Fried Output Transformer??

[Aaron]

Hi Everyone,

I think i've fried my output tranny. Just finished a new build, thought I had double checked everything before turning on. And when I did, output valves made a weird sound and got an instant electric smell.

Shut it off straight away. Triple checked and noticed that I had wired the C.T and an Anode wire the wrong way round

Swapped it over and turned it on, with no valves in, no burning smell voltages are fine. Plug in valves and get a weird static sound and voltages don't measure (my meter only goes to 600V DC).

Have I sent it to it's grave? Is there a way to test with out the valves in?

I wouldn't mind so much if it was a new tranny, but it was out of a '72 100W Marshall Super Lead

Any info would be much appreciated.

Thanks,

Aaron

[martin manning]

First I'd try measuring DC resistance of both sides of the OT primary (should be within ~10% of each other), and deck for a shorts from all windings to each other and to the core steel. If those tests are ok then you could try a test for shorted turns like this: http://www.premierguitar.com/articles/t ... r-tester-1
If that checks out then maybe your problem is somewhere else. I don't know why the OT couldn't stand being connected that way for a short time.

[Blackburn]

If you measure the resistance on both sides of the primary to CT (Amp off, unplugged) and they measure relatively close, it may be okay. That's what I've been told and is an easy way to check.

[Phil_S]

I'm sorry to say, that smell is usually a bad sign, but there is always a chance you got lucky. You can do some tests to see if it is the OT or the tubes. I'd test the OT and if it checks out, I'd assume it is one or both tubes that went bad.

To check the OT there are several things you can do. Pull the output tubes and leave the power off. Discharge the caps.

1) Simple Ohm's test: lift (desolder) the CT from the power supply. Check the Ohms reading on the primary from each outer leg (plate connection) to the CT. They should be approximately the same. If they are equal or very close to equal, that's a good sign. A slight variance between sides is normal. Add the numbers together and then for good measure check the Ohms across the two outer legs and you should get the sum of the halves (or darn close). If any read open or infinite, you probably have the answer you don't want, but we all have to face facts.

2) Check the secondary winding, too. It will be a very low number of Ohms. Just make sure it isn't open or infinite.

3) If the primary passes on #1, you can test for turns ratio. Pull all tubes Use the filament supply from your PT to apply 5VAC or 6.3VAC to the OT secondary and see what the AC voltage appears on the primary. You can use jumper from the filament supply on the tube sockets to power the OT secondary. If the voltage checks out OK, your OT is probably OK.

I'm not sure what the OT ratios are for this piece of iron, but roughly, lets assume it is a 4K primary and 4-8-16 ohm secondaries. The turns ratio is the square root of the impedance ratio. So sqrt(8000/4) = 44.7:1; 8 ohm tap will be 31.6:1 and 16 ohm tap will be 22.4:1. Assuming 5VAC input, expect output of 224VAC, 158VAC, or 112VAC for 16, 8, and 4, respectively. If you don't know the primary impedance, these are OK ballpark numbers. If you know the primary impedance, adjust the expected results accordingly.

4) If it passes the turns ratio test, you really need to look for internal shorts with this thing: http://www.premierguitar.com/articles/p ... r-tester-1

Edit: I'm slow...I see Martin get there first with #4.

Good luck with this.

[pdf64]

Plug in valves and get a weird static sound and voltages don't measure

If measuring plate voltage, putting the meter probe on to it may start the amp oscillating, causing weird readings.
Try just fitting the power tubes.
What static current are they drawing?

I don't see that the initial mixed up connection should have caused significant damage.
Maybe it oscillated heavily and warmed up the screen grid resistors?
Do they measure ok?
Pete

[martin manning]

1) Simple Ohm's test: lift (desolder) the CT from the power supply. Check the Ohms reading on the primary from each outer leg (plate connection) to the CT.

No need to disconnect the CT. The plate leads are not connected to anything so you can get a good resistance measurement with the CT connected.

[Firestorm]

As Martin said, if that's all you did, I can't see how the OT would hate it enough to die. It would just be an oddly designed SE. Did you put signal in it?

But a quick burning smell suggests a short somewhere. Anything else miswired?

[Tillydog]

The 'Light bulb limiter' is cheap insurance....

[/broken record]

[Aaron]

Thanks for the info! I'll check after work.

Aaron

[Aaron]

Okay, did a quick check on the primary and secondary.

Primary
C.T white - Anode red:- 17.9 ohms
C.T white - Anode brown:- 16.2 ohms
Anode red - Anode brown:- 33.1 ohms

Secondary
1.3 ohms on each output

I measured the Twin Reverb transformer that I replaced it with and measured just over double the readings, 36.7 ohms and 39 ohms and 1.4 on the secondary.

I did a continuity test with the multimeter and had no shorts, but will do the premier guitar test later tonight.

Aaron

[Aaron]

Here is a schematic and picture.

When taking the picture I noticed that the 6k8 resistor in the power supply looked a bit burnt. It still measures fine though.

The choke measures 100 ohms.

Aaron

[Phil_S]

I think those readings seem reasonable. There is hope of no damage. Do the neon bulb test for internal shorts to be sure.

[Aaron]

Well, its official...........

I'm gonna go out on a limb here.........

I would have to be the stupidest, slowest, half wit moron you will ever meet!

Wired the common of the secondary to the output of the ohm selector switch. So the output was hard wired together

Got sound now, but hopefully no damage done.
Although thanks so much for the help/suggestions/ideas. You're all winners!!!!!

Now onto the next issues.......

[M Fowler]

We all make mistakes glad you found the problem and the amp lives.

[Firestorm]

Well, its official...........

I'm gonna go out on a limb here.........

I would have to be the stupidest, slowest, half wit moron you will ever meet!

Wired the common of the secondary to the output of the ohm selector switch. So the output was hard wired together

Got sound now, but hopefully no damage done.
Although thanks so much for the help/suggestions/ideas. You're all winners!!!!!

Now onto the next issues.......

Something burned, tho. Can you tell what?