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Old 1st August 2007, 07:58 PM   #1
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Default Series connection of filaments

It is commonly recommended that if employing series connection of filaments in the amp design (2 * 6.3 V = 12.6 V transformer winding), that the tubes be matched and always replaced as a pair.

Does this also hold true for regulated DC filament supplies, whether those supplies be constant current or constant voltage?
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Old 1st August 2007, 08:36 PM   #2
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As far as the filaments are concerned the only matching necessary is the current draw of each 6.3V tube. That must be the same so that each tube receives equal division of the 12.6 volts. And you probably already know this.

Replacing one tube with the same type and not the other is of no consequence as far as the filament supply is concerned. Be it regulated or not. But keeping them matched for other reasons like gain balance might be necessary.

Victor
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Old 1st August 2007, 08:38 PM   #3
engels is offline engels  Israel
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Default serial

The problem is actually that filaments may have slightly different resistances, which results in different voltage drops across each tube.
Say, you have a 12V DC regulated filament suply and you wire two similar 6.3V tubes in series. Then you measure 5.9V across one tube and 6.1V across another tube. If they're "matched" you may measure something like 5.95 and 6.05.

But I'm afraid the term "matching" usually refers to the B+ currents, not the heaters.

Btw, some older tubes have other problems with series heater supply, for example check the 6C8G, but that's another story.
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Old 1st August 2007, 09:24 PM   #4
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Now I think I am more confused. I actually had never meant to suggest that the tubes be matched for filament draw, just that they be matched for gain.

Are you suggesting that ensuring that the tube filaments having the same voltage across them is of no consequence, or that it is necessary for this to be the case to get matched gain from them?
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Old 1st August 2007, 09:29 PM   #5
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Morgan Jones addresses this in his "Valve Amplifiers" book. I don't have the exact statement in front of me, but basically he recommends heaters in parallel (whether in the same or different tube) be supplied with a voltage source, and heaters in series with a current source.

This will be your best defense against varying filament resistances, temperature coefficients, and the like.

While some designs may benefit from balancing triode sections for bias, gm, or mu, I don't hear of too much emphasis on filament balance. It would certainly be that much more difficult to balance both the triode sections and filament at the same time.

You mentioned "it is commonly recommended". I haven't heard this too much, but I can understand the concern about imbalance with a series connection and voltage source. Balance would be beneficial.

To answer your last question, it wouldn't change matters if you use AC or DC, regulated or unregulated, but it would change matters if your are using a voltage or current source (see first paragraph).
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Old 1st August 2007, 11:20 PM   #6
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If 2 tubes of the same type, manufactured in the same time frame in the same plant, are wired with series heaters, there is nothing to worry about.

Series connection of disparate tubes is a problem unless the warmup time is controlled. The "A" suffix seen in 7 and 9 pin miniature types is an indication of controlled heater warmup time. The warmup time became a matter of concern in TV receivers that used length series heater strings.
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Old 2nd August 2007, 03:53 AM   #7
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If you're not using controlled heater warmup tubes (generally -A, sometimes -B... read the datasheet), just use a centertapped 12.6v trafo.
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Old 2nd August 2007, 10:38 AM   #8
engels is offline engels  Israel
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Quote:
Originally posted by zigzagflux
Morgan Jones addresses this in his "Valve Amplifiers" book. I don't have the exact statement in front of me, but basically he recommends heaters in parallel (whether in the same or different tube) be supplied with a voltage source, and heaters in series with a current source.
Exactly!
So if you intend to wire something in series just build a current regulated PS, and don't worry about the voltage.

Yesterday I've missed the part of your post saying "constant current or constant voltage". Sorry for that! When I hear "regulated PS" I always think of regulated voltage.

The old apparatus that used series wired mains powered heaters sometimes had a current regulator tubes, and if you read the datasheets you may pay attention to the fact they didn't vorry about the voltage being 110V or 115V or whatever.

For example, read this: U30
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Old 2nd August 2007, 11:30 AM   #9
ilimzn is offline ilimzn  Croatia
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Another important thing to remember if you are using series connected filaments, designed for series connection (*), but without a constant current source to provide a constant heater current for the chain, you need a 'buildout resistor'. Heater filaments tend to have a much lower resistanxce when cold, which results in a considerable current surge when the heater voltage is applied. In series heater chains, it is the relative differences of these resistances, as well as heating times of the actual tubes, that result in a seizable imbalance of voltages across series connected heaters, simultaneously with the aforementioned current surge. As a result , some tubes' heaters literally flash white hot. Although heaters generallly survive this for a long time (but shorter than under proper conditions), it causes filament material evaporationand contamination of the insides of the tube, hence deteriorated characteristics. It is often visible as blackened areas on the bulb glass between the pins and the bottom mica in tubes used in TVs, for instance. To lessen this effect, series connected strings were not directly connected to a voltage supp[ly equalling the approximate sum of all heater voltages, but rather to a slightly higher voltage, via a 'buildout resistor'. Because this resistor, unlike the resistance of the filament, is a constant, it acts as a current surge limiter, and considerably lessens the effect described above. Of course, the penalty is the ehat developed on this resistor. With a current source supplying the chain, no resistor is necessary, since the current source already limits the current. Use of it, however, does considerably prolong the ehating up time.

(*) Tubes not originally designed for series heater conenction whould in general NOT be connected in that manner as for them it is only guaranteed that the correct applied heater voltage will make the heater operate correctly. There can be considerable variations in the actual current even between different production dates of the same tube version, from the same manufacturer, not to mention across different versions, dates and manufacturers. If series connected, this will result in imporpper heater voltage distribution - one will be too high, one too low. If you must series connect, them you will have to chose and measure tubes to work properly so connected.
There are also a number of tubes that were designed with several heater connection syles in mind. 12A*7 are a good example - they can be connected as 6.3 or 12.6V parallel heated, and just happen to require 300mA and 150mA heater current respectively, which are standard series heater chain currents. Although this looks like great foresight on the part of the tube designers, you should not take this for granted. Many versions of the tube exists, and unless you have the correct datasheet to check if series heater chain application is designed for (keyword: controlled heater characteristics, or the data outright mentions it), do not assume anything. It largely depends on what market the particular version was made for. In Europe it is common to apply tubes like the ECC83 (~12AX7) in a 300mA series chain, also the EF80 and EF184, along with some ECH models that have 6.3V heaters that also pass 300mA under these conditions. But even themn there are great variations in actual heater construction, evident when you replace a tube in a series chain and get the above mentioned white hot filament 'flash'.
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Old 2nd August 2007, 04:39 PM   #10
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Quote:
Originally posted by Eli Duttman
If 2 tubes of the same type, manufactured in the same time frame in the same plant, are wired with series heaters, there is nothing to worry about.

Series connection of disparate tubes is a problem unless the warmup time is controlled. The "A" suffix seen in 7 and 9 pin miniature types is an indication of controlled heater warmup time. The warmup time became a matter of concern in TV receivers that used length series heater strings.

I had a Marantz 2 power amplifier with a pair of 6AU4 wired in series - why I will never know, but in any case I had a lot of rectifier tube failures in this amp due to uneven distribution of filament voltage across the pair. The solution was to find a match to one of the tubes installed in the amplifier such that the filament voltages were equal after some warm up time.. Note that several of the failed tubes met all of the above criteria.
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