Recent mention of the 6s19p 6S19P Push Pull Build has me intrigued with that tube's characteristics and low cost.
A thought experiment - 6.3v x 20 = 126v.
Could you series-string the filaments of 20 6s19p tubes, to feed from standard ~+/- 125vac? Seems very appealing compared to a high-current 6.3v filament transformer.
I know that series-stringing filaments was super common in cost-cutting designs in the mid-to-late consumer product vacuum tube era [and often required tubes with appropriate characteristics for the series arrangement] but I have never heard of someone doing a series string of a ridiculous number of 6.3v filament tubes
A thought experiment - 6.3v x 20 = 126v.
Could you series-string the filaments of 20 6s19p tubes, to feed from standard ~+/- 125vac? Seems very appealing compared to a high-current 6.3v filament transformer.
I know that series-stringing filaments was super common in cost-cutting designs in the mid-to-late consumer product vacuum tube era [and often required tubes with appropriate characteristics for the series arrangement] but I have never heard of someone doing a series string of a ridiculous number of 6.3v filament tubes
First of all: only with an isolation transformer.
Heaters need to be referenced to the rest of the circuitry, so feeding them from the mains would imply some connection between circuit and the mains. Dangerous, don't so it.
About series connecting heater strings: this can be done provided the tubes were designed for a specific heater current and warmup characteristic.
In Philips/Millard naming system, you can recognize those by the first letter: P (300mA) or U (100mA).
Heaters need to be referenced to the rest of the circuitry, so feeding them from the mains would imply some connection between circuit and the mains. Dangerous, don't so it.
About series connecting heater strings: this can be done provided the tubes were designed for a specific heater current and warmup characteristic.
In Philips/Millard naming system, you can recognize those by the first letter: P (300mA) or U (100mA).
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Indeed, it’s so dangerous to run off mains you are at high risk of having this thread shut down but an isolation transformer is fine. Why not consider using a SMPS for the heaters, lighter and more efficient than a regular transformer, you can series / parallel heaters as you like to match with the voltage of your SMPS.
Series operation is for tubes that were specially made to function like that as series filaments are designed for a certain current instead of a voltage. With normal tubes it is not clever to use them in series and it is using the tubes out of specifications and not appealing at all. Imagine what will happen if one or more of the filaments starts to change in resistance ... which they do! Also think of start up current at power on with a number of cold filaments in series that are not 100% the same... Please think what will likely happen and what it will do in the long term. Use theory and logic so a technical way of thinking.
Like mentioned before it is also very dangerous and plain stupid when done without an isolation transformer. It the required isolation transformer is used then what was exactly gained? That is the reason you never heard of someone doing it as it has been done combined with the known results.
Like mentioned before it is also very dangerous and plain stupid when done without an isolation transformer. It the required isolation transformer is used then what was exactly gained? That is the reason you never heard of someone doing it as it has been done combined with the known results.
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what is the construction difference that allows this?
There seems to be a lot of received wisdom in this discussion. My understanding is that series heater tubes are designed to run at a "particular" current, not a "constant" current.
In an old TV heaters were series wired at line voltage and all designed to draw the same current, but with different voltage drops across the types ie larger tubes dropped more volts and so consumed more power, smaller ones less.
ANY indirectly heated tube is just a resistance heater - it neither cares nor knows what is happening either side of it. Provide it with the right voltage and it will draw the correct current, and vice versa.
There may be reasons not to series string heaters, but heater requirement for either constant current or constant voltage is not one of them.
It's not that simple. Yes, it is a resistance heater, but its resistance is substantially temperature-dependent.
Say, we have two tubes with some heater resistances difference.
With parallel connection the heater with lower resistance will initially draw more current, heat up more and increase its resistance, dropping the current. It more or less stabilizes itself within some reasonable limits.
With series connection the heater with lower resistance will simply drop less voltage. The second heater runs hotter because of the bigger voltage, increases its resistance and drops the current, reducing the voltage drop - but that also drops the current through the first heater, furthermore decreasing its resistance and therefore the voltage drop. Depending on the individual parameters of the two heaters it might stabilize somewhere or it might totally go berserk.
