Artificial center tap for elevated heater question

[tubedood]

Hi again folks,
I plan on using an idea I saw regarding elevating the heaters on my 6v6gt tube by using 2 1/2 watt 100r resistors going from the socket tab of the heater wires to the cathode tab of the tube. The schematic it was on shows a transformer without an actual center tap for the heater winding. If the transformer I am using actually has a center tap on it should I cap off the center tap wire... land it on a ground point or use it instead of the artificial center tap resistors?

Thanks as always and have a great Sunday,
Dale

[tubeswell]

Yep its better to use either the 2 x 100R or the CT (but not both) for the heater winding DC reference point

[tubedood]

Hi tubeswell thanks for the lightning quick reply! So if I use the center tap from the transformer I just land it to the tube socket tab for the cathode instead of the merging of the 2 resistors?

[tubeswell]

yep

[Enzo]

Just to be sure, you cannot both ground a center tap and also elevate the heaters.

[tubedood]

Thanks Enzo... my plan since I do have an actual center tap to work with is to not use the 100r resistors as an artificial tap and place the center tap wire on the socket tab for the cathode.

[SoulFetish]

Just to be sure, you cannot both ground a center tap and also elevate the heaters.

That’s right whatever DC point you connect the centertap (or common point for the 2 resistors) then becomes the average voltage for the AC (if we assume some symetry in the AC). Whether that’s 0V earth, or tied to the +Voltage on the cathode pin in a cathode biased amp. But it can’t be both

[SoulFetish]

Thanks Enzo... my plan since I do have an actual center tap to work with is to not use the 100r resistors as an artificial tap and place the center tap wire on the socket tab for the cathode.

I find connecting the heater CT to the cathodes on a tube amp to be problematic. Guitar amps routinely overdrive the output stage. Because of this, the bias voltage is subject to large excursion. This can cause audibly nasty, transient distortion artifacts during overdrive. Seems more likely to be a problem in amps where tubes in a.push pull output stage share a common resistor. It can be mitigated somewhat by bypassing the cathode resistor with a large value capacitor. Probably 220uF or higher depending.

[tubedood]

Hmmm SoulFetish good point to ponder maybe...
This amp is just a lil 5F2-A Princeton with 1 6v6GT. It has the standard 25uf bypass cap on it. Is this not a good choice over the stock heater arrangement?

[SoulFetish]

Hmmm SoulFetish good point to ponder maybe...
This amp is just a lil 5F2-A Princeton with 1 6v6GT. It has the standard 25uf bypass cap on it. Is this not a good choice over the stock heater arrangement?

You probably be fine doing it in this amp. Single ended, not a lot of gain.
Besides, if you run into tbe issue Im talking about you’ll know it. If your heart is set on elevating the heater supply, then ai say go for it. Its not very difficult to swap back if you run into problems

[SoulFetish]

Hmmm SoulFetish good point to ponder maybe...
This amp is just a lil 5F2-A Princeton with 1 6v6GT. It has the standard 25uf bypass cap on it. Is this not a good choice over the stock heater arrangement?

Is the heater suply obviously noisy? If it aint broke...
Quick example, a few fender models use the chassis to carry one phase of the heater supply. Now, this makes me cringe thinking about it, but I haven’t seen any of those amps suffer any noise issues bue to this wiring, so I leave them alone. One big factor is i would to explain to my boss why Im fixing something that isn’t broken during billable hours
But I wouldn’t do a new scratch build like that.

[Enzo]

Also, be aware that elevating heaters ONLY reduces hum caused by heater to cathode coupling in the preamp tubes. it will have no effect on other sources of hum like filter caps or shielding issues. If you are not having a problem with heater-cathode leakage, this will improve nothing.

[trobbins]

I find connecting the heater CT to the cathodes on a tube amp to be problematic. Guitar amps routinely overdrive the output stage. Because of this, the bias voltage is subject to large excursion. This can cause audibly nasty, transient distortion artifacts during overdrive. Seems more likely to be a problem in amps where tubes in a.push pull output stage share a common resistor. It can be mitigated somewhat by bypassing the cathode resistor with a large value capacitor. Probably 220uF or higher depending.

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.

[tubedood]

Thanks all for the helpful hints. Its a new little amp build I am slowly working on that is based on the tweed Princeton. I don't foresee any unexpected humming as the only modification I am going to do is a switchable 1st stage (and possibly second stage) cathode bypass capacitor. I will go ahead and scrap the elevated heater idea, I saw it on another persons 5F1 build and of course I needed it (before I even knew the pros and cons of it). I am planning now on the heater wiring as a standard tweed Princeton.

I am learning a lot in this community. I appreciate you time!
Dale

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

[Enzo]

Take your pick, either to ground or to that cathode.