Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on December 14, 2018, 08:30:14 am

Title: Shielding heater wiring on preamp?
Post by: tubenit on December 14, 2018, 08:30:14 am

Just curious,  has anyone tried shielding their preamp heater wiring by wrapping a wire tightly around the heater wiring when it's "floated" up in the air and then tying one end of that wrapped wire to chassis ground?

Any reason not to try this to see if it further reduces floor noise?

The chassis is normally inside a cab that has shielding on the open side of the chassis.  So, maybe it wouldn't help anything?

However, I noticed the other day, that I could wave my hand about 4 inches above the open chassis and the heater wires would induce hum.  Moving the wire over slightly seemed to resolve this.  But I am still wondering if twisted wiring that is grounded around the heater wiring would make things even quieter?

With respect, Tubenit

Title: Re: Shielding heater wiring on preamp?
Post by: Apexelectric on December 16, 2018, 11:48:37 am

I tried using shielded cable for heaters once and it didn’t seem to reduce the hum over the twisted pair. I had some 18ga shielded cable floating around that I thought I’d give a try to try to reduce some hum in an amp I was having issues with. In the end it didn’t help. Guess my problem was elsewhere. Lead dress made a bigger difference.

Title: Re: Shielding heater wiring on preamp?
Post by: tubenit on December 16, 2018, 01:08:08 pm

Thanks for the response!  I do appreciate it.  IF I get around to trying this, I'll post whether it helped or not.

Tubenit

Title: Re: Shielding heater wiring on preamp?
Post by: Tone Junkie on December 17, 2018, 03:09:27 am

I tried it once and didn't notice any difference . But it was on a medium gain amp I had built several times. so lead dress and layout were tried and true.
Thanks Bill

Title: Re: Shielding heater wiring on preamp?
Post by: jjasilli on December 17, 2018, 09:53:13 am

I haven't tried it either. But see TUT1, Ch 2-12, and Figure 2-15.  KOC states that foil shielding is superior and is preferable to braided or spiral, because it provides 100% coverage.  This can reduce hum up to 60% (presumably re the twisted pair method, but he's vague about this).  #16 supply wire can be daisy changed; or use #20 wire to each preamp tube.  For grounding purposes, the shielding itself should be daisy changed, and ground connected at one end.  It seems to me that this should be the transformer end.  No shielding needed for the power tubes.