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!
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!
Hi,
I assume that this is for a power amp?
Yes, twisting is good, I never had to use shielding.
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.
Old rule is to keep the heater wiring close to the chassis, distance to signal wiring depends on the sensitivity of the signal point.
Why? it looks good but has no practical meaning, (not noticeable anyway).
To the other heater pin, (I am sorry, I can not really understand this question)
Sometimes it can be better to connect the wiper to a DC potential of 50V or more instead of ground
Should not make a noticeable difference for IHT
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
I assume that this is for a power amp?
Yes, twisting is good, I never had to use shielding.
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.
Old rule is to keep the heater wiring close to the chassis, distance to signal wiring depends on the sensitivity of the signal point.
Why? it looks good but has no practical meaning, (not noticeable anyway).
To the other heater pin, (I am sorry, I can not really understand this question)
Sometimes it can be better to connect the wiper to a DC potential of 50V or more instead of ground
Should not make a noticeable difference for IHT
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
Hi,
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.
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,
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.
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,
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
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
Hi,
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
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
Just a couple of suggerstions:
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.
You really don't need a 3kV cap! I usually put 0.1 uF across the heater pins at the valve base.
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.
You really don't need a 3kV cap! I usually put 0.1 uF across the heater pins at the valve base.
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?
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?
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.
Saurav said:
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.
OK, that makes me feel better, because all my heater supplies do that, I think. The IDHT has the CT connected to the B+ voltage divider, and there's a cap going from there to ground. The CT leads for the DHT filament transformers are cut off, but they have a hum pot with the cathode bias resistor and cap connected to the wiper, so that's effectively a center tapped AC ground too. So I think I'm covered in both cases.
AC Heater wiring
The reason it is best to run the heaters witha pot and the slider conected to between 30 and 50 volts DC positive is to stop emmision between the heater and cathode. As the cothode is at a slightly lower temperature than the filiment itself. Using a resistor devider from the B+ and a cap of about 22 uf helps.
Next point is that Leak and QUAD both like almost all oyher British equipement ran EF86 input tubes which were specifically designed with good internal sheilding and twisted then cemented in filiments then they ran the cathode grounded by often a 100 uF cap with the circuit operating in a summing amp mode similar to inverted input methods on modern Op Amps. With centre tapped heater supplies, they got good but not absolutly silent results.
The Telefunken TL72 Microphone amps also us only AC heaters with EF804 tubes which are similar in construction.
The reason it is best to run the heaters witha pot and the slider conected to between 30 and 50 volts DC positive is to stop emmision between the heater and cathode. As the cothode is at a slightly lower temperature than the filiment itself. Using a resistor devider from the B+ and a cap of about 22 uf helps.
Next point is that Leak and QUAD both like almost all oyher British equipement ran EF86 input tubes which were specifically designed with good internal sheilding and twisted then cemented in filiments then they ran the cathode grounded by often a 100 uF cap with the circuit operating in a summing amp mode similar to inverted input methods on modern Op Amps. With centre tapped heater supplies, they got good but not absolutly silent results.
The Telefunken TL72 Microphone amps also us only AC heaters with EF804 tubes which are similar in construction.
I know this is an old thread, but it seems to fit my question.
I have decided on teflon up-grades for two pairs of very sloppy ceramic power tube (6550) sockets in an otherwise pretty decent chinese power amp. Of course if I am going to all the trouble of removing the old ones etc, then naturally I am going to upgrade the connections as well.
The original heater wires were 18 awg tin or nickel plated, so the plan was to replace them with Connex 18 awg silver plated OFC in teflon. The original twisted pair is about 30cm (1 foot) long.
The thinner teflon insulation means the overall diameter of the new wire is much smaller. In fact I could go up to 12 gauge in connex and still have the same o/d as the original. Is there any advantage in doing this?
Secondly, given the better insulating properties of Teflon, does the tightness of the twist become less critical?
David
I have decided on teflon up-grades for two pairs of very sloppy ceramic power tube (6550) sockets in an otherwise pretty decent chinese power amp. Of course if I am going to all the trouble of removing the old ones etc, then naturally I am going to upgrade the connections as well.
The original heater wires were 18 awg tin or nickel plated, so the plan was to replace them with Connex 18 awg silver plated OFC in teflon. The original twisted pair is about 30cm (1 foot) long.
The thinner teflon insulation means the overall diameter of the new wire is much smaller. In fact I could go up to 12 gauge in connex and still have the same o/d as the original. Is there any advantage in doing this?
Secondly, given the better insulating properties of Teflon, does the tightness of the twist become less critical?
David
No advantage in using thicker wire.
No advantage in using Teflon, apart from it not melting when you solder.
No advantage in using OFC or silver plated wire.
Generally, no advantage in changing the heater wiring at all. Changing something which does the job to something else which does the job (but more expensive) is not an upgrade - except perhaps cosmetically, but who looks under the chassis?
No advantage in using Teflon, apart from it not melting when you solder.
No advantage in using OFC or silver plated wire.
Generally, no advantage in changing the heater wiring at all. Changing something which does the job to something else which does the job (but more expensive) is not an upgrade - except perhaps cosmetically, but who looks under the chassis?
OK Thanks. That's actually good news - makes it all simpler.
My thinking was bigger conductor = less resistance = less noise, but there are obviously other issues in play.
Frank, what about teflon insulation? Is there a trade off between the quality of isolation vs. not as tight a twist?
David
My thinking was bigger conductor = less resistance = less noise, but there are obviously other issues in play.
Frank, what about teflon insulation? Is there a trade off between the quality of isolation vs. not as tight a twist?
David
Noise in wire? In an AC heater supply? Teflon having magnetic properties? This has to be trolling..!
I can't be bothered to calculate the Johnson noise for a piece of heater wiring, but I am sure it is very small. Last time I looked, wires do not suffer from shot noise or partition noise - and probably don't suffer from flicker noise or other excess noise. As 6.3V AC does no harm, I think we can be confident that heater wiring noise is not an immediate problem.exwiso said:
One of the delights of audio engineering is that appears to be no boundary beyond which someone will not go in proposing foolish ideas, and others will pick up these ideas and innocently propagate them.
We seem to have quite a few on this forum of late, some are even "Dr.'s"...It's the guru's fault !!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.