On Diodes

[silverfox]

Just started reading this, https://www.dalmura.com.au/static/Power%20supply%20issues%20for%20tube%20amps.pdf ; someone posted a link to earlier.


Anyway: ". . . . and it seems obvious that a ‘1A diode’ is going to be appropriate for a 100-200mA power supply load." Well, yes. After all, I've used those diodes in 100 watt designs. But . . . How is it say, . . . a 100 watt amp those 1 amp diodes are handling 25 watts each? Or is a 100 watt amp only instantaneously generating power? RMS Power?

The phunny phont is phrom a cut and paste elsewhere.

Fox

[AmberB]

Wattage is a combination of volts and amps.  At 350 volts B+, a 1 watt amp can pass about 350 watts of power, as long as it's rated for both the amperage and the voltage.  That's why people tend to use high voltage capable diodes in their tube amps.

Perhaps someone else will jump in and correct me if I've calculated this wrong...

[pdf64]

The rectifier diodes aren’t handling power as such. Rather they job is to pass forward current and stop reverse voltage. Being imperfect devices, when passing forward current, they have some loss, eg 0.6V volt drop x 1 amp = 0.6W.
The diodes will work just the same supplying a 10W solid state amp as a 100W tube amp.

[trobbins]

A rectifier diode in a valve amp has to pass the peak and rms current levels required by the power supply.  A diode has limits on the peak and rms levels it can cope with (whether it's a solid state or valve diode).  However those current values don't directly relate to 'a 100W amp'.  Perhaps you have a 100W amp with a pair of KT88's, that has an idle B+ requirement of 600V and 100mA, such that each KT88 dissipates 600V x 50mA = 30W. 

With a signal, that B+ current will increase - how much will depend on loadline and some other conditions.  A quick estimate could be the KT88 plate current reaching 400mA with a 5kPP output transformer, indicating an output power to the speaker of (Ipk)2 x Rpp / 8 = 0.4 x 0.4 x 5k / 8 = 100W, and the average B+ current could increase to about 250mA average, and the power supply could be delivering 600V x 250mA = 150W to the amplifier.

Note that the article refers to '100-200mA' as a comment, for the reason that once a detailed assessment is done, it could well be that 200mA may be an upper limit for a UF4007 when used in a power supply with a lot of filter capacitance and a low DCR winding transformer, especially if the diodes sit in a hot section of the amp and have little cooling, and the amp is in a hot venue.

PS. The diode current is also not the DC output current of the power supply - it is related, but the relationship depends on power supply configuration, so additional information is needed such as measurement, or simulation, or design calculations, or rule-of-thumb, or checking like equipment.

[silverfox]

The rectifier diodes aren’t handling power as such. Rather they job is to pass forward current and stop reverse voltage. Being imperfect devices, when passing forward current, they have some loss, eg 0.6V volt drop x 1 amp = 0.6W.
The diodes will work just the same supplying a 10W solid state amp as a 100W tube amp.

Thanks. It's been a while and I had forgotten that. I did remember it about 3:30 am.


silverfox.