Is my OT BAD???

[skyyamps]

Hey guys! I think my Push Pull OT might be blown. Are there some easy tests I can run to test it out? The extent of my electrical equipment is a pretty basic multimeter...

Thanks!

[Wayne]

You've got enough equipment to get a good start.

First the usual disclaimer - what I'm about to suggest are tests that need to be done with the amp OFF, the tubes cold, and the filter caps DISCHARGED. Failure to adhere to both of these conditions can result in death to you, or your amp, or both. If that statement makes you uncomfortable, that's a good indication to me that you should seek the help of a qualified tech.

1) Measure the resistance between the plates of the output tubes (pin 3, assuming they're octal tubes such as 6L6, 6V6, EL34, etc.) If your amp has 4 output tubes, measure between the pin 3's of the two outside tubes, i. e. the two the farthest away from each other.

2) Measure the resistance from each pin 3 used above to the +ve lead of the first filter cap. Should be roughly half the resistance measured in step 1.

3) Measure resistance between each pin 3 and ground (chassis). Should be infinite - there may be a brief "blip" or meter deflection as your meter tries to charge the filter.

Let us know how that works out, and please don't go "bzzzzt" in the process. Also, what kind of amp is it and what did it do to make you think the OT was toast?

W

[skyyamps]

I'll get started on those tests, Thanks! It's an amp modeled after the JCM800 using a JCM800 replacement tranny. I just finished building it, and after powering it up and troubleshooting. I am not getting anything to the speakers. I have pretty much tried everything, but have had no luck. I am resorting to testing the OT.

Here's a link to the wiring diagram.
http://www.tubesandmore.com/cemirror/in ... wiring.gif

Also, I'll attach my schematics.

[skyyamps]

Bad news, absolutely no resistance between the two plates or between the plate and B+. I think I have a dead tranny... is there any physical way to tell for sure?

[skyyamps]

One more thing, I've been reading up on symptoms of a blown OT, and one is the Power Tubes will overheat due to the excess voltage unable to pass through the OT. I noticed my EL34s heating up a little too much but disregarded it as bad biasing. I think the OT is bad news.

[Structo]

Sounds like it's blown.

I had a similar issue one time.

I measured and didn't think my readings were correct on the secondaries of the OT.

I tried everything I could think of and was just not getting anything to the speakers.
I was getting ready to cut the OT out of the chassis when a wild thought came to me to try a different speaker cable.

Yep, the cable some how went bad in between the last time I had used it and when it was on the bench.

Bought a new cable and it worked fine.

But since you are measuring infinite resistance on the secondaries, and on the center tap to either side, I would hazard a guess that it is blown.

Do you have any other OT's around that you can measure to get an idea of what to expect resistance wise?

[dobbhill]

Tom:
He wrote "no resistance" which means 0 ohms to my old brain.
Sky- did you mean 0 ohms, or infinite ohms measured across the primaries?
Just trying to help....
D

[skyyamps]

I meant 0 ohms of resistance. I can use the two plates to complete a circuit well enough to light a pilot lamp with a 9V battery. I don't think its the speaker. It's working fine.

[Phil_S]

See if you can jumper the a/c filament voltage to the secondary. That will give you a/c voltage on the primary if it's still working. To do this, pull all tubes and disconnect the speaker before you attempt to power on. You can use a pair of alligator clips from the filaments to the output jack. You can meter for output voltage at the power tube pins.

You can pretty well predict based on turns ratio. Roughly, you'll get 150vac from a 16 ohm secondary, 200vac from an 8 ohm secondary, and 300vac from a 4 ohm secondary. Use the highest ohm secondary available. If you get nothing, the primary is blown.

[Firestorm]

A couple of things (three actually):
Your schematic doesn't show any ground connection on the OT secondary. That's the black "common" wire. If that's not connected, even a good OT won't work.
You have a presence control, but no feedback loop is shown, so there's no point in a presence control. You could do a Vox "cut" control w/o the loop, but that's another story.
Because the the way the presence pot is wired in the schematic, you have no DC ground for the PI's tail resistor. Pretty sure that can't work either.
Maybe it's just the schematic, but if the amp itself is wired this way, it shouldn't work.

[Wayne]

If you're going to use Phil's idea, above, make damn sure you've got a fuse in the primary!

If your OT primary is shorted, current flow could possibly get quite big.

W

[Structo]

You show a 500ma fuse on the line voltage.
That should be a 2 amp slo blo.
The 500ma fuse is usually used for the HT side.

This again is a great place to use a light bulb current limiter when powering up an amp for the first time.
Especially one that you aren't 100% sure of the design.

Let's get back to the OT for a moment.

Unless you have a good meter that will measure low resistance you probably aren't getting a good reading.

Also, running a small light bulb with a 9v battery isn't going to show you much.
The secondary is just one long piece of wire so it is going to show continuity.

I would build a light bulb limiter if you don't have one then make sure your OT is grounded on the secondary (black wire to chassis).

[skyyamps]

Hey, regarding the fuse in the Primary, it is not labeled correctly in the schematic. It should read 2A. I didn't have the Common grounded, so I will try that! Thanks guys!

[skyyamps]

Guys! You rock!!! Turns out I am a moron for the whole presence thing and even worse, the OT ground. But I am happy to say...


MY AMP MADE NOISE!

That's a start. Its pretty low volume and if it gets too loud, it makes a gross farting noise!

SO now we are here. The presence had been removed and the 10K resistor to the presence has been replaced by a 47K to ground. The tubes seem to be running too hot, and I imagine that's a symptom of the bigger problem...

Any ideas again guys?

[Wayne]

If your output tubes are running hot, I would tackle that before anything else - it can get expensive!

What is your bias voltage w/no signal? Also, what voltage do you measure at each cathode (tops of the 10R resistors)?

W