3.3V heaters? Smoke coming from PT?

[Herec]

Also this amp appears to have a Center tap on the PT for the filament supply coded as green/white on the layout so it would not use the resistors that Phil showed but one never knows so that is why he has included them.

Yeah, I think I understand the concept of making the center tap with the 2 100ohm resistors, but I think that layout's been verified without a part like that.

But, at this point, I'm pretty sure the PT is fried. I just hope it wasn't my fault...

[Structo]

Well it's not unheard of to get a bad part.
It can be frustrating especially if you are a bit inexperienced in troubleshooting.
But even pros can have their day.

I am not one by the way.

I was testing an amp after doing so work and I didn't get any output.
Nada.
I was getting ready to yank out the OT when I tested the speaker cable....

Yep, it was an old cable with molded ends that I used on the amps on my bench to a speaker there.
I spent about 30 minutes trying to figure that out.

So, a lot of times it's something so simple we don't think of it right away.

Good luck.

BTW, you can measure the resistance of those windings, not sure what readings are right for that PT but if you have an open or shorted winding it should be apparent.

[Herec]

First, thank you all for helping me figure out what's going on with this thing! I'm a EE major, second year of classes, but we still haven't gotten to transformers.

I just disconnected the wires from the chassis.

There's not continuity between the two different pin sockets in the entire heater harness, which I think means I did them right (the same way as I built my plexi).

The pilot light, either side of it, doesn't have continuity with the chassis.

For the wires on the PT:

The two heater wires have continuity between eachother and to that heater CT wire that was connect to ground like in the layout.

The two yellow HT wires have continuity between eachother.
The two red HT wires have about 140 ohms resistance between them, and 70 ohms each with the HT wire that was grounded (the red and yellow one, the HT CT i think its called?)
There was no continuity between the yellow wire (either of them) and the HT CT wire.

And how do you measure the resistance of the taps? Which tap do you use as a reference? just the same one all the time?

[Structo]

You just measure the two wires for each tap.
Trouble is the reading won't mean much unless you have another transformer to compare to.

You mentioned you are taking EE courses.

I'm curious how much time they spend on tube theory these days.
When I went to school in the mid 70's, the instructor told us we wouldn't spend much time on it as semiconductors did everything a tube does and better with no energy lost in the form of heat.
I think we spent two days on tube theory.

It might be worth it to email Weber and ask them if they could measure one or possibly exchange it.
I haven't bought much from them so I have no idea how helpful they would be in a situation like that.
You never know, they may just send you another PT.

[krash]

If, when I measure from the two terminals, I only get 3.3 V AC, it means that one of the two wires must be grounded because its approximately half the voltage, right?

No. It COULD mean one of the wires is grounded, but if that were the case, and you also grounded the CT, then you would have melted down that half, so it's grounded by virtue of being shorted inside the PT.

But if you have a pilot light or any other thing connected across the 6.3V and one side is OPEN, then both sides would measure 3.3V.

So for example, if I connect only one side to the PT, the CT to ground and the other side of the PT heaters to the circuit, then measure, then both heater taps on every tube socket etc. are going to read the same potential since they are equipotential as long as you have a pilot light in there to bring the open side up to the same potential as the connected side.

[Phil_S]

I'm thinking there is a very good chance Josh is correct about the existence of a short. The question in my mind is whether it really is inside the tranny or whether there is something about what you built that you are not seeing. I think you know how tough it is to do this remotely, without the work piece in front of us.

If I were in this position, here's what I'd do and you've already done some of it.

Leave the primary connected to the mains feed and the fuse. It is a really good idea to have the fuse. Disconnect all the secondaries -- I think you've done this. 140 ohms on the high voltage secondary is reasonable, but I don't know what this PT is supposed to have. It is also a good sign that you've got 70 ohms from each outer leg to the center tap. I'd note it is not usually that symmetrical, as the inner winding has the same number of turns but shorter wire length compared to the outer half of the winding. The filament windings are often very low ohms, sometime less than one ohm and a cheap meter may not give you a reading except to indicate there is continuity. You say you've got continuity.

With all secondary leads safely placed so they don't short to anything metal, turn on the PT and use your meter to see the AC voltage on each of the secondaries. Use clip on leads with the probes so you can be hands free. You can buy clips at Radio Shack if you don't have them. (I use a bit of heat shrink to insulate the bare wires I'm not working with.) Just set the meter to ACV and clip the probes to each pair of secondary leads. Turn it off when you move the probes. While you can do this on a live PT, I think it is not wise, especially for an amateur. If you've got a screw down terminal block, then you can manage the wires and probes without having to turn the thing on and off. In the case of the HV winding, you can also check each outer leg to the CT and expect to see half the voltage.

