What function(s) do these resistors play in this heater circuit?

[mort]

Just curious as I'm not used to seeing resistors [R15 & R16] on the 6.3v circuit.

I'm working on this amp and R15 lights up like a street lamp when tubes are in. I'm guessing it's a bad tube with a heater shorted straight through...


and also that diode and resistor on the bulb... what does that serve?


http://support.fender.com/schematics..._schematic.pdf

[Jazz P Bass]

R15 & 16 are a hum balance circuit.
They are supposed to help balance out any difference in the winding resistance.
They have been used by Fender, almost forever.

The 'diode & resistor on the bulb' are for the 'lamp', which happens to be an LED.

[Enzo]

Almost certain a bad 6V6 or the wiring to it. High voltage being shorted to the heater.


Or the 6V6 has its center post broken off and the tube is in the socket facing the wrong way.

[mort]

Or the 6V6 has its center post broken off and the tube is in the socket facing the wrong way.

I did actually think to check that. I must be learning something

[mort]

Oh wow I didn't think ohming out the tube pins would show me anything, I figured a short would only show itself under load, but there is .4Kr between pins 2 and 3.

Is that a common failure for modern tung sol 6v6's?

[Chuck H]

A few months ago I had a new Tung Sol EL34B fail plate to heater. It's the only time I tried those tubes and I replaced them with something else.

[Jazz P Bass]

I do not believe that it is a brand specific problem.

This is why you really should fuse the heater circuit.

[Chuck H]

Absolutely true! And sage advice placed with consideration where it would be noticed.

I was responding directly to mort's specific subject. And after some looking around I do think it's possible that the new Tung Sol products are fragile in general.

[frus]

I had four 5881's

One shorted plate to heater
The other's getter broke off (!) turnig it into a nice rattle
Third became microphonic
Fourth is still alive and well...

[Jazz P Bass]

Yeah, weed em out!

[Chuck H]

I had four 5881's

One shorted plate to heater
The other's getter broke off (!) turnig it into a nice rattle
Third became microphonic
Fourth is still alive and well...

These were Tung Sol?

[frus]

yes, all new russian tung sol, bought as two matched pairs
I'm not sure now, but I guess they lasted like a year on average
I had a pair of JAN Philips 6L6wgc or something in the same amp for 4 or 5 years, never had a problem (they started to sound a bit thin though, but that's expected)
Right now I have JJ's, no problem with them either

[Enzo]

Fuse the heaters? I just had dinner so I may be slow, but seems to me that the failure current of a 2-3 short is from one side or the other - or both - of the 6v to ground, the fuse in a heater circuit is not to ground but instead is in series with the 6v., and so would not open in these cases.

[Chuck H]

What if you used a 12V fuse? Wouldn't that fail if it suddenly had 400V across it? basically fusing against voltage instead of current. I don't even know but I thought I'd mention it.

[Leo_Gnardo]

What if you used a 12V fuse? Wouldn't that fail if it suddenly had 400V across it? basically fusing against voltage instead of current. I don't even know but I thought I'd mention it.

Hmm wha?? Regardless of voltage, how much current could the HT circuit supply thru a short - 200, 300, 400 mA at most. An HT fuse may have disconnected the hi voltage, but the balancing resistors or pot would likely be lunched by then anyway. Just watched it happen last week. Fender Bassman 100 arced a socket, welding bright, and another welding job on the hum balance pot. Mains fuse popped before I could switch it off. Hey, anybody ever generate ball lightning in an amp? I'm expecting one of these days I'll see it happen.

Fuse voltage ratings are concerned with the width of the gap between the melted metal bits left in the fuse. Keeping in mind the 8000V per inch breakdown of air at sea level pressure & no humidity - why I wondered do they consider these "standard conditions" - the air is always humid at sea level but never mind that for now. Auto fuses are usually rated for 35V although 16V is usually the max you see in a vehicle. They're not recommended for hi voltage work because the current could jump the small gap expected in a burnt car fuse.

Seen too many Sovtek-made TS's fail. I avoid 'em. I'd say shame but New Sensor do not know the meaning of the word.