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Cool old flyback indentification

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Matt Edwards

Sun Jul 04 2010, 02:39PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

I was fortunate enough to find a vary large old flyback. Does anyone have any info on something this big? Input/Output voltage and current capabilities?

Proud Mary Im calling you out!

Proud Mary

Sun Jul 04 2010, 03:43PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Dear me!

By the look of the technology, the Paxolin board with solder tags, and the pre-set, I'm going to guess that it's from about 1960-65.

The two turn winding on what looks to be the otherwise empty leg of the core is the filament winding for a directly-heated EHT rectifier valve.

The pre-set is very likely the focus control.

Could you possibly take another photograph showing the inside of the plastic connector at the end of the short lead coming from the EHT winding?
I would expect this to have a top-cap clip for a single valve inside, (the EHT rectifier) but the connector insulation looks rather blocky for a valve top-cap connector.

From the size of the EHT overwind, I would guess this LOPT came from either a large black and white, or perhaps a colour television. I'll stick my neck out and suggest it may have produced about 25kV at 1 or 2mA, but it is only a guess.

The slug tuned inductor at the tip of your thumb was probably part of the efficiency diode (aka 'booster diode') circuit.

With this type of open-plan LOPT, it is often self evident which wire is the EHT earth return, but if it is not, connect one probe of an ohm meter to the connector sticking up in the air from the top of the winding in your picture, and then touch the other meter probe on each of the other terminal lugs until you get a reading, which I would expect to be upwards of 300R. The EHT winding may be connected internally in series with other windings, so you may find more than one terminal with a similar reading. This is not important unless you are going to use it to repair an old telly, and any of those so connected may be used as your ground return.

Matt Edwards

Sun Jul 04 2010, 03:53PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

It was from an old tv/radio/record player combo.. The two boots were connected to two tubes labeled (6BK4A 3125-52 & 3A3/3B2) i will post a better pic in a bit. I do not have any special plans for it. I was thinking of maybe a cockcroft walton multiplier. Powered by my zvs driver.

Thank you in for the help proud mary! I knew you would be full of good info!

I was wondering what the power ratings on something so large might be.

Proud Mary

Sun Jul 04 2010, 04:06PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

6BK4A was a colour TV shunt stabiliser triode, a primitive voltage regulator that kept the picture tube EHT voltage at 27kV simply by drawing more current and dissipating the unwanted excess voltage as heat.

This means that your AC output from the EHT winding would have to be well over 27kV in order to have enough spare to be shunted away. 6BK4A
could take as much as 60kV on the anode according to the datasheet here:

6BK4A is a valve sought after by amateur X-ray enthusiasts because of the copious amount of radiation it emits. The radiation problem persisted with the next iteration, 6BK4B, but was resolved with 6BK4C, which introduced Pb and other heavy metals into the glass following a public scare about the X-ray dangers of televisions.

I'd say your LOPT was just the job for a C&W, and I look forward to reading how you get on with it!

Matt Edwards

Sun Jul 04 2010, 04:09PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

Thanks! That makes since. That tube looks like burnt toast. Have you ever seen one running? It looks like there is a large amount of arcing onto the inside of the tube.

Proud Mary

Sun Jul 04 2010, 04:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Oidium45 wrote ...

Thanks! That makes since. That tube looks like burnt toast. Have you ever seen one running? It looks like there is a large amount of arcing onto the inside of the tube.

There shouldn't be any arcing inside the shunt-stabliser triode. If the telly has been out of service for many years, the valve may have gone 'soft' - gases locked into the metals have seeped out. Alternatively, if the 6BK4A heater has failed, then you will have the full whack of unregulated EHT voltage on the anode, perhaps 40kV or even more, which would make internal arcing likely. If the heater has failed, then you will have more penetrating X-radiation emanating from the tube, due to cold cathode field emission.

Matt Edwards

Sun Jul 04 2010, 05:50PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

Proud Mary, Here are a few better photographs. Sorry, they were taken with my phone. I have also included a picture of the tube that we were discussing.

Proud Mary

Sun Jul 04 2010, 07:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Your 6BK4A has had a long service life!

The brown discolouration of the glass is caused by both X-ray damage, (common to all glass X-ray tubes - and X-ray producing tubes like yours - as they age,) and the sputter deposition of anode material.

In one X-ray tube failure mode, the metal sputtered onto the inside of the glass envelope becomes dense enough to conduct, promoting erratic discharges and Heaven knows what...

The two big resistors on the paxolin board are likely to be high voltage, high megohm types that may be worth salvaging, though their assigned values may have changed due to heat-accelerated ageing, what with the line output valve, rectifier valve, and booster diode all in close proximity and all producing liberal amounts of heat, in addition to the heat generated inside the resistors themselves.

As for the normal output current of the EHT winding: the 6BK4A could dissipate 1.5mA max doing its job as a shunt stabiliser pegging the EHT to 27kV DC, so EHT current for 24/7 operation must have been about 2mA max, perhaps a bit less.

Insulation does not improve with ageing, so if you mean to spend good money on building a C&W, I would advise you to de-rate it considerably, and operate it in a pot of yellow olive oil.

I think it would be a shame to wreck it by trying to draw long arcs from it, when it could have gone on to give faithful service in years to come by giving it a bit of TLC.

radiotech

Sun Jul 04 2010, 07:42PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The 6BK4 tubes were the bad boys that kicked off the X-ray scandal in color TV's

They got black on the inside as they aged. The high voltage ripped up the anode. To cleanse then, some tubes had 3 getters, to give them a decent lifespan (about 8 months in a well used family TV)

Here is a new 6BK4C/6EL4A made by Sylvania International in the USA along side a well used 6BK4A with no getters but coated inside
with a scourging layer (same as a getter deposits on tube wall) but
evenly sprayed before evacuation.

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