I'm not saying it's 100% not going to work, but it requires some special attention to say the least. Note that current-rated heaters are usually low current/high voltage ones - the convenience of running them directly from the mains is not the only reason for that.
And how they did it ? I saw a schematic in which the filaments were in series mains powered but the rest was insulated using a transformer ... well , that is wrong , there must be a cathode to filament resistive path otherwise there is induced hum and the posibility of cathode-filament short .
So you can't combine both ...
So you can't combine both ...
I'm getting the clear picture that this is not a good idea and I will respect that. I did find one Japanese schematic that showed 16 of them in series on the mains (JP 100V) side of the supply, but in an arrangement where the (mains/primary) side of the power transformer had a center tap, so that it was 8 across each half of the primary. With someone going to that much trouble it seems better to just power them in some more normal way.
Thanks everyone for indulging my thought experiment, it seemed too good/simple to be true and apparently it is...
can be done, has been done ... behind an isolation transformer of course
16 x 6s19p in a single chain, +/-150v doubler from same secondary (OTL)
this tube also has 250v heater/cathode allowance, comes in handy
also solves your heater vs gnd/B+/B- reference issue
controlled heater is relevant if you have vastly different tubes in your chain
like horizontal sweep and small signals as was the case in TV sets
but 20 x same tube type, same manufacturer, same week code, really ...
and what is at stakes ? 2,50 $ if the weakest one decides to play fuse protecting the others
anybody remembers those christmas tree lights, 24 incandescent lamps in a single daisy chain, did they ever go berserk ... billions of them before LED
again, hook them up to transformer secondary only, measure voltage drop across each and every heater
your numbers will vary, between 5.8 and 6.8 v in my case
but you don' t buy just 20 when you plan on 20, I assume ...
and don't wire them up in one big loop, instead daisy chain the feed wire from tube to tube, then run the return the same route back and twist it with the feed
16 x 6s19p in a single chain, +/-150v doubler from same secondary (OTL)
this tube also has 250v heater/cathode allowance, comes in handy
also solves your heater vs gnd/B+/B- reference issue
controlled heater is relevant if you have vastly different tubes in your chain
like horizontal sweep and small signals as was the case in TV sets
but 20 x same tube type, same manufacturer, same week code, really ...
and what is at stakes ? 2,50 $ if the weakest one decides to play fuse protecting the others
anybody remembers those christmas tree lights, 24 incandescent lamps in a single daisy chain, did they ever go berserk ... billions of them before LED
again, hook them up to transformer secondary only, measure voltage drop across each and every heater
your numbers will vary, between 5.8 and 6.8 v in my case
but you don' t buy just 20 when you plan on 20, I assume ...
and don't wire them up in one big loop, instead daisy chain the feed wire from tube to tube, then run the return the same route back and twist it with the feed
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Tubes meant for series string operation had controlled heater time warm up characteristics so that no tube got clobbered with excessive voltage across its heater if it drew less current. The current after warmup was also close to the specified value so that the voltage divided up equitably among the tubes. You won't have that with the tubes you are using, and as a result, reliability will be poor.
Is it really? Please use electrical laws and logic.
The only solution to avoid humongous amounts or filament current is to not make the "system choice" of using 20 tubes. What seemed cheap is expensive after all.
Please see practical results in post #15: between 5.8 and 6.8V per tube.... The perfect explanation why the old Christmas lights one day failed. Anyone who had those remembers looking for the spare bulb
The only solution to avoid humongous amounts or filament current is to not make the "system choice" of using 20 tubes. What seemed cheap is expensive after all.
Please see practical results in post #15: between 5.8 and 6.8V per tube.... The perfect explanation why the old Christmas lights one day failed. Anyone who had those remembers looking for the spare bulb
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If you did a matrix of 4 series strings (5 tubes per string) all going into parallel, that would be 4 amps total @ 31.5 V. The probability of 1 tube out of 5 turning into a hog is much less than 1 out of 20. And a 4 amp, current regulated, or voltage regulated or smps or ac supply that quiesces at 31.5 volts (6.3 x 5) is much safer than mains voltage. I went with 5 because you have 10 tubes per channel, depending on how they are biased you can use a group of 5 going out all at once failure mode to your advantage.
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