What would cause a flyback diode to fail/short?

[ampfab]

I just repaired a bugera 333xl that was blowing the main fuse. The cause of the excessive current draw was a shorted flyback diode connected between the plate of one of the power tubes and ground. I think of flyback voltages or spikes as being caused by intermittent connections. Could this have been caused by a bad speaker cable or wiring connection to the speaker, output transformer connections or bad power tube connections/grounds? All looks good, but I don't know the condition of the speaker cable or speakers that had been used with this amp. I replaced to original diodes with three 1n4007s in series because I wanted to improve upon the original 2000volt diode. Any suggestions?

[Structo]

Aren't those diodes for protection in case a power tube shorts out?

The Trainwreck Express uses three 1N4007 in series on each power tube socket.

I think it was an Ampeg trick that Kenny borrowed and thought it was a good idea.

[Bill Moore]

I just finished a Music Man rehab that had one of those diodes shorted. It had a 5KV 250ma which I was unable to find. Found a Fender type 3KV 250ma and replaced with them. In my search, I also found Peavey had 2KV 250ma availlable.

[ampfab]

I used the trainwreck inspired 3 1n4007s. Just want to make sure I addressed the cause of the failure, not just the symptom of the failure. In other words, was this just caused by the user using a crappy speaker cord, or should I replace the seemingly nicely working power tubes? I suppose I could of had a faulty diode, but want to cover all my bases.

[Bill Moore]

I got advice/direction from several folks as to the function. One link said that the OT would be protected from transient HV, stating that the voice coil could generate substantial voltage (on the tube side) by returning to center after an excursion. Others felt that the main function was to guard the OT against opens in the speaker circuit. I am not a tech, but through research found that many designs are now using these diodes. For some reason the old Twins, and Deluxes didn't need them.

[es345]

Some technical background, these diodes indeed are intended to protect the OT:

Assume the following scenario including a descriptive explanation what happens:

You are running the amp with full output power then suddenly the connection to the speaker gets lost when you have a current peak . Now the OT is working like an inductance . The behaviour of the inductance is to continue delivering the same current but the resistance is open. The result is that the voltage is rising without limitation. Worst case the insulation of the OT is damaged.

Now let us have a look to the primary side of the OT. One side will have a positive peak, the other one a negative, less than 0V. With installed "flyback diodes" the negative voltage will be limited to -0,7V (in case of one SI Diode) and current flow will start limiting the Peak voltage.
As a result also the voltage at the other side of the OT Primary will not rise any more.

What can cause a damage of the diodes?

some possible root causes (there could be other in addition)

- too much current
- loosely coupled parts of the both Primaries resulting in too high voltage even with diodes installed. As a result the maximum allowed voltage of the diode may be violated

If you are willing to spent some of the output power for protection, e.g. 5W in case of a 100W amp:
install a resistor with appropriate powerclass an value in parallel to the LS jack. It will change a little bit the load situation at the LS side but this is not critical for operation.


Hans- Georg

[cbass]

Yes a bad speaker cable or no load will blow those didoes.

[JerryFJA]

bad tube
no load
wrong load
bad cable
running an attenuator