Blown OT??

[lowell]

I was playing a gig other night, in middle of guitar solo my amp cut out completely. I looked in back and the speaker clip had fallen of the speaker terminal... I new it was bad, but not THAT bad until tonight at a gig when I had to turn the amp all the way up and even then it was very low volume and very distorted, to the point that I was far from being able to use it for the gig. I need to do the ot tests on the geo site... just wondering if someone thinks that this quick little situation could fry the OT that fast. It's a 5E3X2 circuit running cathode biased, 5y3 rectifier, 12ay7 preamp, and a Weber xformer.

[Enzo]

Maybe it did, maybe it didn't, but it certainly could.

[PRNDL]

just wondering if someone thinks that this quick little situation could fry the OT that fast

It's unlikely because transformers are fairly sturdy devices (basically wire, tape and iron). Frying them takes enough current to burn either the wire or the tape (by melting the glue, which lets the wires touch).

Most likely something fried in the output section - a tube, grid resistor or an electrolytic.

[Mars Amp Repair]

Well,
I like the PRNDL handle...wonder how many of the younger generation know what that is. You do of course rememeber the PNDLR's I hope! I think the Studebaker's had that one. I think I had a '63 Buick Tempest that was a PNDLR.

Anyway, I have had output transformers flyback without a load & actually hear them arcing inside, so tho possibility not the most likely culprit, the OT certainly could have arced & shorted some windings. It happens. Substitution the easiest way to determine that...unless you have no other xformer that will work temporarily. glen

[lowell]

ok guys, pulled tubes and tested it and both the 8 and 4 ohm taps on the secondary are shorted to the core... damn! Oh well, lessons lessons lessons. I think that the speaker clip must not have been on very well and the entire 3 hour gig was taking its toll on the OT. Is there anything cool that can be done w/ fried OTs?

[Enzo]

Um, did you disconnect the secondary wires from the speaker jack for this? The frame is bolted to chassis, the output winding is usually grounded to chassis at the jack. SO unless you pulled those wores off, it would appear shorted to frame, but not necessarily be.

PRNDL, as Mars mentioned, the danger to on OT without a speaker load is not from current burning it up with heat. It is the lack of termination that lets the transformer act like the ignition coil on your car or the flyback transformer in a TV. It makes huge voltage spikes that will arc over the insulation on the windings. It can kill a tranny in an instant. And that is why many amps have those reverse diodes from plate to ground on output tubes. They are even called flyback diodes. They are there to HELP prevent such arcs inside the tranny.

[PRNDL]

Thanks Enzo ... as you can tell, I have some knowledge about electronics, but am learning a lot about tube amps, which is its own specialty.

[drewl]

Got Mesa Dualrectumfrier which blew a tube when the owner fired it up with no load.....he didn't say how long he had it on....but it blew a 6L6.

[6267]

Most likely the next step is to remove the transformers entirely (desolder instead of just clipping, it will be easier to reinstall). Check the resistances outside of the circuit. Then check the resistors in the output section. I bet it's a screen resistor or a cathode bypass cap. Perhaps a cathode resistor instead, due to low volume. One tube is working into the push pull, and with only push or PUSH-(pull) it's sounding weak and distorted.

[lowell]

Drewl,
I could be wrong but don't see how a disconnected secondary on an OT would have any negative affect on any components on the primary side. Anyone? Is this right or wrong? From how I see it the output tubes are still seeing the same load whether the secondary is connected or not. Someone please correct me if I'm wrong. Although if the OT arcs and the primary gets shorted too then this would cause some problems.

[6267]

Sometimes, but not always, one side of the secondary of an OT may be grounded in addition to running to the speaker. This would read as a short to chassis because the secondaries DC resistance is generally small. If the primary is shorted to chassis, you have a problem. Most blown OTs I've seen end up being open, not short. The DC resistances are much less than the AC resistances as well.

[Enzo]

AN uloaded secondary can destroy an OT with one grand chord on your guitar.

