Another Heater Wiring Question

[TIMBO]

Hi guy's, How do i series wire a 12AX7 with a 6AU6 for a 12v supply. At the moment i have the 12AX7 wired with pin 4,5 together to one leg of the 12v supply and pin 9 connected to pin3 of 6au6 and pin4 is connected to the other 12v leg (artificial CT on supply) When voltage measured across 12AX7 pins i get 3.4v and measured across 6au6 i get 7.8v so i don't think this is right. A little direction please, Thanks

[sluckey]

The tubes are wired correctly and each tube requires the same 300ma current so the voltage should split equally across each tube. Remove the artificial CT and recheck voltages. Is this an AC supply or DC supply? If DC, show us your schematic. Are you measuring voltages directly across each filament right on the tube socket?

EDIT... Try this... Pull the tubes and measure the resistance across the filament pins of each. For the 12AX7, you'll have to short pins 4 and 5 with your probes as you check it. Are the readings equal? If not, the voltage will not split equally.

[TIMBO]

Thanks for that Sluckey, it had me a bit stumped cause i thought that i had it wired right but as usual when you have a closer look at whats in front of you ,i then realise that i did not have a 6au6 in i had 6AM5 in and its heater is 200mA.I still would not have picked up that the different current draw would result in different voltages. I put in a 6AU6 and checked the resistance of each and they were nearly the same this resulted in both pin voltages also being nearly the same.Thanks again for the help 

[sluckey]

the different current draw would result in different voltages.

That's probably one worth commiting to memory if you're gonna work with series filaments. Since there's only one path for current to flow in a series circuit, that current will be the same anywhere in the circuit. So, it's important that tube filaments be rated for the same current. Otherwise, the voltage across each filament will not distribute the way you want it to.

Maybe plexi50 is listening too and will post this on his 'wall of knowledge' along side his other info on series filaments. 

[kagliostro]

Here you can read how to "equilibrate" your heater supply chain

http://www.freewebs.com/valvewizard/heater.html

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.

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.

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 [see the section on power and standby switches].

K

p.s.: TIMBO I don't understand very much the back to back arrangement for the PT, can you explain better ? Thanks

[TIMBO]

Hi K, Again the arrangement of the back to back transformers is another one of Darryls pets and i'm sure he's happy to and some lite. 

[kagliostro]

I was asking because of the way you've draw the connection, also the data of the transformers are a bit confused to my eyes

K

[TIMBO]

Hi guy's, Just a last check with these voltages. I have 5.7v on the 6au6 and 5.3v on the ax7, now the 6au6 has a +/- of .6v so thats ok for it but what is the +/- on the ax7 will 5.3v be ok or would i be better to take the heater supply off the 15v tap because the voltage off the 12v tap actually gives me 11v so if i use the 15v tap and possibly get 14v this will then give me 7v on each ECL84 and a possible 7.2v on the 6au6 and 6.8v on the ax7. What would be the better option. 

K,I know a little bit about the back to back transformers. The first transformer is rated @1A and the other is also rated @1A so when connected together they supply approx. 240v@1A (wall voltage here 240v). I am also using the first transformer for the heater supply.When doing this you have to take all current loads into consideration. I have just added another tube to the circuit so i will have to see how hot the trannys get, my calcs come to just over an 1A. Thanks