"False center tap" blown, why?

[Bowie]

I've built a number of LA2As with the the Hammond 296jx and have used two 1W, 100r resistors to ground as a "false center tap" on the heaters. None of the units have had any issues after years of operation but the most recent one just failed after 3 months, with the two 100r resistors having burnt out. Any ideas as to what may cause this? I should note that these two happened to be old AB carbon comps, because that's all I had on hand.
Thanks.

 

[rackmonkey]

1) carbon comp resistors under specc'ed
2) short at a tube socket that allows HT onto the heater winding
3) not as likely, but a poor connection to chassis

 

[abbey road d enfer]

+1 to rackmonkey's analysis.
I would suggest measuring the voltages, both DC and AC, between heaters and ground.

 

[Tubetec]

Its just a guess , but I'd suspect tube quality might be the underlying issue here .

 

[Bowie]

Its just a guess , but I'd suspect tube quality might be the underlying issue here .

Definitely not the tubes, but thanks for the ideas. I first suspected one may have fail but I re-checked them on my Amplitrex and they are all solid.

Thanks to everyone for the help! I've got some 2w resistors on order for the CT (I have 1/2W but I just don't feel comfortable going that low). I'm going to see if anything changed with the voltages (vs 3 months ago).

Question; if one of the resistors was to blame, would BOTH have burned? They look equally scorched so they went out at the same time, before taking the fuse out. There's a few dropping resistors in the 6.3v path (which I use for lowering the filament voltage) and they seem unaffected.

 

[Tubetec]

Ah yes , before I used half watt 50 ohm resistors as centre tap , I didnt initially realise they were far exceeding their wattage dissapation , there was all kinds of fizzling noises coming from them . In the end I used 100 ohms each 2watt metal film , I also make sure the values are fairly closely matched for symetry .

 

[trobbins]

What is the winding voltage you are using for the heaters? At 6.3VAC, if you have 100R humdinger resistors, then they each dissipate only 0.1W. Even if you used a 12.6V winding (with your dropping resistors), and put the humdinger across the winding and not the 6.3V heaters, each resistor would still only dissipate about 0.5W.

Even with one 100R resistor open circuit, the other resistor won't carry any current, so shouldn't then subsequently fail.

I'd suggest you have another problem with your wiring, and need to do some faultfinding.

 

[radardoug]

I refer you to Ohms law and the power law. Using these, the power in the two resistors from 6.3 volts applied across both, is .19845 watts. So less then a tenth of a watt in each resistor.
If your resistors have burnt up, something else is going on, with significantly higher voltage being applied to those resistors. Draw your circuit out carefully. Measure the voltages.
You say that you have dropping resistors in the 6.3 volt heater chain, why? 6.3 volt heaters are meant to operate at 6.3 volts.

 

[Tubetec]

isnt each resistor dropping 3.15 volts though , or am I just digging a bigger hole for myself by not doing the math homework properly , numbers are a second language to me at the best of times , PRR would have our asses kicked

Under heating the heater is fairly well documented here RadarDoug ,earlier stages in tube gear are the likeiest candidates for it, or maybe you fundamentally disagree with the concept , its fine either way .

 

[abbey road d enfer]

isnt each resistor dropping 3.15 volts though , or am I just digging a bigger hole for myself by not doing the math homework properly , numbers are a second language to me at the best of times , PRR would have our asses kicked

Under heating the heater is fairly well documented here RadarDoug ,earlier stages in tube gear are the likeiest candidates for it, or maybe you fundamentally disagree with the concept , its fine either way .

Instead of speculating, have you measured the actual voltages across each resistor?

 

[Tubetec]

Hi Abbey ,
The issue I had where my 1/2 watt humdinger resistors burned up and got noisey on me was years ago , I vaguely recall I may have had a poor connection at the negative end of the first filter cap after the rectifier but my 2x50 ohm centre tap was downstream of a linear regulator , maybe that was the cause , I'm not sure .
On the one or two other ocassions I saw blown fuses and burnt resistors in the heater supply of guitar amps it was a heater/cathode insulation issue in one of the tubes .

 

[mjrippe]

You have not told us if your heaters are AC or DC.

