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wiring tubes in series

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Aran Everitt

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Joined 2015
#1
hi guys, i just got my very nice OPT and all appears to be very good. i just fired the 3-tube amp up and as you can imagine, all of the 12v pushed into one of the 6v tube's heater. my quick timing may of spared it as i had sort of a kill switch button which i let go of once i saw the flash but for some reason i just can't figure out the way to wire the heaters in series as of course i am not the brightest of amp workers🙁. on the bright side i know what went wrong. since it is a filament, it's 2 wires can be either + or - charged so if i wired it how i thought would of worked, it still allowed a full 12v to pass through 1 tube, instead of both, letting it splitting the difference. hopefully one of you guys can quickly hint me to series tube heater wiring. don't get me wrong. i do know how to wire a series circuit but not one where both the ground and positive are both ways. thanks

Aran
 
#2
Series wiring for heaters is only possible if all tubes require exactly the same heater current. Otherwise, the tube with the lower current requirement will be killed by overvoltage. But be aware that only P-series (European numbering system) tubes are intentionally designed for series heating.

Most tubes are designed for parallel heating with the voltage depending on the tube type, 6.3V tubes are most common.

Without a schematic of your setup and some more details about the tubes you used it will be difficult for the forum members to give you any advice.

Regards,
Andreas
 
#3
thanks for the reply Andreas. ok i will just give you more specific detail. normally lets say a 6.3v heater in parallel is simple - just connect it to a parallel circuit however wiring in series is different. I am only using this for 2 tubes (preamp) so it should be all good in series but like i said i am not familiar with wiring heaters in series (so the 12v would split into 6v and let me run the heaters). Like you mentioned, if one tube requires 12v filament yet the other 2 (same 2 tubes) require 6.3v but connect them through the same power supply this would not work, like you mentioned. to be honest showing the schematic is useless as i didn't consider drawing the heater wiring as i didn't think that this would be a problem. any more help would be much appreciated.

Aran
 
#6
So you wired two tubes of the same type 6n6b-v in series to a 12V supply?

In principle this should work, though the tubes might be suffering some overvoltage stress at startup if they warm up differently. The 'faster' tube will have a higher resistance and get a larger share of the total voltage, heating it even more. Not a good situation.

If series operation is a must, a current-regulated supply would be a better option.

Regards,
Andreas
 
#7
ahh i see. i was atleast expecting to see some life from the 3 tubes but no - all the power consumed in a very bright flash from one 6n16b. very fustrating i must say. do you by any chance know how to correctly wire in series as it was a quick guess for me, like i said i just connected it so it goes + in one end, the two tubes connected at 2 points on the heater, - out the other. no current goes to the other tube. thanks

Aran
 
#8
But be aware that only P-series (European numbering system) tubes are intentionally designed for series heating.
Click to expand...

Andreas, the "A" suffix, for example 6AQ5A, frequently found on American 7 and 9 pin mini types indicates controlled heater warmup time, for use in series strings. In the case of Octal types consult the data sheet. The 6SN7GTB is a good example.

2X tubes of the same type and production run can safely be used, with their heaters in series, even if the type is not specifically rated for series string service. The key in all series strings is avoidance of over voltage on any specific tube's heater.
 
#10
Eli,

thanks for the clarification. I did not express properly what I meant - "P series are the only series heater tubes I know" would have been more on the spot.

I always wondered if there were special designations for series heater compatible tubes in the American or Octal types, so thanks for the information!

Regards,
Andreas
 
#12
Or, in words:

One wire from your power supply to the first heater pin of the first tube. Then from the other heater pin of this tube over to the next tube. Then from the second heater pin of the second tube back to the power supply. You would need three wires in total, and only one wire at each terminal!

Regards,
Andreas
 
#19
If you have some test equipment, test both tubes' heaters for continuity to make sure nothing is fried already.

Some tubes show a bright heater flash at startup, but work absolutely perfect afterwards - maybe you saw this and switched of a little too soon.

Are the tubes special in any way or are you able to replace them in case something goes wrong? If the latter, make sure the wiring is correct and try again.

Regards,
Andreas
 
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