Resistor size artificial center tap

[überfuzz]

Are there any cons in having bigger center tap resistors? I have a 2kOhm multi turned pot i would like to use for this.

[nickb]

Are there any cons in having bigger center tap resistors? I have a 2kOhm multi turned pot i would like to use for this.

The problem comes if you get a short inside the tube from plate to heater. Then a high current can flow which will blow too your resistor and then feed that high volatge to all the other tubes vias the heater circuit and damage them too.

So you need something that will not blow and then, when the current flows, does not have a high volatge drop. The usual two 100 ohms resistors, I use 2W ones, fits the bill.

[überfuzz]

Okay, thanks!

I have an additional question. In order to set the level of a voltage drop in my circuit I use this artificial center tap as an voltage divider, see the attached picture. I hook up the 6Vac taps from the PT.



Is this kosher or should I do it some other way?

Thanks!

[pdf64]

R1-4 are tube heaters? If do what tube type, do the 21 and 10 indicate heater resistance?

[überfuzz]

R1, R2, R3 are tube filament resistance. The 10 is mock load.

[nickb]

It's better to put the balancing resistors on the AC side. 1N914's will not handle the current. I suggest schottky diodes so you can use a resistor, something like half an ohm, in series with the rectifiers to control the final output DC voltage and (probably) lower the ripple.

[trobbins]

That short can also be from pin 2 to pin 3 arcing on the tube socket, which is an increasing risk for amps with higher B+, and why I've seen some vintage amps with a plastic sleeve over each plate terminal and glued to the socket.

Then comes the choice of how to protect the output transformer and power transformer secondary from any such heater-plate fault current flow. One option is to use humdinger parts that will 'fuse' (such as the suggested 2k trimpot), and that then also needs an extra resistor from heater to ground to continue to conduct the fault current but limit heater voltage rise to something like 150-180Vdc or whatever won't cause collateral damage to all the tubes - I recall something like an 82k may achieve that.