Series strung filaments?

[TerrySmith]

I've been playing around with my dumpster tube ST-35 (aka Plinkertron #1) and decided to try series strung filaments for the 29GK6 output tubes. The 47 ohm resistor gets the string voltage to 116v, which is perfect. There's no noise in the speakers and it sounds excatly the same as before!

The chassis is safety grounded and properly fused. Is there any downside to this wiring sceme?

Here's a schematic of what I did:


And the amp:

[Geek]

Yes, h-k leakage currents are enough to cause harm.

Cheers!

[dcgillespie]

I'm not familiar with the H/K rating for these tubes, but usually, any type of power tube made for heater string operation (as these appear to be) were constructed so as to be able to withstand the significant potential placed across these elements in that kind of service. With the old All American 5 radio sets for example, the heater string was arranged so that the detector/1st AF amplifier tube was connected to the side of the line that represented circuit ground, then came the converter tube, IF amp, and finally, the output tube and rectifier, which was connected to the side of the line the rectifier's plate was connected to. This meant that significant H/K potential was in play with the rectifier and output tubes, but these tubes were designed with that issue in mind.

Since your chassis has a green wire ground, that means there will be a direct potential relationship between the heater and cathode elements of these tubes. As long as that potential is within the rating of the tube, then all should be fine. The tube connected to the high side of the line will have the most potential across the elements. The tubes themselves should be rated for at least a 200 volt H/K rating. However the arrangement is still prone to hum. You say the amplifier is perfectly quiet. As long as the H/K insulation does not break down during normal life expectancy operation, then that should be a non-issue as well.

Way to save some bucks!

Dave

[TerrySmith]

I was hoping there wouldn't be any side effects. I got the idea from the Luxman amp that uses 50CA10 output tubes, those are connected across the line also, two each in series. Adds up to 100v as Japan has 100v mains.

The 29GK6's I have look identical to Amperex EL84 tubes and have GE type markings. They sound excellent also! I wonder if the American tube manufactures ever used foreign components?