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Old 25th February 2004, 10:33 PM   #1
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Default Best AC Heater Practices

Greetings,

I'm going to use AC for the filaments on IHT tubes and have found a few tips and suggestions dispersed in this forum and other sources.

I'd like to compile the info that I've seen so far and some questions in this post and ask you to comment on it. The intention is arriving at a comprehensive "Best AC Heater Practices" recipe that can be used by me and others who are implementing AC heating for the first time. So here it goes in no special order:

1. Twist AC-carrying wires tightly together. (Question: does it make sense to use a common shield over the wires and connect one end of it to the ground?)

2. Use solid wire of the gauge just enough to carry the required current. (Question: can somebody provide a reference on the "gauge for current" - e.g., 1A - 16 AWG, 2A - 14 AWG, etc.? I might be totally out of whack in the preceeding example.

3. Keep the wires as far as possible from the signal wires. (Question: is there a more precise definition of the min. distance between them? Does it depend on the current?)

4. When routing the wires bend them at the right angle.

5. (A simple question of a person: if the "hot" AC wire will be connected to the heater pin, where the "neutral" one goes ??)

6. 100 ohm pot between the AC leads with the wiper to ground, adjust for hum.

7. 0.1uF 3KV ceramic cap across the power transformer secondary removes a lot of noise. Actually, this was an an advice for B+, but see #8 below.

8. Use RC network to fight the hum. I understand it as a sort of combining #6 & #7 above (only a resistor instead of the pot), is it correct? How does this network looks like? How are the component values calculated (e.g., what is the min. capacitance value for the ceramic cap)?

9. Anything else?

Your input is greatly appreciated!
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Old 25th February 2004, 11:11 PM   #2
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Hi,

I assume that this is for a power amp?

Quote:
1. Twist AC-carrying wires tightly together. (Question: does it make sense to use a common shield over the wires and connect one end of it to the ground?)
Yes, twisting is good, I never had to use shielding.

Quote:
2. Use solid wire of the gauge just enough to carry the required current.
Why solid? I use 6C33C tubes with 6.5A heater current current in my OTL, solid wire would be really difficult and I fail to see any advantages. The wire gauge need to be big enough to carry the current, I am not familiar with the gauge system but I use a rule of <3A per sqmm wire area.

Quote:
Keep the wires as far as possible from the signal wires. (Question: is there a more precise definition of the min. distance between them? Does it depend on the current?)
Old rule is to keep the heater wiring close to the chassis, distance to signal wiring depends on the sensitivity of the signal point.

Quote:
When routing the wires bend them at the right angle.
Why? it looks good but has no practical meaning, (not noticeable anyway).



Quote:
(A simple question of a person: if the "hot" AC wire will be connected to the heater pin, where the "neutral" one goes ??)
To the other heater pin, (I am sorry, I can not really understand this question)

Quote:
100 ohm pot between the AC leads with the wiper to ground, adjust for hum
Sometimes it can be better to connect the wiper to a DC potential of 50V or more instead of ground

Quote:
0.1uF 3KV ceramic cap across the power transformer secondary removes a lot of noise. Actually, this was an an advice for B+, but see #8 below.
Should not make a noticeable difference for IHT

Quote:
Use RC network to fight the hum. I understand it as a sort of combining #6 & #7 above (only a resistor instead of the pot), is it correct? How does this network looks like? How are the component values calculated (e.g., what is the min. capacitance value for the ceramic cap)?
Haven't seen this, hum pot and rising the center contact of the pot to a higher DC voltage has always worked well for me.

Please note that tubes can be very different with respect to how sensitive they are for induced hum from the heaters, even the same type of tube can be very different depending on the manufacturer, I recently had 12AX7's that where very sensitive to heater hum, much more sensitive then any other 12AX7 I have seen before.

Regards Hans
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Old 25th February 2004, 11:30 PM   #3
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Hi,

Quote:
Why solid?
Because once twisted they stay that way and when pushed aigainst the chassis they do not tend to "relax" and move as multi-strand tends to do.

With the heater current requirements of a 6C33-C that might be a different kettle of fish of course.

As for gauge, as thick as need be but no thicker.

Quote:
When routing the wires bend them at the right angle.
As long as the angle isn't too sharp, otherwise the copper may suffer internal damage.

Other than that, I'm in total agreement with Hans.

