Need Info on 6BK4 Tube

[vintagecollect]

I recently worked on set with a 6BK4 HV regulator/shunt tube. How I know when these are bad???????? My tube tester doesn't have test data for this tube. Tube chart says just 6BK4, I found out later a 6BK4-A-B is available and a 6BK4-C version. Which tube do I use???, what are symtons of bad tube since can't test???

[Chad Hauris]

Measuring the cathode current can help you determine if its weak...measure voltage drop across 1000 ohm resistor in the cathode circuit (400 volts both sides of resistor.) Measure with the set's brightness turned all the way down.

Nominal current is 1 milliamp, (1000 microamps,) equaling 1 volt drop across resistor.

If the 6BK4 is weak you may only get around 300-500 microamps (.3-.5 volts)

Symptoms of a bad tube are usually a picture that changes size when the brightness is changed.

I don't think there is a way to really test it in a tester as you really need high voltage to get it to conduct.

It is recommended that the latest version of the tube be used as it has the best x-ray protection...I think the latest one is 6BK4C/6EL4A.

[frenchy]

I would just recommend getting extras of whatever tubes that remain that could be causing any remaining problems in your set, thru ebay or the radio newsgroup, especially since testers are not foolproof. So you can swap them in, this was the first path taken by tv repair guys - swap tubes first. This is a compromise between getting a complete set of every tube in the set, and just assuming they are all good.

[andy]

...

[Chad Hauris]

Always use the latest suffix letter of a tube type when getting new tubes. It is the most advanced and has the best ratings, such as increased current and voltage ratings.
The 6L6 GC is probably the most dramatic of these evolutions...the metal 6L6 or the curved sided 6L6G is a 19 watt plate dissipation tube while the 6L6 GC is around 30 or so watts. Some of the later model equipment such as a Bogen PA amp relies on the higher ratings of the GC and will overload an old 1930's 6L6G.

Some of the HV tubes such as 6BK4C and 3A3C I believe were designed in the 1970's to control x-ray emission to a certain standard level wheras the older versions were not certified as to x-ray emission.

[holmesuser01]

They were made with leaded glass, for one. I saw alot of RCA tubes.

[vintagecollect]

You Guys are great for info, Thanks!!!!!!, The picture size did change with the contast turned ------that also effected brightness. Recently Horiz Output tube went out and going to swap both tubes as believe 6BK4 caused the latter failure. Once again thanks.

[holmesuser01]

Might want to check your horizontal oscillator tube, and the 100 ohm resistor that runs from the oscillator to the output tube grid (I believe this is it) On alot of RCA's, the resistor will change values and cause a slow-burn of the expensive HOT.

Correct me if I am wrong.

[maxm]

Didn't the 6BK4 add leaded glass with the start of the "B" or "C" versions, I think the first ones used regular glass as they are a lot lighter. I read somewhere that you could get these tubes to glow green if you cranked the voltage up enough...

[holmesuser01]

My 6BK4C glows green sometimes. CTC-10C

The original 6BK4 was regular glass. I have an original dud from 1957. Around 1972 or so they started making these and 3A3C with leaded glass. X-ray problems, they said.

Early 3A3's look just like a 1B3, and I used to have people that checked their own tubes constantly burn out their HV rectifier because someone in the past sold them a 1B3 when they needed a 3A3. 3A3A was a bit larger, I believe.