The filament voltage swings between +53.15V and +46.185V 60 times per second. When you reference (elevate) the center tap to a positive voltage (use 50VDC as an example), the sine wave starts at +50V and rises to +53.15V peak, then falls back thru 50VDC to a +46.85V peak, then rises back up to +50V. The filament voltage swings between +3.15v and -3.15v 60 times per second.
IOW, the sine wave starts at zero and rises to 3.15V positive peak, then falls back thru zero to a 3.15V negative peak, then rises back up to zero. In the common filament circuit with CT connected to ground, the center of the 6.3VAC waveform is referenced to ground (zero volts). Actually, the 6.3VAC waveform is riding on the DC voltage. The DC voltage is simply riding on the 6.3VAC waveform. The DC has nothing to do with heating the filaments. The AC 6.3 is not magically changed to DC. The filaments are still heated only by the 6.3VAC from the PT. The common belief is it is like a half power, but how can it really be half power when you create a OT mismatch? Like when pulling 2 tubes from a Twin, it does decrease output, but it also changes the amp considerably. When running 4 it should be half, correct? I am wanting to avoid, if possible, a mismatch when switching out 2 of the tubes. If the OT for a pair 6v6 is usually on the range of 6k5-8k. I plan cathode bias, but would like to run 4 of them with a switch to remove 2. I have done 2 in a stereo design, but have never used 2 in a mono design and do not know if you can in a traditional way like running a pair with each and a relay to switch out one of the OT's, but still have them combined at the speaker out. I assume he would not have mentioned this if there is not really much of a tonal difference and I do have 2 OT's designed for 2 6v6 so a nice savings for the build. PRR suggested 2, 6v6 running lower plates to achieve 10 watts in response as an alternative to 6AQ5 for the simple reason that Banjo World would have 6v6. This is the first where I will be using multiple relays and I figure it will take me some time, so I a simply gathering information. I have not decided which actual pre the 2 will be, but it will be old tweed, second like a vox EF86 and third will be like a Twin Reverb. One is octal, one is 9 pin, and third will be AB 763. These are areas of interest I would like to play with as I am currently working on a design with 3 different preamps in one amp. Is there an ac elevated and how is it done? Traditional ac center tapped 6.3 or artificial tap
So there are 4 different heater supplies I know, but I do not know the differences, or if there is any difference to speak of. I have made DC heaters, but I an not sure I do it correctly. I have heard of elevated heaters, but do know what it actually is. Also, John mentions he elevates heaters on octal pre-amp tubes. I was wondering if anyone would explain, or have a schematic showing DC heaters.
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