Cheers,
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Old 26th February 2004, 04:09 AM   #4
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Default shielding

Hi,

I was reading through an issue of Radio and Television News, 1948 model, the other day and an amp designer gave this advice. If you want to increase the frequency range of your amplifier, don't shield the signal wires. Instead shield the sources of hum. This included shielded heater wiring and a steel chassis shield between the power supply parts of the amp and the signal carrying parts.

In my personal experience with a 12B4 linestage, gained before I ran across that article, I had stubborn low level hum. This though my power section was at the rear of the amp and my inputs, outputs and tubes all at the front. The signal wires were all shielded and little more than an inch long from input jack to volume control and from volume control to control grid pin. I had the heaters raised to 50Vdc and pretty careful overall wiring layout. Finally in desperation I replaced the heater run with shielded twisted pair. Voila, 70% of the hum gone. The rest left when I moved my first input cap ground off of the end of the ground bus and had just it and the B+ CT together before the bus.

Just my two cents, but I think I will pay more attention to removing the sources of hum in the future, not just to trying to hide my precious signal from it.

Michael
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Old 26th February 2004, 08:22 AM   #5
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Hi,

Quote:
Just my two cents, but I think I will pay more attention to removing the sources of hum in the future, not just to trying to hide my precious signal from it.
In my OTL amp I use only shielded cable from input connector to the first tube grid, all other signal wiring is unshielded and I use AC heaters, total hum an noise on the output is 0.4mV Pk-Pk which is >95dB below full output measured linearly without filter. My experience is that with careful layout and some simple methods it is possible to reach quite respectable hum levels even when using AC heaters without extensive shielding.

BTW, most of the 0.4mV is 100Hz hum from the output stage power supply, I cant see any 50Hz component on my scope, (1mV/display unit + a 20dB amplifier) when I adjust the humpot for min hum.

Regards Hans
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Old 26th February 2004, 08:32 AM   #6
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Wink A little pick...

Quote:
When routing the wires bend them at the right angle.
If you do this with a twisted pair, as in heater wiring, there will be field leakage from the corner. It probably won't make any difference in practice, but I wouldn't do it.
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Old 26th February 2004, 10:53 AM   #7
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Just a couple of suggerstions:

Quote:
6. 100 ohm pot between the AC leads with the wiper to ground, adjust for hum.
Connect the wiper to about 40V above the cathode potential. You can get this from a resistive divider from your B+ supply. Raising the heater above the cathode greatly reduces hum.


Quote:
7. 0.1uF 3KV ceramic cap across the power transformer secondary removes a lot of noise. Actually, this was an an advice for B+, but see #8 below.
You really don't need a 3kV cap! I usually put 0.1 uF across the heater pins at the valve base.
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Old 26th February 2004, 05:09 PM   #8
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To minimise exposure to their hum, AC heater wires should arrive radially at the valve socket. If you have a line of sockets, that means that you also need a line of heater wiring in the same direction. If you put that line of heater wiring close to the sockets, it causes hum, so you move it sideways. Meeting the first two requirements simultaneously forces a right angled bend as the heater wire leaves its line, goes up to the valve, and returns.

Putting 0.1uF across the valves heater terminals won't do much. Putting 0.1uF from each heater terminal to the (earthed) metal chassis substantially reduces RF noise.

Nobody's mentioned it, but the heater supply should be centre-tapped to chassis.

Screening heating wiring can be useful, but it's hard work, and shouldn't be necessary with good layout. How else did Leak and Quad pre-amplifiers have a phono stage using AC heaters?
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Old 26th February 2004, 07:13 PM   #9
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Quote:
Nobody's mentioned it, but the heater supply should be centre-tapped to chassis.
Regardless of whether it's an IDHT or DHT, and/or how the rest of the amp is grounded? I have my chassis connected near the inputs in the front (and it connects to the 3rd IEC prong at the back, of course), and I disconnected the B+ transformer's CT from the chassis. This reduced the hash on the outputs significantly (the amp is a 2A3 SET). Connecting the filament transformer's CT to the chassis would add another point where the chassis and the audio circuitry connect... wouldn't that be a recipe for ground loops?

Also, if one uses a voltage divider from B+ to raise the heater up above the cathode, then I don't think you could ground the CT, right? That's how I have the heaters for the driver tubes setup.
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Old 26th February 2004, 08:34 PM   #10
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by Saurav
Regardless of whether it's an IDHT or DHT, and/or how the rest of the amp is grounded?
Very good point, you are quite right. I was thinking simple IDHT circuits only. What I should have said is that the heaters always need to make some kind of a connection to AC ground (perhaps lifted by a DC potential), and that this connection should be made to a centre tap, rather than one side of the heater supply.
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