 

[Bowie]

You have not told us if your heaters are AC or DC.

AC heaters.

I removed the burnt resistors, checked to make sure my heater wiring was clean, and powered up without tubes installed so I could take measurements but blew a fuse within seconds.
The unit was operating without issue prior to the resistors failing so I guess I'm either looking for something that took out the resistors, or something that was damaged when the resistors went. I haven't been able to find anything so far. Any advice on where to focus my search would be very helpful. I checked the heater leads for resistance with a meter but they read the same as my new/unused PTs.

 

[abbey road d enfer]

AC heaters.

I removed the burnt resistors, checked to make sure my heater wiring was clean, and powered up without tubes installed so I could take measurements but blew a fuse within seconds.
The unit was operating without issue prior to the resistors failing so I guess I'm either looking for something that took out the resistors, or something that was damaged when the resistors went. I haven't been able to find anything so far. Any advice on where to focus my search would be very helpful. I checked the heater leads for resistance with a meter but they read the same as my new/unused PTs.

Then I thibk you should measure the resistance between heaters and ground with the heaters winding disconnected. And check for leakage between this winding and ground; it should be totally floating.
There is also the possibility of this winding being shorted...

 

[Bowie]

Then I thibk you should measure the resistance between heaters and ground with the heaters winding disconnected. And check for leakage between this winding and ground; it should be totally floating.
There is also the possibility of this winding being shorted...

Thank you. I measured at various heater points and grounds, trying to find any leak to ground but found the heaters to be completely isolated. I'm still stumped as to why a fuse blew with no tubes and the heaters floating. Without removing components from circuit, everything looks/tests normal. Any ideas are appreciated as I'm now spending hours staring and probing with no luck.

Silly questions but, I've never built anything without either having a heater CT or creating a false one. Not having one when powering up wouldn't cause the fuse to go, would it?

 

[abbey road d enfer]

Silly questions but, I've never built anything without either having a heater CT or creating a false one. Not having one when powering up wouldn't cause the fuse to go, would it?

No, it shouldn't. I think you have to completely disconnect the xfmr secondaries and start at square one. Does the fuse hold the open current?

 

[Tubetec]

Would it be worth checking for an interwinding short between heater and HT ?
You would think leakage between primary and heater should cause your mains supply to trip out instantly at the breaker.
If your heater winding is disconnected and you still blow a primary fuse it would seem to point to a transformer fault , although the fact that the heater winding itself measures good compared to another new transformer of the same type would seem to rule out shorted turns in the heater winding itself .

With both HT and heater circuits fully dissconnected if the transformer still blows a primary fuse when energised it has to be the transformer .

Theres a simple explanation , you just havent found it yet .

 

[industrialarts]

Would it be worth checking for an interwinding short between heater and HT ?
You would think leakage between primary and heater should cause your mains supply to trip out instantly at the breaker.
If your heater winding is disconnected and you still blow a primary fuse it would seem to point to a transformer fault , although the fact that the heater winding itself measures good compared to another new transformer of the same type would seem to rule out shorted turns in the heater winding itself .

With both HT and heater circuits fully dissconnected if the transformer still blows a primary fuse when energised it has to be the transformer .

Theres a simple explanation , you just havent found it yet .

Just a dumb question - are you sure you have a good ground? If not, it would just be a 200 ohm load across the secondary, which would draw a lot of current to burn up the resistors and finally blow the fuse. Believe me, I've done it.

 

[abbey road d enfer]

Just a dumb question - are you sure you have a good ground? If not, it would just be a 200 ohm load across the secondary, which would draw a lot of current to burn up the resistors and finally blow the fuse.

I don't get it. How could loading a 6.3V secondary with 200r "draw a lot of current"?

 

[industrialarts]

yeah, you're right, I was speculating without doing the math first - that only comes out to less than 1/4 watt power dissipation. I wonder if the resistors are indeed 100 ohm because -

I should note that these two happened to be old AB carbon comps, because that's all I had on hand

not likely, since I don't often see carbon comps get smaller as they age but then I've seen a lot of impossible things happen when it comes to fixing gear