Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on August 31, 2012, 06:32:23 pm
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Hi guy's, This might be a one of those things i am overthinking. I'm using 12v for my heaters PSU and all works well but eventhough the 12ax7 and 6au6 data says that both draw 300mA this is not the case. The 12AX7 with pin4 & 5 connected and measured between this and pin9 gave me 10.9ohms and the 6au6 between pin3&4 gave me 6ohms,this resulted when connected in series,12ax7 running on 5.4v and 6au6 running on 6.3v. Most data says that heaters can run with a +/- of .6v.This maens the 12ax7 is a little low on volts, i don't think this is a real concern for a DIY build but maybe concern for someone else.Thanks
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> gave me 10.9ohms .... gave me 6ohms
These are COLD, right? They increase when hot, and perhaps not the same increase.
> 12ax7 running on 5.4v and 6au6 running on 6.3v.
11.7V, not 12V? Not 12.6V as the specs say?
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Hi PRR, I have a 12v transformer rated @ 2A with two 100r resistor for a centre tap (12.1v across both legs).I have two ecl84 in series and the 12ax7 and 6au6 in series as well. The ecl84s measured between pin4&5 is 5.8v (both 5.8v)and the resulting volts for the ax7 and 6au6 is as said. After being on for a good 5 minutes the voltage does not change. I checked the wireing and it is as the schem.Thanks
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In the RCA datasheet 12ax7 filament voltage is claimed must be within + or - 20%
(for 12.6v = 10.08v till 15.12v / for 6.3v = 5.04v till 7.56)
http://www.tubebooks.org/tubedata/HB-3/Receiving-Type_Industrial_Tubes/6681_12AX7.PDF (http://www.tubebooks.org/tubedata/HB-3/Receiving-Type_Industrial_Tubes/6681_12AX7.PDF)
in the GE datasheet 6au6 filament voltage is claimed must be within + or - 10%
(for 6.3v = 5.67v till 6.93v)
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AU6A.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AU6A.pdf)
the minimum voltage requirement are 5.04v for the 12ax7 and 5.67v for the 6au6, total minimum voltage required 10.71v
so as you have a 12v heater supply you are in the ballpark
Quote from Merlin's web page
Series and parallel: Valves which are normally designed to be run in parallel can be run in series provided you ensure their current demands are met correctly, althopugh series heater chains are not recommeded for audio. For example, an EL84 and ECC83 could be run in series from a 12V supply. The EL84 is rated at 0.76A while the ECC83 is rated at 0.3A, therefore a resistor must be placed in parallel with the ECC83 to pass the additional current without damaging the valve. We want to pass 0.76 - 0.3 = 0.43A through the resistor, and we want the voltage across the resistor to be 6V. Use Ohm's law to calculate its value:
6 / 0.43 = 14 ohms.
(http://www.freewebs.com/valvewizard/heater2.jpg)
The power dissipated will be:
(0.43 * 0.43) * 14 = 2.6W
So we would probably use a 15R, 5W resistor.
All sorts of heater chain combinations can be created in this way.
Series AC heater chains will not benifit from a grounded centre tap [see below], but will benifit from an elevated centre tap, to reduce noise.(note this ! - K add on)
Because the heaters are in series it doesn't matter in which order the valves are wired. However, if one end of the heater chain is to be grounded, then the most sensitive preamp valves should be closest to the grounded end of the chain.
Since the different heaters will often have different warm up times that could put stress on the other valves, a thermistor can be placed in series with the chain, or a resistor switched in and out by a standby switch
as your tubes are both indicated to have a consumption of 0.3A you must do some test and try to find the real current each tube draw, than you can apply the formula to calculate the value of the "balancing" resistor
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in your schematic your PS is draw as it has a 15v winding, if you want to increase the voltage of the heater may be you can try to use that winding with an in series resistor
K
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Thanks K, always good to hear from you.By your calcs i think all is good so i leave it as is. :icon_biggrin:
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A timely thread, I am getting ready for a TOS build, and would like to run the heaters on 12V, would the following wiring work? Ignore the resistance values.
(https://dl.dropbox.com/u/1326040/Heater%20Wiring.gif)
Thanks,
Jaz
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would the following wiring work?
if you connect the 12ax7 for 12.6v seems to be feasible
say a bit more about the transformer, is a 12.6v CT ? Which is the rated current ?
K
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The transformer has two separate 6.3V 2A windings. Pin 4 - pin 5 on the AX7 are designed for 12.6V heater voltage - the current draw is 0.15A per tube.
Jaz
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The transformer has two separate 6.3V 2A windings
If that was my amp project:
I don't connect together the 6.3v windings
put on each winding two 100ohm resistor to have on each winding an artificial CT
connect in parallel the 6v6 tubes to one winding
(current consumption for this winding 0.9A)
connect 12ax7 tubes for 6.3v
connect 12ax7 tubes + 5879 tube in parallel on the other 6.3v windind
(current consumption for this winding 1.05A)
K
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Are you adding the tubes' heater current draw? At 12v draw is 150 not 300.
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Ciao Jjasilli
Are you adding the tubes' heater current draw?
are you asking that to me ?
if so as I told I considered to use two separated 6.3v supply not a 12.6v supply + 6.3v intake
one for Power Tubes, the other for preamp tubes
K
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The transformer has two separate 6.3V 2A windings
If that was my amp project:
I don't connect together the 6.3v windings
put on each winding two 100ohm resistor to have on each winding an artificial CT
connect in parallel the 6v6 tubes to one winding
(current consumption for this winding 0.9A)
connect 12ax7 tubes for 6.3v
connect 12ax7 tubes + 5879 tube in parallel on the other 6.3v windind
(current consumption for this winding 1.05A)
K
Using the windings separately is the standard way to go, but by running the heaters with 12V, we can lower the current, thus reducing the noise/hum generated by them. And since I am not wiring dissimilar tubes in series, I could avoid the un-equal current draw issues as you have indicated above. I think I can even ground the center of the two windings to reduce the hum further...
Jaz
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Ciao Jjasilli
Are you adding the tubes' heater current draw?
are you asking that to me ?
if so as I told I considered to use two separated 6.3v supply not a 12.6v supply + 6.3v intake
one for Power Tubes, the other for preamp tubes /K
The question is to timbo
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Hi Jjasilli,I can get the four tubes total current draw under the 1A transformer rating. Ecl84 .9A when two are in series and when the 12ax7 and 6au6 are using there 6.3v hookup they will draw .15A each. I have found that using the 12v circuit i have no heater hum at all. :icon_biggrin:
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Hi Jjasilli,I can get the four tubes total current draw under the 1A transformer rating. Ecl84 .9A when two are in series and when the 12ax7 and 6au6 are using there 6.3v hookup they will draw .15A each. I have found that using the 12v circuit i have no heater hum at all. :icon_biggrin:
That's what I am aiming for as well - minimum hum.
Jaz
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Your idea to lower hum is based on the fact that at 12.6v current consumption is lower
My idea to use two separate PS at 6.3v is based on the fact that final tubes draw a good amount of current so I've think it will be better to separate the current request
As there are separated windings (and not a 12.6v winding with CT) the PS can be arranged in both the way
K
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Your idea to lower hum is based on the fact that at 12.6v current consumption is lower
My idea to use two separate PS at 6.3v is based on the fact that final tubes draw a good amount of current so I've think it will be better to separate the current request
As there are separated windings (and not a 12.6v winding with CT) the PS can be arranged in both the way
K
That makes sense as well, I will keep that in mind for the bench test to see which way works better.
Thanks,
Jaz
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I will keep that in mind for the bench test to see which way works better
and we are interested to know results
K