Humdinger. - UK Vintage Radio Repair and Restoration Discussion Forum

adibrook · 11th Feb 2006, 8:14 pm

I've been looking at some schematics of old directly heated stuff (like a 300B amp for example) and I'm a bit confused about the heater/cathode arrangements.

The normal way to wire up a directly heated power triode seems to be like this.

http://img.photobucket.com/albums/v405/adibrook/dh1.gif

But I dont understand what the 100R pot is for. Technically, if the heater winding and the HT winding have no common earth...than heater current and ht current can co-exist in one set of wires...like directly heated rectifiers. Right?

So why not just do this?
http://img.photobucket.com/albums/v405/adibrook/dh2.gif

Or is there something I'm missing here?

Thanks.

Paul Stenning · 11th Feb 2006, 8:18 pm

The pot is to minimise hum. With your second curcuit one end of the cathode has the full heater AC voltage on it, which would cause quite a lot of hum on the audio.

With the pot adjusted for minumum hum there would be about half the heater AC voltage on one emd of the cathode and an equal and opposite AC voltage on the other end, so they cancel out. Some sets would have a pair of resistors of equal value instead of the pot which is probably fine for lower power applications.

dazzlevision · 11th Feb 2006, 9:26 pm

Hello,

This arrangement is quite common even in indirectly heated valve circuits. The pot is often referred to as the "humdinger" or "hum bucking" adjustment.

Regards,

Dazzlevision

adibrook · 11th Feb 2006, 9:59 pm

Ok. Thanks!

If I used a DC power supply for the heaters would that still be necessary?

I'm thinking of using a voltage regulator. It'll need to sink a bit...but hey I have a whole metal Chassis to play with so I suppose heat dissipation isn't that much of a problem.

It's just that I dont particularly like pots at high currents.

dazzlevision · 12th Feb 2006, 8:53 am

Hello,

If you use a FULLY smoothed dc supply, then there is no ac component in the heater supply to balance out with a humdinger pot.

Regards,

Dazzlevision

Paul Stenning · 12th Feb 2006, 10:45 am

However you would then have to account for the DC voltage on the cathode when calculating the biasing.

Why not just usetwo equal value wirewound resistors? There might be a small amount of hum but that probably wouldn't be an issue with the type and volume of music you will be playing through it!

adibrook · 12th Feb 2006, 1:56 pm

Ok. That sounds good. Thanks!

I suppose the hum would be the same irrespective of volume, and since I'll always have it at clipping it wouldnt really matter.

Now I need to find some directly heated stuff..

11th Feb 2006, 8:14 pm	#1
adibrook Retired Dormant Member Join Date: Dec 2004 Location: Weymouth, Dorset, UK. Posts: 377	Humdinger. I've been looking at some schematics of old directly heated stuff (like a 300B amp for example) and I'm a bit confused about the heater/cathode arrangements. The normal way to wire up a directly heated power triode seems to be like this. http://img.photobucket.com/albums/v405/adibrook/dh1.gif But I dont understand what the 100R pot is for. Technically, if the heater winding and the HT winding have no common earth...than heater current and ht current can co-exist in one set of wires...like directly heated rectifiers. Right? So why not just do this? http://img.photobucket.com/albums/v405/adibrook/dh2.gif Or is there something I'm missing here? Thanks. Last edited by Darren-UK; 15th Jun 2007 at 6:54 pm. Reason: Tidying.

11th Feb 2006, 8:18 pm	#2
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: directly heated cathode biasing. The pot is to minimise hum. With your second curcuit one end of the cathode has the full heater AC voltage on it, which would cause quite a lot of hum on the audio. With the pot adjusted for minumum hum there would be about half the heater AC voltage on one emd of the cathode and an equal and opposite AC voltage on the other end, so they cancel out. Some sets would have a pair of resistors of equal value instead of the pot which is probably fine for lower power applications. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

11th Feb 2006, 9:26 pm	#3
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,611	Re: directly heated cathode biasing. Hello, This arrangement is quite common even in indirectly heated valve circuits. The pot is often referred to as the "humdinger" or "hum bucking" adjustment. Regards, Dazzlevision

11th Feb 2006, 9:59 pm	#4
adibrook Retired Dormant Member Join Date: Dec 2004 Location: Weymouth, Dorset, UK. Posts: 377	Re: directly heated cathode biasing. Ok. Thanks! If I used a DC power supply for the heaters would that still be necessary? I'm thinking of using a voltage regulator. It'll need to sink a bit...but hey I have a whole metal Chassis to play with so I suppose heat dissipation isn't that much of a problem. It's just that I dont particularly like pots at high currents. Last edited by Darren-UK; 15th Jun 2007 at 6:52 pm. Reason: Tidying.

12th Feb 2006, 8:53 am	#5
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,611	Re: directly heated cathode biasing. Hello, If you use a FULLY smoothed dc supply, then there is no ac component in the heater supply to balance out with a humdinger pot. Regards, Dazzlevision

12th Feb 2006, 10:45 am	#6
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: directly heated cathode biasing. However you would then have to account for the DC voltage on the cathode when calculating the biasing. Why not just usetwo equal value wirewound resistors? There might be a small amount of hum but that probably wouldn't be an issue with the type and volume of music you will be playing through it! __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

12th Feb 2006, 1:56 pm	#7
adibrook Retired Dormant Member Join Date: Dec 2004 Location: Weymouth, Dorset, UK. Posts: 377	Re: directly heated cathode biasing. Ok. That sounds good. Thanks! I suppose the hum would be the same irrespective of volume, and since I'll always have it at clipping it wouldnt really matter. Now I need to find some directly heated stuff.. Last edited by Darren-UK; 15th Jun 2007 at 6:50 pm. Reason: Tidying.