AC heaters done right….how to get it done

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Doxie man
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AC heaters done right….how to get it done

Post by Doxie man »

Here is the method I use when wiring up the filament string in my amplifiers. It will yield the most quiet, safe and balanced heater circuit you can achieve in an AC powered filament circuit. The reasons to do so are as follows:
1. Using the (2) 1% precision toleranced 100R 1/2W resistors will balance to the two halves of the circuit better than the factory supplied CT. Should a short occur in the filament string , the resistor(s) will act as sort of a fuse and “cook off” protecting the power transformer. While it isn’t a 100% guarantee damage won’t occur, it is much better than having the supplied CT directly coupled to ground and a short occur.
2. Instead of referencing chassis ground through those resistors, simply reference ground to the power tube’s cathode / cathode resistor. This will elevate the heater string above chassis ground reducing noise potential.

I’ll attach a diagram and a pic of what I have mentioned here and think it’ll be a useful bit of information for the guys who are unaware of this very simple yet effective technique.

Parts needed: (1) stand-off/terminal strip (2) 1/2W 100R 1% resistors
* factory heater CT is simply capped off and safely secured ( unused)
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Last edited by Doxie man on Sun 08/29/21 7:33 pm, edited 1 time in total.
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Doxie man
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Re: AC heaters done right….how to get it done

Post by Doxie man »

Hope this was helpful. I use this method exclusively when doing AC powered filaments 👍
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JMPGuitars
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Re: AC heaters done right….how to get it done

Post by JMPGuitars »

I still don't see an example of why your artificial CT is better. I see your OPINION of that, but not any actual real data or sources. I've also never had a PT heater CT be as far off as 1% from each halve.

Elevated heaters is often a good practice for noise issues, which is likely necessary in your case with the heater wiring method you prefer. If the end result works for you, that's cool. But that doesn't make it a superior or necessary method for all. I've personally had zero amps that needed elevated heaters. I've tried it a couple times with no perceivable difference.

To each their own, a lot of people like elevated heaters. This is the first time though that I've ever heard that opinion on preferring an artificial CT to a real one. I'd like to hear some proper engineers weigh in on this one.

Thanks,
Josh
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Re: AC heaters done right….how to get it done

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JMPGuitars wrote:
Sun 08/29/21 7:52 pm
I still don't see an example of why your artificial CT is better. I see your OPINION of that, but not any actual real data or sources. I've also never had a PT heater CT be as far off as 1% from each halve.

Elevated heaters is often a good practice for noise issues, which is likely necessary in your case with the heater wiring method you prefer. If the end result works for you, that's cool. But that doesn't make it a superior or necessary method for all. I've personally had zero amps that needed elevated heaters. I've tried it a couple times with no perceivable difference.

To each their own, a lot of people like elevated heaters. This is the first time though that I've ever heard that opinion on preferring an artificial CT to a real one. I'd like to hear some proper engineers weigh in on this one.

Thanks,
Josh
What happens if there is a short in the filament string with the CT directly coupled to chassis ground ? It’s pretty obvious what happens. Now….run the heaters through 2 balanced resistors to reference an elevated ground and those resistors will cook off in a quick second potentially saving the PT from a catastrophic event. I learned this method from a very experienced and knowledgeable tech that works and builds more amplifiers than you or I ever will. I do realize it’s an unlikely scenario but I like to cover every base I can when I’m building a circuit. If you do some digging and research you’ll verify it. I have seen PT’s that we’re not balanced as good as if I used the CT but ymmv of course. I also have a nice way of taking the strain off the CF tube on startup in other circuits using a diode and resistor. It’s the little things we learn from more experienced builders that makes us better 😉
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geoff 1965
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Re: AC heaters done right….how to get it done

Post by geoff 1965 »

Talking of strain on start up,I just watched a tech servicing a laney tube amp on YouTube and noticed they use a thermistor on the ac mains input.
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Re: AC heaters done right….how to get it done

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geoff 1965 wrote:
Sun 08/29/21 10:55 pm
Talking of strain on start up,I just watched a tech servicing a laney tube amp on YouTube and noticed they use a thermistor on the ac mains input.
Those are good to install yes. If your using your amplifiers in a well known environment with little chance of voltage spikes like at your usual playing spots like home, friends place etc. it’s not a necessity. It comes more into play and usefulness with touring guys who roam all over the place. Either way it’s not a bad idea to install them….it’s extra insurance is all.
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Re: AC heaters done right….how to get it done

Post by JMPGuitars »

Doxie man wrote:
Sun 08/29/21 9:43 pm
What happens if there is a short in the filament string with the CT directly coupled to chassis ground ? It’s pretty obvious what happens. Now….run the heaters through 2 balanced resistors to reference an elevated ground and those resistors will cook off in a quick second potentially saving the PT from a catastrophic event. I learned this method from a very experienced and knowledgeable tech that works and builds more amplifiers than you or I ever will. I do realize it’s an unlikely scenario but I like to cover every base I can when I’m building a circuit. If you do some digging and research you’ll verify it. I have seen PT’s that we’re not balanced as good as if I used the CT but ymmv of course. I also have a nice way of taking the strain off the CF tube on startup in other circuits using a diode and resistor. It’s the little things we learn from more experienced builders that makes us better 😉
I mean...If you want to be careful, do a good job wiring your heaters and this whole argument is dead. I can't even recall a newb ever having this problem so severely that they blew a PT.

