Unknown Xfmr fail test

[jeff]

 I boosted a Xformer from a "Cathode rejuvenator tester". Looks like test equipment to be used on TV picture tubes. Voltage ~ 350-0-350 unloaded. Any guesses if I could use it to build a little SE 6V6 amp?
(approx current draw for testing picture tubes???)

Size it about right and I don't mind blowing it if it can't handle the job so I'd like to try it, if it works it works, but is there a safe way to stress test it?
In other words should I wire up an amp and keep an eye on it to see if it smokes or is there a method to slowly, little by little, stress it to see what it's capable of?

[eleventeen]

Sure. You say your two halves are 350 or so, thus we expect 350 * 1.4 for a no-load output with a SS bridge. That's 490 volts. Let's say that will fall to 450 volts with a nominal load. A SE 6V6 amp might want 30 mils, with a preamp tube or two. 450 volts / 30 mils = 450 / .03 = 10K ohms, a 15K resistor will simulate your 30 mil load. .03 * .03 * 15,000 = 13.5 watts. 30 mills is probably at the top end of things, so if the thing can survive your torture test, it should work. You may want to get your 15K dummy load resistor in the form of several 5K/5 watt resistors in series (or other such arrangement)...that way they could be maybe a little more useful for something else. Instead of buying those specific values at an electronic store, sometimes you find an ebay seller selling 10x or 20x NOS power resistors for cheap.

I prefer this approach to a big fat rheostat...a 25 watt rheostat isn't 25 watts when you have it dialed down to a low percentage of its full resistance and are only using 10-20% of its total rotation. 

[HotBluePlates]

I boosted a Xformer from a "Cathode rejuvenator tester". ...

Agree regarding the dummy load for testing.

I don't know about your particular piece of gear (you didn't say exactly what this came from), but you should know something about cathode rejuvenators in general...

The idea is that over time the cathode has reduced emission due to a number of causes. The rejuvenator operates the cathode at higher-than-normal temperature (usually by running the filament at a higher-than-spec voltage) for a brief period of time. When the cathode is returned to normal operating temperature (by returning to spec heater voltage), hopefully cathode emission has increased.

Most tubes that seem near the end of their life due to reduced emissions or Gm tests may get some temporary benefit from this process. But you never know if it will work, will work well enough, or just be a waste of your time.

All that to say that given how these things work, you haven't given us enough info to know anything about the current capacity of the transforme you mentioned.

[eleventeen]

Excellent point by HBP. These were popular though generally cheapo devices (to use the term loosely) for 50's-60's TV repairmen to see if they could get a few more months out of a (CRT) TV tube before having to make that much-dreaded replacement. They worked by burning crud off the emitting coating of the cathode via arcing, which either got worn or poisoned in some way. I am sure many were sold, I can't speak to whether or not they were more than trivially effective. It would surprise me some if one had enough ooomph for a little tube amp, but the physical size of the tranny would be one clue and you're the one with the thing in front of you. These were the kinds of things that budding TV repairmen bought out of the back of Radio-Electronics or Popular Electronics in the 50's.

More saliently, the *filaments* of CRTs of that era were in many cases driven by a tiny winding coming off the flyback. Not necessarily a conventional 6.3 volt heater like all the other tubes. Ergo, before I became committed to building a little amp just because this freebie transformer dropped out of the sky, I would check the heater voltage (if any?!) to see if you actually have something usable.

[jeff]

The unit is marked Cathode Rejuvenator Tester Model 350 B & K MFG. CO Chicago, Ill.
 The Xfmr measures 2 3/4" x 2 3/4" x 2 1/2"
There's 120 input taps(black)
350-0-350(red,yellow/red,red)
4 wires all internally connected giving different voltages depending on which two you measure across
Green, green harring, yellow, bone yellow herringbone(Although thew yellow may actually be orange, hard to tell)
 but there is 6.3 across two of them. Which I didn't get at first, but seems to jive with what HBP said
"The idea is that over time the cathode has reduced emission due to a number of causes. The rejuvenator operates the cathode at higher-than-normal temperature (usually by running the filament at a higher-than-spec voltage) for a brief period of time. When the cathode is returned to normal operating temperature (by returning to spec heater voltage), hopefully cathode emission has increased."

There was a needle gauge marked ua 0-1000 with 300-1000 being "GOOD"
  5 way Selector switch:
  CONT-SHORT
  EMISSION
  DYN-LO
  DYN-MED
  DYN-HI
a pot labled CUT OFF marked
 OFF(CCW)
 GOOD(between 10-2 O'clock and)
 LIFE TEST (full CW)
And 3 momentary switches marked
  LIFE TEST
  DYNAMIC INTENSIFIER
  EMMSSION
And 3 bulbs with a chart to diagnose marked
  H
  G-1
  G-2
Other than giving you the diagnostic chart that's all the info I have

[jeff]

All that to say that given how these things work, you haven't given us enough info to know anything about the current capacity of the transforme you mentioned.

I guess that's what I'm trying to test for since I don't know.

 I get what you are saying about the dummy load. But what should I be looking for?

 I'll calculate a dummy load based on my expected current draw.
 Then start with a load greater than that and gradually decreace the dummy load measuring the voltage as I go???
 If so what am I looking for to rest assured that this Xfmr is usable???
 Voltage drop???
 How much???
 Excess heat??

Do I rectify to DC first or hook the dummy load to the raw AC???

