Artificial centre tap - resistance to ground

[trobbins]

That is quirky. As well as the 470 ohm to ground on each heater side, there would also be a 330 ohm resistor to ground via the bridge. The sidac probably has a very low on voltage, but there is no secondary side fuse that would blow (I doubt the heater fuse would go if the B+ was the source of the current), so that leaves the primary side fuse.

Some faults are transients, like OT transient flashover, or a heater-cathode whisker, and so the sidac is there to avoid further breakovers I guess. They have 2x 1N4007 flyback diodes on each plate to ground.

It is most likely that they put the sidac in to reduce field faults.

[Merlinb]

That is quirky. As well as the 470 ohm to ground on each heater side, there would also be a 330 ohm resistor to ground via the bridge.

Yes, I too am frequently amazed at how many 'professional' circuits make this beginners' error. i.e. putting a ground reference on both sides of a rectifier, thereby creating a fairly low-resistance path right across the rectifier. Only ONE ground reference is allowed or required!

[R.G.]

Yes, I too am frequently amazed at how many 'professional' circuits make this beginners' error. i.e. putting a ground reference on both sides of a rectifier, thereby creating a fairly low-resistance path right across the rectifier. Only ONE ground reference is allowed or required!

Sadly, I often find that "professional" does not necessarily imply "competent".

[trobbins]

The technical merit to implementing a humdinger on each heater supply would be that they are only connected for a small % of time when the diode bridge conducts. For the majority of time, one of the heaters would be effectively floating if it didn't have its own humdinger. Perhaps that is why they have used relatively high humdinger resistances. So I can't see any detriment to that design.

My comment on quirky was really aimed at the sidac use, and trying to appreciate why they had introduced it.

[pdf64]

Might the SIDAC be acting to prevent a power tube short (eg plate or screen grid to heater) pulling the heater line too far away from ground potential?

[Enzo]

MAybe more to keep such a failure from burning up the CT resistors.

[trobbins]

When a fault current starts, if it raises the voltage across the humdinger to above 120V (ie. there is some energy source behind the fault current), the sidac then protects the humdinger, and I guess for a short transient event then the sidac would survive. What events does that then cover. Maybe the primary fuse would blow if the event was sustained. If the event subsided quickly enough not to damage anything, then all would return to normal.

[deci belle]

I've been reading all the threads that came up on my search for "fusing heater taps" tonight.

I've fused a bunch of things and installed parallel 10k cathode resistors, dual reversed diode strings and surge suppressors on all my amps (3) per RG's and Merlin's suggestions, but on my next amp, I want to fuse the 6v windings too.

RG said on this thread that "Even a poor man ought to save up and put one fuse in each side of the heater winding."

Will someone please help me find a link so I can find out the value of fuses to use in the next amp I build (5e4)?

That would be really awesome!!

[trobbins]

Such a fuse creates extra heat; creates extra chance of failure (due to the fuse itself, or its holder, or solder joints); and has a very low likelihood of ever being used for its intended purpose. Any rewiring mistakes that may happen are usually managed by a variac or light-bulb tester when powering back up. Any heater breakdown internal to a valve (eg. helix/double helix) would likely burn itself open.

The normal maximum current through a fuse should typically be no more than 80% of fuse rating. Heaters have high levels of turn-on current - I have only seen one reference which suggests an initial peak of at least 3-4x rated level, with a 200% level at 1 second. If that was typical then a slow-blow fuse of 1.5 to 2x the normal max heater rating should be ok.

[deci belle]

hi trobbins!

Such a fuse creates extra heat; creates extra chance of failure (due to the fuse itself, or its holder, or solder joints);

I know that much for sure, like when 1/2 my PT winding went AWOL~ haha!!But last time a screen resistor burnt cuz a tube died, nothing expensive took a hit.

Thank you very much for that information in the 2nd paragtraph, trobbins— that helps a lot!