Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Baguette on February 28, 2011, 07:10:40 am

Title: Twisted pair heaters on single ended amps
Post by: Baguette on February 28, 2011, 07:10:40 am: Hey there geeks,

The concensus is to run the 2 heater wires as a twisted pair to reduce hum. The reasoning, if I'm not mistaken, is that the hum from the 2 different phases will cancel out... That make sense in a push pull amp. But how about a single ended amp?

I've made my SE amps with twisted pair heaters for years, just because it's a habit of mine. But isn't it superfluous as there's no phases to cancel each other out, right?

Is there any advantage using twisted pair heaters Vs. one side heater / one side grounded as in the old amps?
(I'm talking SE amps here)

Thanks for sharing!
Title: Re: Twisted pair heaters on single ended amps
Post by: RicharD on February 28, 2011, 08:18:34 am: Forget the amplifier for a moment and focus just on the 6.3VAC. If you use a grounded center tap, or a virtual center tap to ground via a pair of 100 ohm resistors, then the filament AC is truly differential with respect to ground. The real question should be, in a SE amp should you center ground or corner ground your filaments? I don't think it matters. Whatever ends up the quietest. Either way, you should twist AC pairs to increase CMRR (common mode rejection ratio). This is not an absolute, but it certainly doesn't hurt.
Title: Re: Twisted pair heaters on single ended amps
Post by: jjasilli on February 28, 2011, 08:24:20 am: One advantage of PP is that it cancels out noise of all kinds including heater hum. But it's still better to eliminate, or at least reduce, hum to begin with. This is even more important in SE amps, because they lack the noise cancellation of PP.
Title: Re: Twisted pair heaters on single ended amps
Post by: Fresh_Start on February 28, 2011, 11:00:35 am: I'll go further and say that a twisted pair will benefit from an elevated voltage reference for the virtual center tap in a single-ended amp. Based on one build, but it made a difference I could hear.

Cheers,

Chip
Title: Re: Twisted pair heaters on single ended amps
Post by: kagliostro on February 28, 2011, 11:33:37 am: I think that twisting filament wires is useful both in SE and PP amps

twisting those wires will give you some "control" on the hum signal straggler from the wires

if you twist it one wire will shield the other and the low frequency dispersed is less

Kagliostro
Title: Re: Twisted pair heaters on single ended amps
Post by: jjasilli on February 28, 2011, 03:24:55 pm: Quote from: Fresh_Start on February 28, 2011, 11:00:35 am
I'll go further and say that a twisted pair will benefit from an elevated voltage reference for the virtual center tap in a single-ended amp. Based on one build, but it made a difference I could hear.

Cheers, Chip

Good point. My only SE amp is a VibroChamp. I converted the filament supply from one wire/ground to twisted pair. The amp became dead quiet (though it wasn't that bad stock). But if there's still heater hum, I'd take further steps too.
Title: Re: Twisted pair heaters on single ended amps
Post by: Baguette on March 01, 2011, 05:04:28 am: Thanks guys,

I'll do the amp with twisted pairs and a virtual center tap elevated to the power tube cathode.
It happens my layout has two unused terminal right next to the power tube cathode so they'll get the 100ohm resistor. I like easy builds ;)

Thanks for sharing.
Title: Re: Twisted pair heaters on single ended amps
Post by: sluckey on March 01, 2011, 06:17:55 am: No need for separate terminals for the two 100Ω resistors if you mount them on the socket directly between the filament and cathode pins.