[sluckey]

Sometimes you find stuff that has no value in the guitar amp world.

[jeff]

Sometimes you find stuff that has no value in the guitar amp world.

That's exactally what I'm trying to find, if this is one of those times or not.
                          

[John]

There was a needle gauge marked ua 0-1000 with 300-1000 being "GOOD"

I am probably way off base, but that seems to say that you won't get much current/amperage.

But, easiest way to check is to alligator-clip a couple power tubes to the 6.3v windings and see what happens. If it smokes then you know. 

[HotBluePlates]

The unit is marked Cathode Rejuvenator Tester Model 350 B & K MFG. CO Chicago, Ill.  ...

That's the bit needed to see if there's a schematic out there. Maybe there is, but I couldn't find it. I did find that any high voltage was probably for G2 on the picture tube, but I have no sense of how much current those would draw in use in this tester.

The Xfmr measures 2 3/4" x 2 3/4" x 2 1/2"  ...  350-0-350(red,yellow/red,red)

I get what you are saying about the dummy load. But what should I be looking for?

 I'll calculate a dummy load based on my expected current draw.
 Then start with a load greater than that and gradually decreace the dummy load measuring the voltage as I go???
 If so what am I looking for to rest assured that this Xfmr is usable???
 Voltage drop???
 How much???
 Excess heat??

Do I rectify to DC first or hook the dummy load to the raw AC???

You rectify to d.c., just like you are building the power supply for your SE 6V6 amp (because that's what you're doing).

350v * 1.414 = 494vdc, so you're gonna need a tube rectifier to bring the d.c. voltage down. Got a 5v winding handy with 2-3A available? If no, you might have a negative indicator for this whole project.

As Eleventeen said, you're gonna want 450v or less (maybe well less). 12w/450v = ~27mA. Again as Eleventeen said, you need some extra current for screen and preamp, so call the total 30-35mA.  450v/35mA = ~13kΩ.

So you have a tube rectifier, a filter cap (maybe 20uF @ 500vdc), and place a ~50kΩ and then progressively down to a 13kΩ resistor from the filter cap + to -. If the d.c. volts across the cap stays at ~450v in the face of the lowering load resistance, then the PT can deliver the current demanded.

450v²/13kΩ = 15.5w  -> You'll probably need a 50-100w resistor to act as your dummy load. You would double the dissipation to get a safe rated value, but you'll probably buy an aluminum clad resistor, which assumes you'll have the resistor bolted to a substantial heatsink to meet its rating. If you just have it hanging loose, it will burn up at a much lower (than rated) dissipation value.

As time goes by, I ask myself more & more if I save anything by going through the trouble to assess whether "cheap iron" can be used. I prefer to cut to the chase and buy the part that I know will work.

[eleventeen]

"As time goes by, I ask myself more & more if I save anything by going through the trouble to assess whether "cheap iron" can be used. I prefer to cut to the chase and buy the part that I know will work."

Definitely something to be said about that sentiment, though they are pricey. It's sort of different if you grab an organ transformer set that you can see powered 36 tubes and 2 x 6L6's and a 5U4 and you're looking to build say a Deluxe or a Super...in that case, you only add 8 lbs of iron to your finished weight but can be confident that the tranny has the juice to run your build.

My bugaboo is with chassis. I am always trying to recycle old chassis from this or that and cheaping out on buying a new one. I don't mind that much doing the drilling and filing to make tube-socket holes (don't have a punch set) in steel....but there is a massive time consumption factor and the thing will rarely look that great should it turn out (sonically) nice.

And pots. Expensive.

[jeff]

"350v * 1.414 = 494vdc, so you're gonna need a tube rectifier to bring the d.c. voltage down. Got a 5v winding handy with 2-3A available? If no, you might have a negative indicator for this whole project."

I was hoping to get away with using a 6X5 or 6X4. Actually I think there originally was a 6X4 in it.

First things first, I'm gonna string up a 6X? 6V6 12AX7 and see if the heater can suppy enough.
If so I'll procede with your suggestions, if not, no need... (Although I do have a spare 6.3@450mA xfmr)

Thanks

[HotBluePlates]

I was hoping to get away with using a 6X5 or 6X4. Actually I think there originally was a 6X4 in it.

First things first, I'm gonna string up a 6X? 6V6 12AX7 and see if the heater can suppy enough.

Okay, but you know the 6X4 should be on its own 6.3v winding, right?

I looked up the 6X4 data sheet cause I wasn't sure, and this tube does have separate heater and cathode. There is a rated maximum of 450v from heater to cathode when the cathode is positive (how you'll be running it). You will always exceed that on startup.

So it may not fail right away, or even for a long time. But sooner or later, that 6.3v winding will arc through the 6X4 heater-cathode to B+. No idea if you'll kill the preamp tubes' heaters along the way. The 6X4 seems better suited for a lower B+ or for an independent heater winding just for the rectifier.

Same rating limitation for the 6X5.

[jeff]

Okay, but you know the 6X4 should be on its own 6.3v winding, right?

Nope I did not know that Thank you very much.

Well maybe it wasn"t meant to be, I'll keep thinkin

[6G6]

Maybe SS recto, single ended 6L6 and use the seperate filament trans you already have?

[HotBluePlates]

Maybe SS recto, single ended 6L6 and use the seperate filament trans you already have?

But he's gotta test the transformer to know if it can supply 5mA or 500mA...