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Filament Supply - No Center Tap
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Author:  Steph [ 12 Mar 2017, 08:41 ]
Post subject:  Filament Supply - No Center Tap

Hello all,

I did a quick search and didn't see exactly what I was looking for. If this has been covered and you have a link to share that would be great so as not to duplicate.

I'm working on an all tube project with tube rectifier supply. I hope to use a power transformer that I have on hand however, on the 5V secondary there is no center tap. In my old transceiver days we would tie one side of the filament loop to ground. SSB isn't particularly quiet so we didn't pay much attention but in the audio world I'm not sure if this will be sufficiently quiet. I'm familiar with creating an artificial center tap using a couple 100 Ohm resistors to ground as an alternative.

Anyone with opinions on best approach?

Thank you

Author:  Suncalc [ 12 Mar 2017, 10:12 ]
Post subject:  Re: Filament Supply - No Center Tap

OK you need to be very specific here. Are you talking about the supply for the rectifier tube, or the supply for the other tubes? It makes a difference. Since you reference 5V and most audio tubes are 6.3v or 12.6v, I am assuming that you are talking about the rectifier filament supply.

In most of the octal 5V rectifiers, the filament is either a filamentary cathode (5U4, etc.) or the unipotential cathode is tied to the filament at pin 8 (GZ34). In either case, the filament runs at the B+ voltage out of the rectifier and should never be grounded.

The filament loop that runs the audio tubes (so long as they are all unipotential cathodes) may be center tap grounded, grounded through a pseudo-center tap with two 100Ω resistors (as you mentioned), or one side can simply be tied to ground. In general I use either the center tap ground (if available on the transformer) or the two 100Ω resistors. This has given me the best results.

But keep in mind that if you have any tubes running at elevated cathode voltages (cathode follower, SRPP, cascode, WFC, etc.) then you need to make sure that the cathode to filament limits on each tube are not violated. There are times when you may want to elevate the cathodes (i.e. positive bias) to control noise, but this is usually only required in very high gain circuits and is usually better handled by DC on the heaters.

Does this help?

Author:  Steph [ 12 Mar 2017, 11:20 ]
Post subject:  Re: Filament Supply - No Center Tap

My apologies and right you are.

I was a bit side tracked when I wrote this question up. My transformer has 5, 6.3 and 720 secondaries. I meant to reference the 6.3.

My power transformer will feed a tube rectifier and will be used to supply 720 VAC and 5 VAC to a 5U4G. The third set (6.3 VAC) will be used for additional pre-amp and power amp filaments.

My concern is more with the 3rd set of windings for the 6.3 VAC line which has no center tap. My hope is to keep the noise and hum down to a minimum. In very few previous audio projects I've always had the benefit of a center tap and have never had any issues with hum from the filaments. With this transformer having no center tap I'm looking for a best practices approach. I've read about this being done a number of ways; with balancing pot, an artificial center tap using a pair of resistors, I've also seen a couple of examples where people have used a simple resistor between the lines or some much older simper designs where one side is tied to ground.

I appreciate any advice you might have.


Author:  Suncalc [ 12 Mar 2017, 11:45 ]
Post subject:  Re: Filament Supply - No Center Tap

I'd just go with the two 100Ω resistors. It's simple and it works.

This was the whole idea behind the original lacewood amp design. It once again demonstrated that filament hum is much more urban myth than fact. If the power supply is properly filtered you should have no hum; even with floating filaments. The pseudo-center tap just gives an added level of protection.

Author:  Steph [ 12 Mar 2017, 11:51 ]
Post subject:  Re: Filament Supply - No Center Tap

Wonderful...I thank you. Have a great day.

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