Reverse diodes plate to ground - really need them??

[LooseChange]

Just curious... Are those "protection" diodes that go from power tube plate to ground really needed? Or are they an overkill UL/CSA requirement?  I've had more than one amp with these gone bad. I replace them but would the OT had blown if these diodes were not there?

Most modern amps have a HT fuse too, isn't that enough??

Thanks!!

[FYL]

The jury is still out on this.

[Merlin]

This was discussed recently at MEF:
http://music-electronics-forum.com/t20957/

They aren't strictly necessary. Lots of people do without them, especially if there's a permanent resistor hooked up to the secondary to protect against a disconnected speaker.

[Bierschinken]

Hello,

a thing that is often ignored; the peak which damages the OT while running without a load endures only a nick of a second and standard diodes are too slow to open up and let them pass to ground.
Therefore you have to use realy fast diodes such as booster diodes used in old tube-monitors.

- Swen

[Merlin]

a thing that is often ignored; the peak which damages the OT while running without a load endures only a nick of a second and standard diodes are too slow to open up and let them pass to ground.

Ordinary diodes are fast enough to clamp the voltage across a relay coil, so I would expect the same to apply in this application?

[Bierschinken]

Ordinary diodes are fast enough to clamp the voltage across a relay coil, so I would expect the same to apply in this application?

Honestly I´m not familiar enough with those things but to my extend the relay coil differs from the OT in the way they work.

If you turn off the voltage from the relay, the coil starts a process of selfinductance to maintain the currentflow. So the reverse-voltage raises "slow" but steady to the point where the diode breaks through and shunts to ground. So a standard diode is sufficient.

On a OT it appears that there is not such a steady process of selfinductance but a very fast impulse-like inductance with pretty strong peaks.
And it´s those peaks which only last hundredth of a second or so which are too fast for standard and "UF"-Type Diodes.

As I´ve sai, I´m not an expert on this, but this is how I think it does work 


- Swen

[LooseChange]

I asked this question over at the Powerscaling.com board and they came up with a "no" on the diodes but instead adding a "tethering" resistor 330k in it's place.

[Tiny_Daddy]

I haven't seen those diodes do anything except short out and blow the fuse. Most amps don't have them.

[sluckey]

"tethering" resistor

Now there's a term I've never heard.

[LooseChange]

KOC terminolgy.