On the other hand, some people actually have had issues on the HT, and that's why some people like to incorporate a 300mA to 500mA fuse on the HT CT, though I've never seen an issue with this either personally.

For the record, and in general, when making an argument, saying "trust me, a guy smarter than me said so" is not a valid argument. Believe, I've wanted to use that argument too. lol (usually I'm referring to Zaphod_Phil, but I have the benefit of linking to his posts for reference) hahahahah

I highly recommend you do some more reading on this site, on the ValveWizard site, and on Randall Aiken's website.

Thanks,
Josh
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Re: AC heaters done right….how to get it done

Post by Doxie man »

JMPGuitars wrote:
Mon 08/30/21 6:54 am
Doxie man wrote:
Sun 08/29/21 9:43 pm
What happens if there is a short in the filament string with the CT directly coupled to chassis ground ? It’s pretty obvious what happens. Now….run the heaters through 2 balanced resistors to reference an elevated ground and those resistors will cook off in a quick second potentially saving the PT from a catastrophic event. I learned this method from a very experienced and knowledgeable tech that works and builds more amplifiers than you or I ever will. I do realize it’s an unlikely scenario but I like to cover every base I can when I’m building a circuit. If you do some digging and research you’ll verify it. I have seen PT’s that we’re not balanced as good as if I used the CT but ymmv of course. I also have a nice way of taking the strain off the CF tube on startup in other circuits using a diode and resistor. It’s the little things we learn from more experienced builders that makes us better 😉
I mean...If you want to be careful, do a good job wiring your heaters and this whole argument is dead. I can't even recall a newb ever having this problem so severely that they blew a PT.

On the other hand, some people actually have had issues on the HT, and that's why some people like to incorporate a 300mA to 500mA fuse on the HT CT, though I've never seen an issue with this either personally.

For the record, and in general, when making an argument, saying "trust me, a guy smarter than me said so" is not a valid argument. Believe, I've wanted to use that argument too. lol (usually I'm referring to Zaphod_Phil, but I have the benefit of linking to his posts for reference) hahahahah

I highly recommend you do some more reading on this site, on the ValveWizard site, and on Randall Aiken's website.

Thanks,
Josh
Been there, done that years ago….not new to this stuff 😂
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Re: AC heaters done right….how to get it done

Post by JMPGuitars »

Doxie man wrote:
Mon 08/30/21 9:25 am
Been there, done that years ago….not new to this stuff 😂
Nothing wrong with a refresher. I revisit stuff all the time. ;)

I even rewatch the soldering videos in my signature's soldering thread. There's always room for improvement and clearer understanding.

Thanks,
Josh
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Re: AC heaters done right….how to get it done

Post by Doxie man »

JMPGuitars wrote:
Mon 08/30/21 1:03 pm
Doxie man wrote:
Mon 08/30/21 9:25 am
Been there, done that years ago….not new to this stuff 😂
Nothing wrong with a refresher. I revisit stuff all the time. ;)

I even rewatch the soldering videos in my signature's soldering thread. There's always room for improvement and clearer understanding.

Thanks,
Josh
I couldn’t agree more with you on what you just said Josh ! As I age…I forget **** all the time. 🤦‍♂️ I got really burned out last year building/experimenting and designing higher gain stuff…..I had forgotten just how relaxing and refreshing returning to the simple circuits I started with many years ago ❤️
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Re: AC heaters done right….how to get it done

Post by zaphod_phil »

JMPGuitars wrote:
Mon 08/30/21 6:54 am
Doxie man wrote:
Sun 08/29/21 9:43 pm
What happens if there is a short in the filament string with the CT directly coupled to chassis ground ? It’s pretty obvious what happens. Now….run the heaters through 2 balanced resistors to reference an elevated ground and those resistors will cook off in a quick second potentially saving the PT from a catastrophic event. I learned this method from a very experienced and knowledgeable tech that works and builds more amplifiers than you or I ever will. I do realize it’s an unlikely scenario but I like to cover every base I can when I’m building a circuit. If you do some digging and research you’ll verify it. I have seen PT’s that we’re not balanced as good as if I used the CT but ymmv of course. I also have a nice way of taking the strain off the CF tube on startup in other circuits using a diode and resistor. It’s the little things we learn from more experienced builders that makes us better 😉
I mean...If you want to be careful, do a good job wiring your heaters and this whole argument is dead. I can't even recall a newb ever having this problem so severely that they blew a PT.

On the other hand, some people actually have had issues on the HT, and that's why some people like to incorporate a 300mA to 500mA fuse on the HT CT, though I've never seen an issue with this either personally.

