Duplicate PT Secondary for Rectifier Only

[anrque]

Hi, I've searched for this on Google and forums and seem to find partial answers, so I figured I'd ask the question direct here for the exact thing I am seeing.

On a small Voice of Music PA I bought with a 6x4 rectifier, the power transformer has a 2nd 6.3V secondary with only the rectifier heaters connected. I also saw a schematic for an old Teisco practice amp this morning that showed the same thing. Unlike the 5Y3 that uses 5V, the 6x4/EZ90 uses 6.3V. So why a 2nd secondary to feed just the rectifier? The current required for the 6x4 is only .6A unlike a 5Y3 that draws 2A.

[sluckey]

Precaution. Heater to cathode short is a fairly common tube failure. When you put that 6X4 on the same winding as the other tubes you risk putting B+ on the filament string which can be disastrous. Best to just keep the 6X4 isolated from the other circuits.

[anrque]

So, simply keeping the rectifier heaters in isolation to the other tubes heaters?

[tubeswell]

So, simply keeping the rectifier heaters in isolation to the other tubes heaters?

... because under ordinary operation, the rectifier cathode sits at the B+ voltage (i.e. several hundred volts) and so if the rectifier filament was powered by the same filament winding as the heaters for the other tubes (and the heater winding for the other tubes normally only sits about 3.15VAC above ground potential with a pair of 100R balancing resistors), it would mean the heater-to-cathode voltage on the 6.3V rectifier would end up being several hundred volts - increasing the likelihood of heater-cathode insulation failure - causing the B+ to short to ground. This isn't too bad if you have a rectifier (like an EZ81) that can handle high H-K voltage. 6X4 and 6X5 aren't as reliable as EZ81s in this regard.

[anrque]

Hey, thanks folks! Every day I learn a new little nugget about our old tech.