3.3V heaters? Smoke coming from PT?

[Herec]

Hi guys,

Just finished my Supro Model 24 kit from turretboards.com. Turned on the power switch, no tubes in, and was measuring the heater voltages, and I got 3.3V difference between the two heater pins on all the tube sockets, and even when I measured where the heater wiring starts (the stripboard) to go to the tube sockets and to the pilot light. The pilot lights up fine. There's conductivity between the two pins also, which I'm not sure whether there should be or not.

After awhile, I had the amp on for about 5 minutes, i noticed smoke, and it seemed to be coming from the side of the PT where the heaters are along with the mains (the mains measured at 120V). The PT also seems very hot.

I traced the heater wiring using a sharpie and its connected to the pins is supposed to be (i used green for both.)

There's also conductivity between the heaters and the chassis.

Here's the schematic and layout. I'll get some pictures up soon.

Schematic: http://www.turretboards.com/Supro%20kit ... ematic.pdf

Layout: http://www.turretboards.com/layouts_sch ... layout.pdf

[M Fowler]

This is a dumb question bear with me here but did you have a speaker or dummy load plugged into that amp?

Mark

[Herec]

I didn't have any tubes plugged in, didn't think I needed to if there were no tubes installed yet.

EDIT: When I said tubes in the original post, I meant tube sockets. Hope that clears the confusion!

[krash]

Sounds like either:

* you grounded the center tap AND one side of the heater winding, so you have a short on half of the heater winding (wiring error, break in he insulation near a socket, etc.)

* you wired up the heaters with the CT and one side, and grounded the other side. However this doesn't explain the smoking PT.

Here's what I'd do (I recently had to debug a quite similar problem):

1. solder a 15 or 30W light bulb across the mains fuse and remove the fuse. This will limit current into the amp and allow you to debug, hopefully without frying the PT.

2. disconnect all of the PT wires from the amp and tape them off. DO NOT SKIP THIS STEP

3. power up the amp with the light bulb in place of the fuse and measure the AC voltages from each PT heater wire to ground. You should not have a voltage from any of the heater winding terminals to ground.

4. measure the AC voltage between each of the heater winding terminals: + to CT, CT to -, and + to -. Depending on how bright your light bulb is, you should at least see a volt or two between the + and - and half the voltage on each leg to the CT.

5. Power off the amp, and now just connect the heater CT to the circuit (ground). Fire up the amp and measure the voltages from each leg of the heater winding to ground. They should each be the same AC voltage, probably a volt or two.

if this test all passes then it may indicate the heater winding is not the problem. If you simply had it miswired it will reveal the problem.

While the PT is disconnected, double check all of your heater resistances (remove all tubes). You should have just the pilot light resistance (in my amps, that's about 11 ohms). Be sure and check both legs to ground, as well as both legs to each other. You could have the two heater wires shorted somewhere and they'd both measure infinity to ground.

If you determine the heater winding is not the problem, then it's time to debug the other windings.

Sounds to me like one side of your heater winding is shorted to ground.

Now might be a good time to pick a lower mains fuse value. You should blow the fuse before you get smoke from the transformer. If you actually had smoke from the transformer then the chances are very good that you melted the insulation in there somewhere and have a shorted winding. I know that's not good news.

FWIW I have had UF4007 diodes in the HT circuit go short and cause symptoms just as you describe by breaking down the heater winding insulation and shorting it inside the transformer. Sux. I have a bad PT here, good HT winding, good 5V, and shorted 6.3. $60 paper weight.

[Phil_S]

Those links are restricted access.

You let the smoke out. We joke about this even though it's usually not funny to the victim. You can't put it back in. Many of us have BTDT one time or another. You may have hosed the filament winding. Fear not, if there is room on your chassis, you can mount a separate filament transformer and these are relatively cheap.

What do you mean when you say there is conductivity to the chassis? Did you ground the heaters? I'm guessing this wasn't done right.

Often, the heater circuit is simply implied on the schematic. Typically, heaters are wired in parallel. Imagine the two tranny wires are the rails on a ladder and the tube sockets as the rungs. The heater/filament string needs a ground reference. This is done by using a pair of 100 ohm resistors. One end of the resistors are grounded on the chassis, making a "V" with the pointy side at the chassis. The other end goes to the filament wires. This is an artificial center tap. The connection can be made just about anywhere. The common places are at the lamp or the last tube socket.

Did you make an artificial CT? If you just grounded the wires, it's not surprising you let the smoke out. The winding may still be OK, so put in the CT and try again.

I've attached a picture showing the artificial CT at a tube socket (lower right, two resistors from the heater pins to ground).

[Phil_S]

I have a bad PT here, good HT winding, good 5V, and shorted 6.3. $60 paper weight.

Sorry to hijack this. What's the HT voltage and any other info? I might take it off your hands depending and if we can reach an agreement. PM me if interested.
Phil

[Herec]

I checked the heater wiring from socket to socket, its on the correct pins, so the problem must be in the area where the PT connects to the turret board which provides the heater voltage for the pilot light and then on to the first tube socket.

