OT Test

[ajeffcote]

What is the easiest way for a bumbling nooby to test an OT for shorts? My test equipment consists of one DMM from Sears.

[spacelabstudio]

Set up your DMM to measure resistance.  Then make sure the leads that are on the same winding have some finite, non-zero resistance between them.  Then make sure the leads on different windings have infinite resistance between them.

[kagliostro]

Give also a look here

http://www.el34world.com/Forum/index.php?topic=12884.0

Kagliostro

[ajeffcote]

Give also a look here

http://www.el34world.com/Forum/index.php?topic=12884.0

Kagliostro

Thank you. I saw that before I posted. I just don't have the switch or the light.

Set up your DMM to measure resistance.  Then make sure the leads that are on the same winding have some finite, non-zero resistance between them.  Then make sure the leads on different windings have infinite resistance between them.

thanks you also, and I will try this. I thought maybe that would work.

[RicharD]

Marshall trafo de saída

[ajeffcote]

Let me add some info to my original request, please. I think one of the output tubes, a TAD 6L6GC was shorted accidentally by a loosely draped ground buss connecting the speaker jack, and both 6L6's to ground. And there are no screen resistors installed as per the Marsh instructions. So I'm thinking a burnt winding in the OT.
Will the DMM testing be sufficient for a catastrophic failure, supposing the winding is now open? Or am I even thinking about this correctly?
By the way, I intend to install screen resistors on the 6L6's. Standard seems to be 470 Ohm 3-Watt+. I have a bunch of 5 Watt resistors but no 470's. I have 2 Watt 470's. I think I have some 1K, or 1.2K 5 Watts. Will they do? I would think that as long as they are that close it wouldn't matter a hair's worth.

[HotBluePlates]

Will the DMM testing be sufficient for a catastrophic failure, supposing the winding is now open? Or am I even thinking about this correctly?

Yes. Unsolder the OT wires from whatever they connect to, then make your measurements.

OR, do one of 2 things:
1. Remove the output tubes from their sockets, measure resistance of the primary from end to end (one plate pin to the opposite side plate pin), and from end to center-tap (one plate pin to B+ node connected to OT center-tap; repeat other side).
2. Remove the output tubes from their sockets, probe plate pins to measure B+ voltage. Presence of B+ means the winding is not open.

You can also measure the resistance of the secondary side with the speaker unplugged. Remember to plug the speaker back in before operating the amp.

If you suspect a fault related to the sockets/jacks, then unsolder the OT leads from these components before making your tests.

By the way, I intend to install screen resistors on the 6L6's. Standard seems to be 470 Ohm 3-Watt+. I have a bunch of 5 Watt resistors but no 470's. I have 2 Watt 470's. I think I have some 1K, or 1.2K 5 Watts. Will they do?

Don't quote me, but I think the old Fender amps used 1w carbon comp resistors, if they were used at all. I say that based on the size of the resistors installed.

We have to assume either a voltage or current (or better, both) to know whether a given wattage rating is sufficient for the resistors. For ease, I'll assume your screen node is 450v. The 6L6GC data sheet says for a class AB amp with 450v plate, 400v screen, that screen current is 2.8mA per tube at idle and 11mA per tube at maximum output.

470 ohms * 2.8mA = 1.32v
1.32v * 2.8mA = 0.0036w at idle

470 ohms * 11mA = 5.17v
5.17v * 11mA = 0.057w

Judging from this, even a 1w resistor should be sufficient. Your 2w resistors should remain nice and cool. If, however, you used a higher resistance, the power dissipated in the resistor would rise accordingly.

Yes, screen current might be slightly higher with 450v screen instead of 400v assumed by the data sheet, and it may be higher or lower with various individual tubes. However, I think we've shown that with a resistance as low as 470 ohms, we don't need a high wattage rating.

[ajeffcote]

Thank you HotBluePlates, that's what I was looking for.