Burned Artificial Center Tap

[Ambugaton]

So I am installing a new PT on a deluxe lite build and  before installing it I was just looking through and cleaning some of my wiring. I noticed that the 2 100ohm resistors were fried. I think they were only 1/4 watt. Just curious as to what that means and what the cause was?

[kagliostro]

100R resistore must be 1/2W at least

http://www.valvewizard.co.uk/heater.html

(But this also may be due to one other problem)

Franco

[sluckey]

As K said, it could be as simple as the resistors were undersized.

A common cause for those resistors to be fried even if they are 1/2 watt size would be a short between pins 2 and 3 on the output tube. This puts B+ onto the filament winding so basically B+ across those 100Ω resistors, so they pop. This could be a permanent short which may also kill the PT HT winding. Look for a carbon trace between pin 2 and 3 on the output tube sockets, or check for low resistance between those pins. But the short may have just been a momentary arc between pins 2 and 3. Many times the only evidence that an arc has occurred will be the fact that those resistors fried. The most likely cause for an arc (short) between pins 2 and 3 would be playing guitar loudly through an amp with no speaker connected. This may happen because the speaker just ain't plugged in or could occur if the speaker is momentarily disconnected (dirty jack, bad cable, etc) while you are playing Deep Purple.

So, examine those tube sockets and be sure there is no continuity between pins 2 and 3. Why did you replace the PT? If it failed it could be related to the fried resistors.

[Ambugaton]

Thanks, I will check pins 2 and 3. Replacing because the previous pt failed. The ht windings did in fact short so this is definitely looking to be the probable suspect.

Only thing I don't understand is that with undersized resistors... Shouldn't they have pipped early and saved my pt? Just don't understand how the resistors would fry and still short out the ht windings?

[sluckey]

Shouldn't they have pipped early and saved my pt? Just don't understand how the resistors would fry and still short out the ht windings?

That's hard to say. Did the resistors burn and fall apart or did they just burn and create a short circuit? Do you know that the amp has been played without a speaker? This could be a good reason to use a shorting jack for speaker connection if you are not currently doing so. This will protect you from an unplugged speaker cable at the amp but offers no protection against open cable or blown speaker.

[Ambugaton]

They didn't fall apart just seems they just shorted out. I will look into a shorting jack. I know the dangers of not having a speaker connected so I can say if it did happen I was not aware of it.

[PRR]

A 1/4W 100r resistor will stand 5V for weeks. Taking half of 6.3V they should live forever.

Something happened to blow them. And not any little 2mA preamp tube short, this had to be a HIGH-current fault, >50mA. I am thinking an output tube short. This may manifest as a PT melt-down. But if you replace the PT and the tube shorts still or again, you lose the 19 cents of resistors and 89 bucks of PT again.

[Ambugaton]

Ok so my next question is: should I move forward with standard start up procedures or spend more time determining the cause?

[Ambugaton]

Before the last pt went out the amp was blowing fuses without power tubes installed, that may have been after the centertap fried though.

[Ambugaton]

So I decided to plug it in to a limiter and got 330VAC on the Rectifier but it seems I have an issue with the 5VAC. No heat on the tube. Hmmm

Overall:

Light bulb limiter does not glow, pilot light does, 330V on 4 & 6 of 5Y3, 2.8VAC on each filament, getting 5VAC on pins 2 & 8 on the 5Y3... but doesn't seem the filament is heating up. It is a verified good 5Y3 that was just borrowed from another working amp.

[sluckey]

Pull the tube. Check resistance between pins 2 and 8 on the tube. Spray some electronics cleaner into the socket and quickly insert the tube several times to wipe the socket pins clean. Any better?

Put another tube in. Any better?

You got 330vac with it plugged into a limiter. Wonder how high that will be when you plug into the wall? Plug the amp straight into the wall with no output tubes. Measure pins 4 and 6. Does the tube lite? Any better?

How was your meter connected when you saw 2.8vac on each heater? You don't have a center tap connected to ground on the 5V winding do you? The correct way to measure filament voltage on that 5Y3 is to measure from pin to pin, not from pin to ground. Remember, there will be B+ voltage on that filament when the tube ever lites up.

[Ambugaton]

- Tube has been pulled and cleaned, 0 resistance between pin 2 & 8.

- Don't have a spare 5Y3 at the moment but will fix that

- Without limiter I get 355V

- I don't have a centertap for the 5VAC, just 6.3VAC and HT windings

- Tested the heaters for other tubes from pin to ground... and oddly enough now I can't seem to get anything on the 5Y3 whatsoever from pin to pin. I think a tube swap should be my first move.

I have the 5VAC windings going from pin 3 with diode to pin 2, and pin 7 with diode to pin 8. From pin 8 to filter cap. I am starting to question that now.

[sluckey]

I have the 5VAC windings going from pin 3 with diode to pin 2, and pin 7 with diode to pin 8. From pin 8 to filter cap. I am starting to question that now.

Well, that's the problem. Connect the 5v wires directly to pins 2 and 8 and the tube will shine. The only way that could ever work would be if the diodes shorted. And that's possible if you used 1N4007s that are only rated for 1 amp.

Can you show us an as built schematic?

[Ambugaton]

Huh, maybe I was confused as to how to install safety diodes then? I thought you put them in line with each 5vac winding? I just thought the unused socket terminals would be easy.

It is built using your Deluxe Lite schematic

[sluckey]

Safety diodes are connected between the high voltage wires and pins 4 and 6. Have these diodes been wired like this from day one?

[Ambugaton]

Ohhhh... stupid mistake. I added these when replacing the pt, wasn't like that before.

Removed diodes and all seems fine now

[trobbins]

Just for background info - what identified that the PT had gone bad?  Was it blowing AC mains fuses?  Was a particular winding measuring zero or open resistance?

[Ambugaton]

It was blowing fuses so I disconnected the ht windings from the rectifier and light limiter showed I had a short and meter showed 2 ohm resistance between the windings.

[trobbins]

Can you recall which windings?

[Ambugaton]

It was the high voltage (B+) winding

[Ambugaton]

So just in case it is beneficial to someone down the road:

Originally the amp was ss rectified. After being built it worked fine for months until one day it started blowing fuses. I put it on a light bulb limiter and it was showing that I had a short. I removed power tubes and got the same results. I removed the ht windings from rectifier and got the same results. After testing the ht windings I realized they were wide open.

When it was all said and done I assume that I had a diode failed in the rectifier circuit which I have read can also take out a PT. I replaced the transformer and wired in a tube rectifier (did not change any other components) and the amp is back to functioning. This time around I will add safety diodes to the ht windings. I also did not have an ht fuse in this amp... but that would have probably saved me $60 for sure.

[sluckey]

After testing the ht windings I realized they were wide open.

That's odd. An open winding should not blow a fuse.

[Ambugaton]

I never had the ht windings disconnected and off of the limiter so I do not for sure it would have blown a fuse when it was disconnected. It for sure popped 2 fuses while connected with and without power tubes installed.

I think what happened was... diode failed and popped fuse. I removed power tubes and powered up again... popped fuse - but it maybe also opened up the ht windings this second time? Cause after that I could not get any b+ on the windings at all.

New pt installed and ss rectifier removed... amp works perfectly. I (being a complete novice) did not take the time to test each diode before removing them so I will never know for sure.

[sluckey]

What about the fried center tap resistors? Ever find any evidence why?

[Ambugaton]

I still can't figure out why. They were only 1/4 watt but it was mentioned that they should have been fine. The set of power tubes I was using before the pt swap were plugged in last night and were fine. No evidence of any shorts on power tube sockets.