Question about heater wiring

[TonewolfAmps]

I have a power transformer with 12vct heater taps I wanna use for a build. It's gonna run 4x EL34s and 3x 12ax7s. I'm gonna wire the EL34 heaters in two series pairs. Do I ground the 12v heater center tap when using it on tubes wired in series?

The amp I scrapped the transformer from also ran 4x el34s wired in series pairs but they had the center tap not connected and taped off.

[Leo_Gnardo]

Do I ground the 12v heater center tap when using it on tubes wired in series?

You could do that. Or use a "hum dinger" pot to fine tune the hum rejection in your amp. For that I'd use a wirewound pot say 200 to 500 ohms. Isolate the center tap of the transformer, connect the pot wiper to ground, and each of the other pot electrodes to one side or the other of your filament supply. 100 ohms is used generally in 6.3V filament amps, and that would be OK with a 1 watt rated WW pot. With 12.6V to balance, probably best to choose a higher ohm rating or higher wattage rating.

[pdf64]

I suggest to definitely use the CT, connected to pin9 of the ECC83s and the nodes of the EL34 heaters. Because any discrepancy in the EL34 heater impedances, which will dominate in the arrangement, might otherwise put their heater voltages out of spec.
Of course it should all even out but it doesn’t seem a good idea to leave it to chance.
The only way to get in control of that is to use the CT and its facility to deliver current, something that resistive balancing arrangements can’t achieve.
The CT can also be connected to the desired heater circuit reference point, eg 0V common or a DC elevation.
Or balancing resistors or a humdinger pot be used similarly. Whatever, the CT should be connected to the actual heaters to ensure correct heater voltage.

[ChopSauce]

The CT can also be connected to the desired heater circuit reference point, eg 0V common or a DC elevation.
Or balancing resistors or a humdinger pot be used similarly.

In prefered order ?

[pdf64]

No, just randomly listed
Actually I’ve never had to deal with 12V heaters. Offhand, they might be a somewhat different case for that stuff than 6V types. eg lower heater current in the wiring, but from the perspective of each section’s heater, its current is single ended rather than balanced. Hence perhaps a better case for DC power, if only for the input stage.

[Phil_S]

Am I missing something here? The solution seems deceptively simple to me. Ground the CT. Wire the power tubes in series-parallel pairs so they will use 12V. Wire the 12AX7's for 12V. I know we usually wire the 12AX7's for 6V, but they can just as easily be wired for 12V and are intended for either way.

[ChopSauce]

Of course. So you suggest the best practice would be grounding the CT, when it exists, rather than leaving it isolated and grounding an artificial CT made of two 100R resistors?

[pompeiisneaks]

Of course. So you suggest the best practice would be grounding the CT, when it exists, rather than leaving it isolated and grounding an artificial CT made of two 100R resistors?

I think this is about opinion. I've heard a decent debate about the benefit/detriment of both, and can't say I've had anyone clearly state to me that one is better than the other.

One suggestion was that the center tap can sometimes be a bit 'off' from dead center and create a slight hum imbalance whereas a pair of 100 ohm resistors tend to be close enough to not cause imbalance... BUT I don't know personally that I've seen this. I've used both, and seen neither be a major issue. I'm seriously debating it on my bassman build right now. I'm currently wiring up the chassis connections before dropping in the board and can't decide which route to take...

I have also found in my builds so far that heater hum is so low of a problem on MOST builds that it doesn't make a ton of sense to stress it too much. Having done straight heater wires, without twisting, and having no real hum at all to speak of, I've become a lot less worried about them. Thus why, for speed, I might ground my center tap and move on

~Phil

[Phil_S]

If there is a CT, ground it and see how it works. If there is 50/60Hz hum, you can always lift and isolate the CT and install a pair of 100Ω resistors for a faux CT or install a humdinger pot. Grounding the CT seems like the simplest thing to do, so why not?

[ChopSauce]

Thanks! I was confused by seing so many examples of good amps with an artifical center tap.

One suggestion was that the center tap can sometimes be a bit 'off' from dead center and create a slight hum ...

Yes, I remember that now, thanks also! I also tried straight heater wires with "hum-wise success" ...

[pdf64]

I have a loosely formed opinion that DC elevation of the heaters might inhibit the likelihood of pin 2-3 arcs etc, in octal output valves.

[Phil_S]

I have a loosely formed opinion that DC elevation of the heaters might inhibit the likelihood of pin 2-3 arcs etc, in octal output valves.

Sure, move the CT from ground to a DC elevation source!