Here's the links that hopefully work now.

Schematic: http://www.turretboards.com/Supro%20kit ... ematic.pdf

Layout: http://www.turretboards.com/layouts_sch ... layout.pdf

[krash]

I have a bad PT here, good HT winding, good 5V, and shorted 6.3. $60 paper weight.

Sorry to hijack this. What's the HT voltage and any other info? I might take it off your hands depending and if we can reach an agreement. PM me if interested.
Phil

It's one of these:

https://taweber.powweb.com/store/025130sch.jpg

270-0-270, 340-0-340, 207V winding with SS rect on the 125V primary makes about 330VDC in my amps.

However, the 6.3V winding is shorted inside the transformer, so it would take some creativity to eliminate it. If you just hook up the primary and fuse only and apply power to the primary it blows the fuse instantly.

Originally it was just open on one side of the 3.15-0-3.15 winding so I was getting ~3.3V from one side to ground, but during the course of debugging the amp it went ahead and failed short.

Also I have a big old Edcor PT of similar specs only about 50% larger with an open heater winding IIRC. Same root cause.

[Herec]

Would it matter which heater tap I connected where on the pilot light?

Both heater taps are supposed to go to the pilot light, but I didn't make sure it matched the same configuration as the layout.
i.e. the heater which is on pin 9 of the preamp sockets is going to the same side of the pilot lamp in the layout

(which i can't tell which side of the pilot lamp in the layout matches up with which side on the actual pilot lamp fixture.)

but that's not in the schematic, so it doesn't matter, right?

[Phil_S]

I have a bad PT here, good HT winding, good 5V, and shorted 6.3. $60 paper weight.

Sorry to hijack this. What's the HT voltage and any other info? I might take it off your hands depending and if we can reach an agreement. PM me if interested.
Phil

If you just hook up the primary and fuse only and apply power to the primary it blows the fuse instantly.

Oh...I misunderstood. I thought maybe only the 6V secondary was hosed, in which case, I would be able to rewind it. If the fuse blows, I think I'm losing interest. You see, the filament windings are usually on the outside, and typically about 20 turns of fat wire, so they are easy to do. It's still a lot of work, as you've got to take the whole tranny apart. Once you get down into the HV secondary or the primary (usually inner winding), there is too much very fine wire and all that varnish to deal with. You need a mechanical winder, counter, and more knowledge than I have.

[M Fowler]

The layout you have is different from the one I have and mine shows a better layout of the filament wiring. Pay attention to the power tube wiring the 6973 tubes

Oh that is the lite version never mind.

Just wire the PT filament supply to the pilot then to the power tubes and keep the wires going to the same pins on all tubes in tightly twisted pairs. Such as a green wire from pin 4 on power tubes to pin 4-5 on the remaining preamp tubes and pin 5 on power tube to pin 9 on the remaining preamp tubes. It doesn't matter which wire just pick one and be consistent from tube to tube.

[Phil_S]

Would it matter which heater tap I connected where on the pilot light? <snip for brevity>

I think you are confused. Please forgive me if I've misread your post.

The layout diagram show a terminal strip or a turret strip. Two of the terminals are for the 6.3V feed from the PT. You connected each of the 6.3V wires (really 3.15V each) to individual terminals, right?

This is where you branch the parallel circuit. It is wordy to describe what's done. I drew a picture. This isn't really a layout diagram, it's more of a pictogram. (Note, I messed up on the blue line, so ignore the place where it was obviously erased.) Wires should be twisted, like in the photo I posted before.

[Structo]

Another reason to use a light bulb limiter when powering up a new amp for the first time.
You obviously had a very high current draw somewhere that let the smoke out.

Once out, you can't put it back.

I am probably repeating what you already know but, the 6.3v is ac, alternating current.
Some old amps like some early Fenders grounded one side of that.
It really is not a good method to use and should be avoided for a number of reasons.

Each side of the filament winding is 3.15v or there abouts.
To measure the heater voltage you should be measuring between those two wires and should come close to 6.3vac +- .2v

So when checking continuity of that winding and you found contact between ground and one of those wires, there is your problem.

You either use a heater center tap provided on the PT or make an artificial one with a 100 1w resistor off each leg to ground.
Some say it is more balanced than a center tap in many cases.

If it is a typical Fender type pilot light, the two connections are isolated from ground.

[M Fowler]

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.

[Herec]

Would it matter which heater tap I connected where on the pilot light? <snip for brevity>

I think you are confused. Please forgive me if I've misread your post.

The layout diagram show a terminal strip or a turret strip. Two of the terminals are for the 6.3V feed from the PT. You connected each of the 6.3V wires (really 3.15V each) to individual terminals, right?

Yes, I connected each of the wires to induvidual terminals. I've been looking, but I can't find a path to ground.

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?

What are the chances there was a pre-existing short in the PT?