tube hum in Aikido

cbutterworth · 8th March 2007, 03:45 AM

It is possible that heater AC is getting into the signal on my Aikido. I am using 0.01uF ceramic caps across the heaters, which John Broskie recommends for AC. I have tested a variety of tubes and have made the following observations:

If I use my 6SN7GTB's as the output tubes, I get no 60Hz. I have a pair each of Tung-Sol and GE.

Any of my other tubes in the output position cause hum - two pairs of Sylvania 6SN7GTA.

When I use the Sylvania as input to the GE, I get no hum, but the bass is weak compared with Tung-Sol / GE. I have yet to try the bass using Sylvania / Tung-Sol.

Somewhere I read that the GTB tubes have their getters on the bottom as opposed to the top. Could this be having an effect? Would DC heaters take care of this problem? What about placing caps between heater pins and GND?

Are the newer, low-noise tubes such as Sovteks likely to show an improvement?

Regards,
Charlie

SY · 8th March 2007, 11:11 AM

I can't answer this directly (I use DC on my preamp's heaters), but the small ceramic caps are not for hum reduction, but rather for bypassing RF and other high frequency hash.

cbutterworth · 8th March 2007, 03:49 PM

Sy,

OK on the ceramic caps across the heaters. Bruce Anderson thinks that caps from each heater pin to GND would improve things.

Anyway, I have tried to find a suitable heater DC circuit. So far, I am thinking:

12.6VAC transformer into a bridge rectifier (some diodes are probably better than others), paralleled by a 1mF cap (electrolytic?); into a 7812 regulator; another paralleled 100uF cap (electrolytic?) - should give 12VDC.

This would (through jumpering on the Broskie board) allow for 6.3VDC and/or 12VDC heaters. If I use four 6SN7s, I need 2.4A of current. Fairchild produce a positive voltage regulator (MC78T12CT) which can handle 3W.

What do you think? I have searched for DC heater circuitry, but haven't really been able to find much.

Regards,
Charlie

Bruce Anderson · 8th March 2007, 05:45 PM

Charlie, Charlie, Charlie

You know I only recommend gonzo, over the top mods. The cap from each heater pin to ground is only a part of the total bypass I recommended in my post of 3/5/07. I did mention that Bottlehead does that in their phono pre and because they build to a price point they only use parts when they feel it required (in that case for a 6DJ8 phono pre). For our personal one off builds we are not limited. Just my 2 cents. Feel free to ignore.

8th March 2007, 03:45 AM	#1
cbutterworth diyAudio Member Join Date: Oct 2006	tube hum in Aikido It is possible that heater AC is getting into the signal on my Aikido. I am using 0.01uF ceramic caps across the heaters, which John Broskie recommends for AC. I have tested a variety of tubes and have made the following observations: If I use my 6SN7GTB's as the output tubes, I get no 60Hz. I have a pair each of Tung-Sol and GE. Any of my other tubes in the output position cause hum - two pairs of Sylvania 6SN7GTA. When I use the Sylvania as input to the GE, I get no hum, but the bass is weak compared with Tung-Sol / GE. I have yet to try the bass using Sylvania / Tung-Sol. Somewhere I read that the GTB tubes have their getters on the bottom as opposed to the top. Could this be having an effect? Would DC heaters take care of this problem? What about placing caps between heater pins and GND? Are the newer, low-noise tubes such as Sovteks likely to show an improvement? Regards, Charlie

8th March 2007, 11:11 AM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	I can't answer this directly (I use DC on my preamp's heaters), but the small ceramic caps are not for hum reduction, but rather for bypassing RF and other high frequency hash. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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8th March 2007, 03:49 PM	#3
cbutterworth diyAudio Member Join Date: Oct 2006	Sy, OK on the ceramic caps across the heaters. Bruce Anderson thinks that caps from each heater pin to GND would improve things. Anyway, I have tried to find a suitable heater DC circuit. So far, I am thinking: 12.6VAC transformer into a bridge rectifier (some diodes are probably better than others), paralleled by a 1mF cap (electrolytic?); into a 7812 regulator; another paralleled 100uF cap (electrolytic?) - should give 12VDC. This would (through jumpering on the Broskie board) allow for 6.3VDC and/or 12VDC heaters. If I use four 6SN7s, I need 2.4A of current. Fairchild produce a positive voltage regulator (MC78T12CT) which can handle 3W. What do you think? I have searched for DC heater circuitry, but haven't really been able to find much. Regards, Charlie

8th March 2007, 05:45 PM	#4
Bruce Anderson diyAudio Member Join Date: Jun 2005 Location: Tempe, AZ	Charlie, Charlie, Charlie You know I only recommend gonzo, over the top mods. The cap from each heater pin to ground is only a part of the total bypass I recommended in my post of 3/5/07. I did mention that Bottlehead does that in their phono pre and because they build to a price point they only use parts when they feel it required (in that case for a 6DJ8 phono pre). For our personal one off builds we are not limited. Just my 2 cents. Feel free to ignore.

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