Transformers are like gears, they transform high voltage and low current to low voltage and high current. And like gears, it all matters what they are connected to and what they are doing. Gears have no inherent speed. Transformers have no inherent impedance. Your OT may say 3000 ohms on the primary, but the tubes see 3000 ohms ONLY when the secondary has the specified load.

SO to say the primary has the same load no matter what is not correct. The primary sees whatever load the secondary has, but transformed accordingly. In our example, you put that specified 8 ohm load on the secondary, the tubes see the 3000 ohm. (I am making up an example here, I don't know your numbers) But connect a 4 ohm load to the same secondary, the tubes no longer see that 3000 ohms. That is why we make various taps on the secondaries and why we try to match the speaker to the tap. The tubes do not see the same load no matter what.

Now someone else might be able to explain the physics better than I, but when there is no load, a large signal on the primary can result in a huge voltage spike. Without the load to dampen the waveform, the primary is free to act as an inductor, and it does the same thign the ignition in your car does. That inductance tries for all its worth to keep the current flowing, and it does this by ramping the voltage up with the energy at hand. That is what inductors do.

If that voltage spike is large enough, the primary winding will arc between windings or from winding to frame. And at that point the transformer is destroyed.

NEVER operate a tube amp without a load.

[6267]

I heartily agree with enzo's post, but I'd add that goes for transformer coupled reverb drivers too! Although easier to replace and far less costly, leaving your reverb tank unattached while an amp is on the bench can be a recipe for nasty flyback on the preamp B+ lines and bye-bye reverb transformer and possibly tube as well. Sucks when you go to play the show later too and find out you have a nasty hum and crackle generator instead of reverb.

Transformer impedance is a reflected one, based on the square of the turns ratio (i.e. a 120v to 5v transformer will reflect a 4,608 ohm impedance with a load of 8 ohms -- use those old power transformers as bad output transformers!). However, the speaker is not a pure resistive load of 8 ohms. It may range from half of that to a couple times that at varying frequencies. The danger to output sections apparently is too high a load on the secondary, including the infinite impedance of an unconnected secondary (unloaded is seen by the primary as an infinite impedance). Although I do not plan on trying it, I've heard that an output section can deal with a dead short between the speaker wires much better than having them unconnected (do not try at home!).

[Bruce / Mission Amps]

AN uloaded secondary can destroy an OT with one grand chord on your guitar.

Transformers are like gears, they transform high voltage and low current to low voltage and high current. And like gears, it all matters what they are connected to and what they are doing. Gears have no inherent speed. Transformers have no inherent impedance. Your OT may say 3000 ohms on the primary, but the tubes see 3000 ohms ONLY when the secondary has the specified load.

SO to say the primary has the same load no matter what is not correct. The primary sees whatever load the secondary has, but transformed accordingly. In our example, you put that specified 8 ohm load on the secondary, the tubes see the 3000 ohm. (I am making up an example here, I don't know your numbers) But connect a 4 ohm load to the same secondary, the tubes no longer see that 3000 ohms. That is why we make various taps on the secondaries and why we try to match the speaker to the tap. The tubes do not see the same load no matter what.

Now someone else might be able to explain the physics better than I, but when there is no load, a large signal on the primary can result in a huge voltage spike. Without the load to dampen the waveform, the primary is free to act as an inductor, and it does the same thign the ignition in your car does. That inductance tries for all its worth to keep the current flowing, and it does this by ramping the voltage up with the energy at hand. That is what inductors do.

If that voltage spike is large enough, the primary winding will arc between windings or from winding to frame. And at that point the transformer is destroyed.

NEVER operate a tube amp without a load.

Another good reason to always have a 270 ohm to 470ohm 5-10 watt resistor across the secondary of your output transformer.

[Mars Amp Repair]

yep,
Just looked at a TSL that the cust ran at high volume with no load. He said he was just using the loop for something & didn't know it was a bad thing to do.

One EL34 had air in it but there were no visible cracks anywhere on the bottle. I'm guessing it arced inside the base of the tube & possible cracked between the pins. Wild...glen