For the record, and in general, when making an argument, saying "trust me, a guy smarter than me said so" is not a valid argument. Believe, I've wanted to use that argument too. lol (usually I'm referring to Zaphod_Phil, but I have the benefit of linking to his posts for reference) hahahahah

I highly recommend you do some more reading on this site, on the ValveWizard site, and on Randall Aiken's website.
I particularly recommend the latter one. Not everything Valve Wizard says is reliable, as he tends towards a kind of theoretical paranoia, while Aiken is 100% solid.:)
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Re: AC heaters done right….how to get it done

Post by zaphod_phil »

Doxie man wrote:
Sat 08/28/21 9:01 pm
Here is the method I use when wiring up the filament string in my amplifiers. It will yield the most quiet, safe and balanced heater circuit you can achieve in an AC powered filament circuit. The reasons to do so are as follows:
1. Using the (2) 1% precision toleranced 100R 1/2W resistors will balance to the two halves of the circuit better than the factory supplied CT.
There is actuallyi zero need for the elevating DC to be applied in a balanced manner.DC elevation is just as effective when applied at either end of the heater supply chain (ie either 6.3V lug on th PT) .
Doxie man wrote:
Sat 08/28/21 9:01 pm
Should a short occur in the filament string , the resistor(s) will act as sort of a fuse and “cook off” protecting the power transformer. While it isn’t a 100% guarantee damage won’t occur, it is much better than having the supplied CT directly coupled to ground and a short occur
But when using DC elevation, the heater string isn't referenced to ground! Instead it's connected to the elevating DC supply. :)
Doxie man wrote:
Sat 08/28/21 9:01 pm
2. Instead of referencing chassis ground through those resistors, simply reference ground to the power tube’s cathode / cathode resistor. This will elevate the heater string above chassis ground reducing noise potential.
Yes, I've also used that trick myself.However, it has a drawback, that the DC elevation voltage is limited to the power tube cathode voltage, which for a marshall18W type amp is around 12 to 14 volts, while at least 30V is typically recommended for elevation.

Also please note that heater elevation is not about referencing to ground or even elevating the heaters above ground potential, but rather it's biasing the heaters to a positive DC voltage level high enough above all the cathodes' voltages (including the PI's 70V in an 18W amp) so that the heaters don't emit any electrons onto the inner surface of the cathodes, which would introduce hum. I'm a huge believer in DC elevation, myself and have used it very successfully in many amp builds over the years.
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Re: AC heaters done right….how to get it done

Post by TriodeLuvr »

DC heater elevation may also be necessary to avoid exceeding a tube's heater-cathode rating in a cathode follower or totem-pole (pseudo-SRPP) configuration. One of the driver stages in a hi-fi amp I'm rebuilding uses a 6BL7 with the two triodes in series. The upper triode has more than +500V applied to the anode, so its cathode operates at about +250V. Applying roughly +100V to the filament supply (shared only with another 6BL7) keeps both of the series sections within the tube's +/- 200V H-K rating.

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Re: AC heaters done right….how to get it done

Post by TriodeLuvr »

This thread raises a question I've been thinking about lately. If the filaments of 6.3V preamp tubes are raised to +70V, is the center tap still needed? Connecting one side of the filament to +70V will mean the other side never falls below +61V at the peak of the wave. I think that's probably sufficient to prevent electrons at the filament from flowing to the cathode.
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Re: AC heaters done right….how to get it done

Post by zaphod_phil »

Yes, that's just what I was saying earlier. And so there's no need for a centre-tap on the PT's heater winding, whether real or virtual. However, keep in mind that the cathodes of the LPT-PI in a typical Marshall 18W are at 70V, so you may need to elevate a bit higher than that.
A handy way to get a heater-elevation bias voltage is to tap it off from the 220k filter cap bleed resistor, by splitting it into a 50k|170k 1W potential divider pair, with the 50k to ground and a 16uF cap in parallel with the 50k. That should give you 77V clean elevation voltage. :)
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Re: AC heaters done right….how to get it done

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zaphod_phil wrote:
Fri 01/21/22 6:52 pm
Yes, that's just what I was saying earlier. And so there's no need for a centre-tap on the PT's heater winding, whether real or virtual. However, keep in mind that the cathodes of the LPT-PI in a typical Marshall 18W are at 70V, so you may need to elevate a bit higher than that.
A handy way to get a heater-elevation bias voltage is to tap it off from the 220k filter cap bleed resistor, by splitting it into a 50k|170k 1W potential divider pair, with the 50k to ground and a 16uF cap in parallel with the 50k. That should give you 77V clean elevation voltage. :)
Yes, I see now that you did say an elevated heater system doesn't need to be balanced. That makes things a little easier, eh? It's worth nothing that some tubes like the 12AT7 have an h-k limit of less than 100V, and it isn't always clear from the data sheets whether that might include the AC component of an unbypassed cathode. This limit requires care if the entire string will be elevated on the basis of LTP voltage. Actually, I would expect signal levels at the LTP to be high enough to forego this method of hum reduction. Just raising the voltage to suit the preamp should be enough. Thinking about it further (which usually gets me in trouble) this might be a valid reason for balancing the string, even if it's elevated. It might be that some of the tubes in the string will experience a positive heater, others not. Just thinking out loud here...

Jack
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