Artificial center tap for elevated heater question

[Chuck H]

Edit: So now I take it my center tap on the heater windings will go to a ground point?

This has already been explained, but let's be clear about it. Since we've been discussing the false (or virtual) center tap that uses a resistor from each side of the filament winding AND the actual center tap of the winding...

You would use the false center tap circuit OR the actual winding center tap but not both. If you go with the false center tap then the actual center tap would not be connected to anything. If you use the actual center tap then you would not wire up the false center tap at all. The false center tap OR the actual center tap can be connected to ground for no elevation OR the top of the power tube cathode for elevation. But you will only use the false CT or the actual CT. Not both.

[tubedood]

Thanks Chuck for the clarification. You explained it much better than I! Exactly what I was understanding.

[mozz]

Standard tweed Princeton wiring would have 1 leg of the filament grounded. If your transformer has the filament center tap, ground it, twist the other 2 wires and run them to your tube filaments, simple.

[Rhodesplyr]

In terms of elevating the heaters, the idea, as I've read about it from various sources, is to make it so that the heater AC waveform never crosses the cathode voltage of the tube in question. Some sources claim better results with elevating the heaters to ~30V or higher.

You can do it by tying the heater center tap to the output tubes' cathode bypass, or you can wire up a simple voltage divider from B+ with a couple of resistors, plus an additional capacitor at the divider junction.

Like Enzo says, this only eliminates ONE source of hum, but it's an old trick. RCA used it in 50s mic preamps, and the famous Pultec EQP-1A equalizer also uses the technique. (I know because I had to repair it in one EQP-1A where one resistor had opened.) The Hammond Organ Company used it in early preamps beginning around 1939.

Eliminating other hum sources will depend on good layout, grounding, and wire dress. Elevated heaters is not a magic bullet.

[Enzo]

You want it high enough that the cathode is always less positive than the heaters.

[Chuck H]

You want it high enough that the cathode is always less positive than the heaters.

THAT^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The only other qualification would be in cases where the cathode is hugely elevated . As is the case with cathode follower circuits. Common in Marshalls, early Fenders and most tremolo circuits that use two triodes. Some Rusky preamp tubes for a number of years couldn't handle the over spec cathode to heater differential. Not sure how it is now, but in amps that use cathode followers I usually elevate (at least) the preamp tubes to reduce the differential for reliability assurance. No reason a "customer" should need to consider tube origins when replacing tubes.

***Customer is in quotes because although I have made several amps for people I am NOT an amp maker by trade.

[tubedood]

Thanks guys, this website is such a great resource. I appreciate all of your advice.

Dale

[Enzo]

Generally the H-K thing mainly affects stages with low signal levels, like input stages. Those are not usually cathode followers. If your amp uses a cathode follower, it is probably best not to elevate the whole heater string. Or do what Marshall and others do, rectify and feed DC to the heaters of the first tube or two.

[SoulFetish]

I'd seriously doubt that using an output stage's bypassed cathode bias as a supply for heater elevation has got anything to do with amp output signal distortion. The elevated DC voltage just moves the voltage difference between heater and cathode to a region where the parasitic resistance between heater and cathode is typically substantially higher than if there was no elevation voltage. That parasitic resistance is commonly so high in most first-stage valves that elevation has no noticeable affect on hum level, unless you have a very high gain amp, and can listen up close to the speaker with no signal, or you have a spectrum analyser connected to the output, or you are using a particularly bad valve for the first stage.

If the elevation voltage has some signal on it, then that needs to be compared with the heater voltage signal for starters, and its frequency content, as its the heater voltage signal that can sometimes couple in to the input stage as hum, especially if heater wiring is not too good, or there is no humdinger pot to minimise any such hum. A bypassed cathode inherently suppresses audio band signals, and any audio signal would definitely be way less than 6.3Vrms, and rolling off at 6dB/oct. And if the amp is in gross overload, then I can't imagine that you would somehow selectively notice any residual sneaking back in to the input stage at a very low level.

Using an unbypassed output stage cathode bias as the elevation voltage supply would never be recommended by anyone I'd suggest.

Ive seen it. Also, i would add that I’ve only seen it on shared cathode resistors. The reason is when the amp is operating in the class A portion of the cycle the voltages through the resistor essentially cancel each other out. but when one tube goes into cutoff, and the output stage is operating in class B, the loadline shifts vertically and there is much more current through the resistor. This causes high frequency voltage transients to couple into the signal which you can see. I was using a 440Hz signal and I could see the effects on the waveform clearly when overdriving the output tubes. I didn’t experiment much with partial bypassing, since fully bypassing seemed to stabilize the amp I was working on.

Edit: sorry, i see now you were talking about fully bypassed cathodes. That seemed to be fine in the limited experiments I ran. So I agree, I personally just don’t like using that node as a voltage reference to elevate the heaters to avoid any of those issues all together.