HT and filament heating from the same transformer

[Thermionic Idler]

One thing Lynn Olson recommends is to have filament transformers be separate from the HT transformer, at least for DHT's like 300B's. This is because you (apparently) get rectification switch-noise from the HT secondary winding capacitively coupling into the low-voltage filament circuit.

When I built my 300B's originally, I had all the filaments fed by separate transformers - 5V for the 300b filaments themselves, and a 12.6V transformer feeding the 6V6 heaters in series, plus the 12SN7.

The KT88 heaters draw 1.6 amps each so that arrangement won't work as the existing filament transformer isn't large enough. However, the new transformers each have a 6.3V secondary that is rated for 3 amps.

So I figured that pragmatically, it would make sense to use them for the KT88 heaters (one each). KT88's aren't directly heated, and they are amplifying larger voltages than the 12SN7 (which will still have its own dedicated transformer, as will the 300B's) so I figured they would be the least sensitive devices in the chain to such noise. Plus the power supplies will include snubbing and UF4007 rectifiers to reduce the switching spikes as far as possible anyway.

I guess the question is - is this theoretical coupling of rectification noise into the heater winding really that much of an issue in reality? Context - we're talking all-AC heating.

[Andrew]

If you're spending all that money on iron, it would seem false economy to AC heat, even indirectly heated valves.

[Paul Barker]

Lynn is right.

The effect still could affect IHT’s but only detected by Nick or someone like that with very fine test equipment. Normal humans wouldnt notice it.

But dht its audible and you go mad trying to locate the problem. The problem is picking up the switching noise of the ht.

All of my heaters and filaments are not from the ht transformer. Except the diode filament winding, but the diode is already up in the heat of the switching action, and a part of it. That noise is what you are filtering out of youre power supply. The noise youve just filtered out you are now injecting through youre dht.

[Nick]

If you're spending all that money on iron, it would seem false economy to AC heat, even indirectly heated valves.

Indeed. I would bet there is more hf noise and distortion on your mains than you would create from adding diodes. So I think you are creating a false choice. Worrying about added noise from rectification.

[andrew Ivimey]

All those Western Electric circuit diagrams, transformers and valves..but now we have caps and clever devices. Me, I'd like to cut caps and trust in the weight of windings...

Karna, without the clever ccs and so on, works and is lovely.
Monoblocking and d.c. heaters... and the rest. How long have you got?

If you have time, try it and see.

[Nick]

All those Western Electric circuit diagrams, transformers and valves..but now we have caps and clever devices. Me, I'd like to cut caps and trust in the weight of windings...

Karna, without the clever ccs and so on, works and is lovely.
Monoblocking and d.c. heaters... and the rest. How long have you got?

If you have time, try it and see.

Ahh, so its all about the feels

[Mike H]

If you're spending all that money on iron, it would seem false economy to AC heat, even indirectly heated valves.

Indeed. I would bet there is more hf noise and distortion on your mains than you would create from adding diodes. So I think you are creating a false choice. Worrying about added noise from rectification.

Whenever I get a look at my mains on a 'scope (from a transformer winding) it's invariably ghastly - where I live now (Boston) it looks like a rounded and lopsided triangular wave, with a lean to the right, almost like a sawtooth, quite strange. Where I used to live in Essex I often saw what looked like TRIAC chopping artifacts, but nothing to do with us! Didn't have light dimmers and no power tools in use.

[Morgan Jones]

I guess the question is - is this theoretical coupling of rectification noise into the heater winding really that much of an issue in reality? Context - we're talking all-AC heating.

YES. But it's not just HT rectification interference, it's any common-mode interference on the mains, and there's plenty of that. Note that it's a common-mode issue - doesn't matter whether you have AC or DC heaters.