OT failures anyone?
Moderators: pompeiisneaks, Colossal
OT failures anyone?
Has anyone experienced an output transformer failure on a real Trainwreck or clone? I was just looking at a thread about the diodes on the power tube plates of an Express. Did the amp that failed have the diodes? They are a form of output transformer protection. There are other forms I'd like people here to try to see if any make an audible difference.
Re: OT failures anyone?
I've not had failures....
So, what causes the failure and what do the diodes protect against?
The diodes protect against the case of having no load attached to the output of the OT. With no load, the OT looks like a near pure inductance to the power tubes. As the power tubes try to alter the current in the OT, the OT responds with a very high voltage in it's attempt to keep the current constant.
It's this very high voltage that causes damage. You can get arc-over inside the power tubes or across windings within the OT that can cause permanent damage.
The diodes, which are reverse biased in normal use and have no influence on operation or tone, provide a shunt path for the OT inductance to dump energy without causing high over-voltages in the event the amp is played without a load attached.
One alternate solution is a 200 ohm, 3W resistor across the output jacks to ensure there is always some load, even when a speaker is not plugged in. This resistor could be wired to a switching jack if you want it removed from the circuit when a speaker is plugged in.
So, what causes the failure and what do the diodes protect against?
The diodes protect against the case of having no load attached to the output of the OT. With no load, the OT looks like a near pure inductance to the power tubes. As the power tubes try to alter the current in the OT, the OT responds with a very high voltage in it's attempt to keep the current constant.
It's this very high voltage that causes damage. You can get arc-over inside the power tubes or across windings within the OT that can cause permanent damage.
The diodes, which are reverse biased in normal use and have no influence on operation or tone, provide a shunt path for the OT inductance to dump energy without causing high over-voltages in the event the amp is played without a load attached.
One alternate solution is a 200 ohm, 3W resistor across the output jacks to ensure there is always some load, even when a speaker is not plugged in. This resistor could be wired to a switching jack if you want it removed from the circuit when a speaker is plugged in.
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LeftyStrat
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Re: OT failures anyone?
I like the diode approach or the nonswitched resistor better. Someone can always plug in a speaker cable into the head and forget to connect the other end.
Not that I have ever done that.
Not that I have ever done that.
It's never too late to have a happy childhood.
Re: OT failures anyone?
I'm not crazy about the diodes approach, as I would would doubt the diodes could take abuse for long, and then you'd be dead in the water until you replaced the diodes. The "always on" 220 ohm is safe, easy, and lasts indefinitely, abused or not!
--mark
--mark
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LeftyStrat
- Posts: 3117
- Joined: Fri Jan 14, 2005 6:58 pm
- Location: Marietta, SC, but my heart and two of my kids are in Seattle, WA
Re: OT failures anyone?
Well I can vouch for the diode approach being problematic. Here's a review from HC of the ACC 165 tube amp that only used one diode:
"It had two or three meltdowns in the 10 years I used it exclusively. The diodes would fry and then the fuse would blow and you were done. My older brother/amp tech has snipped the diodes and there have been no more problems. He thinks that the diodes were there to protect the power amp section in the event that the speaker cable was unplugged. "
Note the attached schematic where they are crossed out.
"It had two or three meltdowns in the 10 years I used it exclusively. The diodes would fry and then the fuse would blow and you were done. My older brother/amp tech has snipped the diodes and there have been no more problems. He thinks that the diodes were there to protect the power amp section in the event that the speaker cable was unplugged. "
Note the attached schematic where they are crossed out.
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It's never too late to have a happy childhood.
Re: OT failures anyone?
Paul is right about the situation when no load is connected but, a speaker can also generate spikes when severe overdrive creates a dead zone in the crossover region. It's a long story but I designed a solid state power amp that could easily produce 100V spikes with +/- 28V rails. A resistor across the speaker will limit the spikes. An RC snubber is also effective. Trainwreck and Dumble amps get driven pretty hard so it made me wonder.
Try connecting a scope across the speaker while playing. Set the clipping level to about 2 divisions and the spikes will look like grass growing out of the waveform ( if the amp has no protection ).
I think the 820 ohm feedback resistor in a Black Face Fender amp limits the spikes to save levels.
Try connecting a scope across the speaker while playing. Set the clipping level to about 2 divisions and the spikes will look like grass growing out of the waveform ( if the amp has no protection ).
I think the 820 ohm feedback resistor in a Black Face Fender amp limits the spikes to save levels.