On a piece of test equipment that I have, it started to sizzle when I powered it up. I turned off the equipment immediately and upon investigating found the power transformer to be extremely hot. Checked the power transformer and thankfully, all windings are still intact. On checking the high-voltage winding (center-tapped), I found one half of the winding to have a very low resistance of about 7-8 ohms whereas the other half was about 160 ohms. However, upon removing the rectifier tube (a 6X4), the resistance values are approximately equal which led me to believe that there was a short in the rectifier tube. Checking the resistances between the tube pins, I found that there is a very low resistance (a few ohms) between one of the two plates of the rectifier tube and the heater pins (specifically between pin 6 and the heater pins 3 and 4) confirming a short between the plate and the heater. The other plate shows infinite resistance with the heater pins.
I have heard of a heater-cathode short, but not a heater-plate short. How common is this with the 6X4?

Most unusual but not not impossible.

A heater to cathode short in a 6X4 results in massive plate current deriving from the
oxide emission on the cathode which melts the plate inside out in a short time. The 6X4
has very close spacing K-P (high perveance) hence the short. Arc to ground.

Break the tube and take it apart and look.

Hi Steve,
I am aware of the heater-to-cathode shorts in the 6X4, but in my situation, I am talking about a heater-to-plate short.
Are you saying that the 6X4 is also prone to heater-to-plate shorts because of the close spacing between the cathode and
plate?

Plate to cathode short is possible in a 6X4 but not heater to plate. Plate has close spacing to cathode. Metal can warp from excessive heat and short elements. Not likely heater to plate unless cathode is also involved.

Could have melted a hole in the cathode, clearing its short.

You may be right, Alan, because I can find no short between the plate and the cathode, only between plate and the heater pins.

shake the tube upside-down looking for flecks of debris in the glass envelope. might could operate the tube upside-down but not likely

Look at the base end of the tube. There are thin ribbons that connect the elements to the base pins. See if any look burned out or shorted to other pins.
Don

What is the test equipment involved, (schematic & voltages ?) Was this a scope ?

tube tests on the old tube:

Heater resistance. Does it light up ?
Does the other plate and the cathode/heater test good as a half wave rectifier with
settings that leave the cathode pin open, i.e. like 5Y3 settings?

Some 6X5 tubes were prone to taking out transformers, and the 6X4 has very
similar specs with a slightly higher plate current. 6X4 tubes lasted into 60s as
rectifiers in cabinet radio/phono with 6AR5 outputs (Philips ?). I can't remember
any particular gripe about 6X4s though. Akai used them right from Terecorders to
M8 models.

I think Don has it: the cathode is shorted to the plate and heater, but has lost its connection to the outside world.

Do check for probable cause. I did have a heater rectifier EZ82 in a set virtually vaporised, within the envelope (bits everwhere) and that was cuased by one, of several gramaphone needles, lodging in its socket underneath.

Marc

Don't think we need to put the 6X4 to death after doing such a good job on the 6X5 before the GT version. Still a couple of 1N007's in series for each leg counts for belts and suspenders.

Check the filter caps for shorts. If they fail they can take out the rectifier.

Check filters...yes, That fits into probable cause..... It takes a fairly decent short to destroy a rectifier, if it was not an internal issue. That we do not know, but need to establish.

Marc

Here is an update...

I do not have the proper equipment to check the electrolytic filter caps for shorts, so I decided to go ahead and replace them, which I did. Got a new 6X4 tube (which I tested for heater-cathode or other shorts using an ohm meter) and turned on the equipment using a dim-bulb tester which did not indicate a problem. I switched the dim-bulb out of the circuit, but upon turning the equipment on, there was crackling and a violet flash in the rectifier tube. I immediately turned off the equipment and upon testing the rectifier, I see that it now has a heater-cathode short. I have not replaced any of the other caps in the unit. I am thinking that something was drawing a lot of current, causing the heater-cathode short, but not sure where to look. Looking under the chassis I do not see anything obvious like burnt components. Here is a link to the schematic http://bama.edebris.com/manuals/motorola/t1034a Any help will be appreciated.

Don

This is an interesting one. You are only allowed a certain amount of volts between cathode & heater before there is a flashover.

Measure the AC voltage on the HT secondary (no tube) CT to chassis. Be aware that across the pins could be around 700VAC & some modern meters will not handle it.

It is almost as if you have an open circuit, or voltage doubler, rather than a short causing the heater to flash due to exessive voltage on the cathode? Check the wiring & for dry joints & open circuits in the early part of the PSU.

If there is a mistake with the filter caps, say 1000uF on the cathode, that will rip it appart. & if I got the right object. I would have a good look at that rectifier in the heater circuit.

Marc

My first guess is that your input filter capacitor is too large.

The AC voltages on the HV secondary are 379 V from each leg of the secondary to center tap. This is with a line voltage of about 122 Volts (the schematic calls for 340 V, but that is with a line voltage of 117 V). I have double-checked the ratings on the filter caps and they are all OK. I replaced the 3-section 20-20-20 mfd electrolytic with 22-22-22 mfd caps. I don't think this is excessive or enough deviation from specification to cause any problems.

When you say "have a good look at that rectifier in the heater circuit" are you referring to the selenium rectifier CR1? How would that affect the B+ voltage?




Alan, as I have stated above, the input filter capacitor is pretty close to what is specified in the schematic.

Don

The circuit is not really clear, but it looks like there are two windings involved with that rectifier giving 12VAC to it. I am wondering (thinking aloud) if there is a fault /miswire open, that allows back EMF from the 1000uF electrolytics, or could the 6x4 be getting 12V on the heater, which would fuse it?

Voltmeter may pick that up.

You will also have to check all of the other tubes for shorting.

Marc