If the filament winding is bad, you'll know right away. With no load it will run a bit higher than rated. I'd expect 6.5-7.0V on the 6.3V winding. If you only get 3.3V then you know you've got an internal short. So, check this one first.

Do this much. It isn't much effort, and post results.

[Phil_S]

As I pressed the submit button, I had a completely different thought. I keep ignoring the layout drawing, which shows a center tap on the 6.3V secondary.

Is there a center tap on the 6.3V winding?

If there is a center tap, it is quite possible that you've mixed up the CT with the outer legs. If you did, 3.3V would make perfect sense. This is something you can identify in making the live ACV readings with all secondaries disconnected. Just be safe.

[Herec]

Ok, just for clarification:

On this PT

There's 2 black wires for the primary, one goes to the power switch and the other to the actual mains from the wall.

There's 5 HT wires, two yellow and two red that go to the rectifier socket, and the red and yellow striped which I assume is the HT CT that gets connected to ground.

there's 3 wires for the filaments (heaters):

One is green and yellow striped, which is connected to ground on the layout, I assume this is the CT that's being reffered to.

There are two plain green wires, which are the two wires that get connected to the tube sockets for the heaters, and I assume these wires are the actual heaters.

If I have the CT connected to ground, and one of the green wires connected to ground accidentally but also to the tube sockets, and the other wire connected only to the tube sockets, would this be a problem?
Would the AC voltage that is supposed to go to the tube sockets just go to ground at that point because of least path of resistance etc. to ground, thus causing the short?
Or would it not matter because there's continuity between the two heaters and the heater CT (green/yellow wire)?

If, when I measure from the two terminals, I only get 3.3 V AC, it means that one of the two wires must be grounded because its approximately half the voltage, right?

No. It COULD mean one of the wires is grounded, but if that were the case, and you also grounded the CT, then you would have melted down that half, so it's grounded by virtue of being shorted inside the PT.

But if you have a pilot light or any other thing connected across the 6.3V and one side is OPEN, then both sides would measure 3.3V.

So for example, if I connect only one side to the PT, the CT to ground and the other side of the PT heaters to the circuit, then measure, then both heater taps on every tube socket etc. are going to read the same potential since they are equipotential as long as you have a pilot light in there to bring the open side up to the same potential as the connected side.

So since I had the pilot light wired up, but it was getting the same feed as the tube sockets with potentially one of the heater wires going to ground, would the theory still hold?

I'll get to testing the wires of the transformer hopefully in the next few days.

Two general questions as well:
HT is an equivalent term for high voltage secondary?
The high voltage secondary can have a CT?

[tictac]

The heater winding should only have the center-tap grounded, if you grounded one of the other leads that would be a problem...

TT

[Herec]

Doing the PT check now, disconnected all wires except for the ones connected to the rectifier socket, but they're not really connected to anything, just makes it easy to take voltages. And the mains ones of course. The 2A fuse that's been in there all along is still present.

I've left this PT running for about 5 minutes now.

No smoke.

Not getting hot. Just kinda warm, but just like my other amps that work fine.

Measured between the two green heater wires, 6.3VAC, perfect.
Measured between the two and the CT, get around 3.3VAC for each.

Measured between the two red high voltage wires, get about 635VAC.

Measure between the high voltage secondary and the CT for it (red/yellow wire), get about 315VAC. I think I eventually wanted 325V on the output plates, but would the 315VAC go up if the rectifier was installed?

wtf?

I think I know what the problem was, though.

Just for kicks, I checked the continuity to ground between each of those 3 turrets on the turret board that the two heater wires go to. Everything's been disconnected from them. Its just 3 turrets on a turret board.

One of the turrets on the board has continuity to ground. There's nothing connected to it. I haven't unbolted it from the chassis, but there must be some scrap of metal or stray piece of solder that's underneath it.

I'll remove that tiny turret board and take a look, making sure when I put it back in to put it on board stand-offs.

The PT has been on for probably 20 minutes now. No smoke. Pretty much the same readings on the HT, give or take a couple of volts, and 6.4VAC between the two heater leads, pretty much the same reading between the heater wire and the heater CT.

The smell i smelled with the smoke yesterday smells like the gunk that's on the outside of the PT (i put my hand on it and smelled my hand, very scientific.)
Maybe the smoke wasn't coming from directly inside the PT windings, just from some manufacturing residue that won't smoke unless the PT got extremely hot like it did yesterday?

So, is this PT actually still good?

[Structo]

Is an eyelet touching a standoff ?

