Blackstar HT Club 50 with uneven heater voltages

[mxrshiver]

the amp blew one of the EL34's so bad it's not conducting at all when tested, though the tube's filament still draws current, albeit at fairly high current, about 1.7A. it gives a lovely purple fireworks show when i apply DC test voltages in my Etracer. also blew the 500mA HT fuse. owner relates it was possible the amp was operated into a broken speaker cab, so an infinite/no load could have caused this havoc.

popped a new matched pair of EL34's in the amp, and discovered a large discrepancy in the filament voltage for the two halves of the winding - about 6.1V and 6.5V respectively. the 6.5V side was the one with the EL34 that got blown, so i figured it may have been related. nothing else seems to have failed, bias voltage is healthy and the current through both tubes together is somewhat cold at 44mA, less than the recommended 50mA. but the one with the higher filament voltage is definitely conducting much heavier than the other even though this is a pair matched to 2%, indicated by very different voltage drops across the two halves of the OT primary, even accounting for the different DC resistances of the two halves.

i don't have a schematic, and it being a Blackstar, i don't even dare to hope anyone has one to share... but here's what i can glean without pulling the board. there is no filament center tap coming from the transformer. it seems to be a 12.6V winding where pin 2 of one power tube is connected to one leg, pin 2 of the other to the other leg, and the pin 7's are grounded, ideally making 6.3V on either side. i've tried switching the connectors for the filament winding from the power transformer, and the voltage discrepancy switches with them. this is confusing to me seeing as there's no CT on that winding, so i'm not sure how one side would be giving higher voltages anyway, but the discrepancy does seem to be sourced at the transformer. i also tried pulling the power tubes, and the discrepancy remained, with much higher voltages overall of course. tried lowering the voltage of the higher side with a .1 ohm 5W resistor, but it lowered both sides proportionately.

i'm mostly wondering whether or not this is something to be concerned about, and could possibly indicate a failing power transformer. i plan on at least lowering the voltages to 6.3V/5.9V, but i'm concerned and confused as to how the transformer is creating this discrepancy in the first place.

[sluckey]

The two filaments are connected in series across the 12.6V winding, therefore the same current has to flow through each filament. The pins 7 are connected to ground simply for hum reduction and are not a factor in the uneven voltage split. The difference in voltage drop across each tube is due only to the fact that the two filaments are not equal resistance. The one with the higher voltage will also have a higher resistance filament. Swap the tubes around and you will see that the higher voltage is always on the same tube. The PT is not causing this discrepancy so don't be concerned.

[mxrshiver]

ugh, thank you Sluckey, you were right. somehow, since the ground reference is in between the two tubes, i wasn't processing the filaments as being in series. this does sort of seem like a recipe for one tube wearing out faster than the other, though? is there a benefit to supplying the filament winding ground reference this way?

[sluckey]

this does sort of seem like a recipe for one tube wearing out faster than the other, though?

Not really. The current through the filament is more important than the voltage across it. And the current is the same through both filaments. Nothing to be concerned about.

is there a benefit to supplying the filament winding ground reference this way?

There's a benefit for the manufacturer. He was able to provide a free ground reference, ie, not center tap or artificial center tap.

[Latole]

The two filaments are connected in series across the 12.6V winding, therefore the same current has to flow through each filament. The pins 7 are connected to ground simply for hum reduction and are not a factor in the uneven voltage split. The difference in voltage drop across each tube is due only to the fact that the two filaments are not equal resistance. The one with the higher voltage will also have a higher resistance filament. Swap the tubes around and you will see that the higher voltage is always on the same tube. The PT is not causing this discrepancy so don't be concerned.

What a weird wiring !!!!

[sluckey]

What a weird wiring !!!!

Is that part of the Blackstar schematic? If so, mxrshiver could benefit if you post the entire schematic. I'd like to see it also.