I have read about diodes used to protect the rect tube

[catnine]

Can anyone explain to me how diodes between the PT 5 volt leads and the plates of the rectifier tube allows the amp to still work only at a higher SS B+ if the rect tube shorts out. Hows does the voltage get from the diodes on the rect plate to the anodes to power the B+ rail . does this mean that when the rect tube shorts out that the plates and anodes in the rect tube short they are now touching and this is how the amp works off the diodes. What if only one plate and anode short together?

I never understood this concept I just have a fuse between the pin 8 of the rect tube and the B+ junction . This fuse would blow if there was a short after the rect pin 8 but would it also blow if the rect tube shorted internally?

I have seen schematics with fuses in the 5 volt leads and also after pin 8 and diodes in series with the 5 volt leads to the rect plates and even fuses in the 6.3 heater taps .

Fender and no other amps I'ver ever seen had all this protection .

[Diablo]

I think you have it wrong. The two diodes go between the transformer HT leads and the two anode plates. The idea is that if the rectifier shorts - either one or both anodes touch the cathode - this would send AC down your B+ rail, and that would be bad. By sticking the diodes in the circuit, you will only get DC on your B+ rail, even in the event of a rectifier short. However, the DC voltage will be higher because the resistance of the diodes + shorted rectifier tube will be less than that of a good rectifier.

[Enzo]

Yeah, what Diablo said.

If your diodes were from plate to cathode (heater) on the rectifier tubes, they would simply conduct and the tube would become invisible. if the tube then shorted, it would be the same as shorting across a diode. And fuses would blow.

If the rectifier tube shorts, then it becomes a wire, essentially. SO with protective didoes between transformer and recto tube plates, if the tube turns into a wire, you still have the diode to do the rectifying. And the substantial voltage drop across the typical rectifier tube is replaced by the half a volt drop across the diode.

[catnine]

Originally Posted by Enzo

Yeah, what Diablo said.

If your diodes were from plate to cathode (heater) on the rectifier tubes, they would simply conduct and the tube would become invisible. if the tube then shorted, it would be the same as shorting across a diode. And fuses would blow.

If the rectifier tube shorts, then it becomes a wire, essentially. SO with protective didoes between transformer and recto tube plates, if the tube turns into a wire, you still have the diode to do the rectifying. And the substantial voltage drop across the typical rectifier tube is replaced by the half a volt drop across the diode.

Ok but what happens when the tube becomes a wire yet you have the 5 volt cathode heater getting the full HT voltage fed into it. Or does the HT say 330 VAC take the path of least resistence and power the amp and not flow into the 5 volt cathode winding of the PT?

[Diablo]

Originally Posted by catnine

Ok but what happens when the tube becomes a wire yet you have the 5 volt cathode heater getting the full HT voltage fed into it. Or does the HT say 330 VAC take the path of least resistence and power the amp and not flow into the 5 volt cathode winding of the PT?

If you use two diodes where we described, you don't get 330 VAC on the cathode winding. You only get DC (around 330V) on the cathode winding. That DC voltage will not produce a large current on the cathode winding, because the cathode winding doesn't connect to ground. The DC voltage just goes down the rail and powers the rest of the amp.

[Enzo]

When the tube is operating properly, your 5VAC winding has 400-500vDC or however many on it.

[catnine]

Originally Posted by Enzo

When the tube is operating properly, your 5VAC winding has 400-500vDC or however many on it.

Yes that's right it does , sorry I was not thinking this through .

[mike_mccue]

What is a suitable diode for this application?

Will a IN4007 do the job?

thanks,
mike

[Robert M. Martinelli]

1N4007s PIV is 1000 V, so I think they're OK if working up to 500 V, because the diodes should be able to withstand at least twice the voltage; for peace of mind ( or if your amp operates over 500 V ) you can always put two 1N4007s in series to each PT leg ( at the cost of an "additional" .5 V drop on each leg ) so that the PIV becomes 2000 V.

Cheers

Bob

[mike_mccue]

Thank you.

best,
mike

[Diablo]

I use IN5408 diodes on my rectifier sockets. They can handle a little more current (3A), and they're still plenty cheap.

[mike_mccue]

Thank You.

best regards,
mike

[Don Moose]

Originally Posted by Diablo

I use IN5408 diodes on my rectifier sockets. They can handle a little more current (3A), and they're still plenty cheap.

Um, that would be 1N5408 (one N five four zero eight) - the search goes a lot better.

Hope this helps!

[Old Tele man]

...placing the SS-diode in SERIES with each rectifier plate, provides protection to both the PT and following circuits. It protects the PT from "seeing" a dead AC-short across that leg should the rectifier short; if the rectifier goes open, nothing (literally) will happen as the SS-diode won't be able to conduct either.

...placing the SS-diode in PARALLEL with each rectifier plate provides "operating" protection against the rectifier plate going open, but does nothing to protect against a rectifier plate shorting.

[TheTinMan]

Originally Posted by Diablo

I use IN5408 diodes on my rectifier sockets. They can handle a little more current (3A), and they're still plenty cheap.

I use UF5408 diodes for just about everything. "Ultrafast" is supposed to have less switching noise IIRC. 1,000 volts, 3 amps, and one part to stock. The cost difference might matter if I were building thousands of amps but I'm not.

Chip