If it were me, I would pull the bell cover off and take a gander inside to see if any of the wires are fried.

BTW, the PT should not get warm with no load on it.

[Herec]

On the PT, the filament wires would be the ones on the outside, correct? I'll pop the cover off later.

I've got a bad feeling it's burnt and thus not really good anymore, because the PT was getting warm with no load on it (wires weren't attached to anything).

If the PT is still putting out normal voltages, why can't it still be used (just wondering)?

[jjman]

I would expect more than 6.3vac on the heater wires under no load. I had over 7volts on a tranny under no load but it was a vintage one intended for 110vac (not 120.) Running the output tubes with the heater circuit connected would be a good test to see what the heater voltage does. No need for any centertap on the heater circuit nor DC on the tubes to test only the heater voltage. It would go down even further after also connecting the rectifier's 5v heater or/also the DC.

[krash]

normally I see about 6.8V on a loaded heater winding and about 7.x volts with an open heater winding on most of my transformers, which have 125V primaries connected to line voltage approx. 125V so I do think open circuit 6.3V on the heater winding is probably not right... but the transformer will get a little warm with no load on it.

If you had a short from one side of the 6.3V to ground (through mechanical contact, etc.) then you may have fixed it. If you have good voltages then it's probably ok. The "smoke" may have just been some lacquer getting hot and outgassing... dunno if you can (yet) tell the difference between that and burnt transformer smell

[Phil_S]

Doing the PT check now, disconnected all wires except for the ones connected to the rectifier socket, but they're not really connected to anything, just makes it easy to take voltages. And the mains ones of course. The 2A fuse that's been in there all along is still present.

I've left this PT running for about 5 minutes now.

No smoke.

Not getting hot. Just kinda warm, but just like my other amps that work fine.

Just kinda warm might be OK. Depends on exactly what you mean by kinda warm. Like Tom says, under no load, it might possibly take the chill off cold iron, but not much above room temperature. See comment about pulling the end bell, below.

Measured between the two green heater wires, 6.3VAC, perfect.
Measured between the two and the CT, get around 3.3VAC for each.

Really, this is very good news. The CT goes to ground. Don't use the 100 ohm resistors. IMO, there is nothing wrong with the PT.

Measured between the two red high voltage wires, get about 635VAC.

Measure between the high voltage secondary and the CT for it (red/yellow wire), get about 315VAC. I think I eventually wanted 325V on the output plates, but would the 315VAC go up if the rectifier was installed?

A 5Y3 will give you about 1.1x. Voltage will drop when you put the tubes in the circuit. So it would be 1.1x whatever the 315 drops to. I'm guessing you'll end up with around 330-340VDC. This is close enough to 325 and I wouldn't fool around trying to drop the voltage.

wtf?

I think I know what the problem was, though.

Just for kicks, I checked the continuity to ground between each of those 3 turrets on the turret board that the two heater wires go to. Everything's been disconnected from them. Its just 3 turrets on a turret board.

One of the turrets on the board has continuity to ground. There's nothing connected to it. I haven't unbolted it from the chassis, but there must be some scrap of metal or stray piece of solder that's underneath it.

I'll remove that tiny turret board and take a look, making sure when I put it back in to put it on board stand-offs.

Yes, insulation is a good idea based on what you found. You might take a good look at the bottom of the turret strip. If it is just a turret poking through to the metal chassis, consider a strip of insulation or a dollop of silicone gel on all of the turrets. Stand offs work, too. Use what you have, just be safe.

The PT has been on for probably 20 minutes now. No smoke. Pretty much the same readings on the HT, give or take a couple of volts, and 6.4VAC between the two heater leads, pretty much the same reading between the heater wire and the heater CT.

The smell i smelled with the smoke yesterday smells like the gunk that's on the outside of the PT (i put my hand on it and smelled my hand, very scientific.)
Maybe the smoke wasn't coming from directly inside the PT windings, just from some manufacturing residue that won't smoke unless the PT got extremely hot like it did yesterday?

I'm with Tom on this one, gunk/smell isn't good. Probably some burnt insulation. Remove the end bell and sleeve the burnt lead with some heat shrink tubing. Do this sooner rather than later. You don't want a short to the end bell. It could cost you the transformer and more. Why take the chance? Maybe you'll find nothing, but better safe than sorry.

So, is this PT actually still good?

Yes, it seems to have survived, but concerns remain. There is a chance it is fine but I can't say without seeing it in front of me.

Just a note, the two yellow leads are not high voltage. They supply 5V for the filament of the rectifier. The two reds and the red/yellow are the HV winding with the striped one as the CT.