False Ground Resistor Question

[wattsup]

What all can cause the 150 ohm false ground on the heaters to fail?

[Leo_Gnardo]

If there's been a shorted tube, usually an output, that can sizzle filament balance resistors.

[wattsup]

What if the tubes are all good?

[tubeswell]

Oops - reply posted in wrong thread, sorry.

[TUBEDUDE]

Is it a fresh build? If so, another inadvertent ground reference. Any pics, schematics?

[Leo_Gnardo]

What if the tubes are all good?

What's the wattage rating of your resistors? And what construction - that is metal film, metal oxide, carbon film, carbon comp, wirewound? One of the bugs in currently made small format resistors is they often don't hold up the way you expect 'em to, regardless of power law theory. IOW "It's only dissipating a quarter of a watt, a half-watt resistor should do" doesn't seem to hold up well with today's miniaturized resistors.

Note those tough old 60's Fender amps that seem to last near forever used half-watt carbon comp 100 ohm resistors in this application. They hold up just fine for decades. Can't say I've ever seen one fail except that a dodgy output tube sent a surge of current thru, or a tube mis-plugged because it was missing its locator key or forced in despite locator key bump not aligned with its notch. If you're not already using them, I recommend carbon comp R's for this application.

Also, if your tubes measure good on a tube tester, be aware most testers don't put much hi voltage on tube plates & screen grids, usually about 175V. Output tubes can pass the "pinball machine" test but still break down when real amp voltages are applied, 400 to 700V depending on the amp. All it takes is one quick arc and goodbye balance resistors.

Speaking of arcs, have a close look at both sides of your output tube bases and the mating surface on the tubes. If there's ever been an arc on the socket or tube base, it often leaves a carbon path that allows current to jump places it isn't supposed to go. The most often found arc spot is between pins 2 and 3 - that can send hi voltage from plate to filament & wreck your balance resistors too.

[wattsup]

What if the tubes are all good?

What's the wattage rating of your resistors? And what construction - that is metal film, metal oxide, carbon film, carbon comp, wirewound? One of the bugs in currently made small format resistors is they often don't hold up the way you expect 'em to, regardless of power law theory. IOW "It's only dissipating a quarter of a watt, a half-watt resistor should do" doesn't seem to hold up well with today's miniaturized resistors.

Note those tough old 60's Fender amps that seem to last near forever used half-watt carbon comp 100 ohm resistors in this application. They hold up just fine for decades. Can't say I've ever seen one fail except that a dodgy output tube sent a surge of current thru, or a tube mis-plugged because it was missing its locator key or forced in despite locator key bump not aligned with its notch. If you're not already using them, I recommend carbon comp R's for this application.

Also, if your tubes measure good on a tube tester, be aware most testers don't put much hi voltage on tube plates & screen grids, usually about 175V. Output tubes can pass the "pinball machine" test but still break down when real amp voltages are applied, 400 to 700V depending on the amp. All it takes is one quick arc and goodbye balance resistors.

Speaking of arcs, have a close look at both sides of your output tube bases and the mating surface on the tubes. If there's ever been an arc on the socket or tube base, it often leaves a carbon path that allows current to jump places it isn't supposed to go. The most often found arc spot is between pins 2 and 3 - that can send hi voltage from plate to filament & wreck your balance resistors too.

That's good to know. Thanks Leo_Gnardo I'll pull the tubes and check to see.

[schaublin65]

Hi,

normally I like to use 2 watt resistors but in this position I prefer 1/2 watt so they are "fusistors" if things go wrong

take care..

John

[jjman]

Touching a probe to both pin 2 & 3 on a 6v6 6L6 EL34.....Oops...

[wattsup]

Well power tubes were def shorted - BOTH of them. Had a guy at a local electronics shop put them on his tube tester. He said they were so bad that they made his tester smoke. He wasn't kidding either!