ohms approx 800-900mA AC
7-14 115V 3.2 ohms approx 650-700mA AC
5-10 117v 5.0ohms approx 500-550 mA AC

Hence using this transformer to produce about 450mA DC @ 500V under rated load is possible. you will however be stressing winding 5-10 much harder than the rest. however the extra core heat won't be much of a problem considering other windings won't likely be used at all.

Knowing that the maximum current through a winding is proportional to its resistance you could solve the maximum permissable current through the windings. that's what i did in the above. please correct me if im wrong.
 
#7
Correct me if I'm wrong, but these trafos have no center taps and while you can series the secondaries together, you're creating a Full Wave Bridge (FWB) to establish a ground. FWBs, while very efficient, have the downside that current supply is halved and some would suggest that the current rating should be 250% of what you expect to draw.

Someone please confirm or correct me on this.
 
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