6V6 filaments in series

[Joshua_G]

Is there any known problem or should there be any problem feeding 2 6V6 tubes filaments in series from a constant current source?

[tomchr]

I'd think you would get more capacitive coupling between the two 6V6 cathodes. If your application is sensitive to this, you might want to run them on separate filament strings.

~Tom

[Soonerorlater]

Hi Joshua-G,
I too would be interested in members views on this subject.

I have a pair of power transformers that have a heater output of around 13v instead of the usual 6.3v. I was thinking of heating a 12ax7 directly and a pair of push-pull El84's in series. My thinking was that it might be an advantage if a heater fails in one EL84 the other EL84 in the pp pair would be powered down as well.

Also a ccs might be a better way for me to drop the heater voltage a little than a resister.

Brgds Bill

[Joshua_G]

Thanks Tom, I see your point.
Now, additional question: when there are 2 6V6 tubes, 1 for each channel in a stereo amp and each has cathode bias by a resistor shunted by an electrolytic capacitor. In such a case, as far as I see it right now, both cathodes are AC shunted to the ground. So, does it make any difference that the 2 cathodes are AC coupled by about 12pF?

[Tom Bavis]

The 6V6GTA is rated for use in series heater circuits, with controlled warm-up time. I think this will be less of an issue with constant current, since there won't be a turn-on surge.

[Joshua_G]

The tubes I have are 6V6GT Tung-Sol reissue. They will be fed by CCS, either each individually from a different CCS, or both in series from one CCS.
My question is about the capacitive coupling between the 2 cathodes when the filaments are in series, how much is it relevant when both cathodes are AC shunted to the ground?

[SemperFi]

unless used in a first stage of a high gain circuit, there will be no problems whatsoever.

There will be no crossover or whatever. Just be sure the heaters are positive in respect to ground.

btw, they are heaters, not filaments

[Joshua_G]

Thanks.
The tubes will be used in a 1 tube per channel line stage.
The heaters will be elevated about +40 Volt above ground.
Each heater will be shunted to the ground by two 0.1uF capacitors, one from each side of the heater – to avoid possible RF modulation.

What's the difference between heaters and filaments?

[tvrgeek]

Heaters/filaments, anodes/plates, tubes/valves. Is this just not historical variations on syntax or which side of the pond you are from?

[tomchr]

Originally Posted by Joshua_G

Thanks Tom, I see your point.
Now, additional question: when there are 2 6V6 tubes, 1 for each channel in a stereo amp and each has cathode bias by a resistor shunted by an electrolytic capacitor. In such a case, as far as I see it right now, both cathodes are AC shunted to the ground. So, does it make any difference that the 2 cathodes are AC coupled by about 12pF?

You might be able to measure a difference in channel separation, but I doubt it'll be audible. However, as others have pointed out, you need to watch out for oscillations. I second the advice of using a cap in the range of 1.0~100 nF from each heater pin to ground (in older gear this is done right on the socket). Too little cap, the amp might oscillate due to the coupling through the heater string, too much cap, the current source might oscillate. Also, decouple the electrolytic cap with a 0.1~1 uF film cap and a 10 nF ceramic will make sure the cathode sits at AC ground even at higher frequencies where the electrolytic is way past its resonance frequency.

~Tom