High Heater Voltage in Dlator

[SoundPerf]

Hope someone can give some help.
I built a Dlator which is working great and sounds great, but I have an issue with 7.7VAC on the heater line. I'm guessing mainly due to having 124VAC mains and a slightly out of spec transformer. I'm using a separate 6.3v-0V @ .6A tranny.

I wasn't as concerned at first, but I bought a NOS 12AX7 and started reading about running heaters at too high voltages and got worried I was killing my nice tube. The thing is some of the stuff I'm reading has only served to confuse me more due to conflicts (possibly perceived) or the fact that most are refering to multiple tubes (pre & pwr) being heated by one supply.

After measuring the current on the heaters, it's seeing 330mA. Is this something to be concerned with or not? I understand that I can place small resistors in line to drop the voltage and have figured out what values to use. Another thing that got me a bit confused is based on the 330mA current and 7.7VAC I calculated ~23 ohms heater resistance. Does this seem correct?

Thanks for any help and advice.

[ecisthebest]

on spec, your transformer should be seeing 117vAC, and you're seeing 124vAC (6% increase). transformer is out of spec by a bit too much, being 7.7vAC (which is off by 20% or so). heaters want to see 5.8v~6.9v, at most 10% from the 6.3vAC norm. I think 7.7vAC is a bit too much.

330mA is 30mA higher than the datasheet 300mA, i think you should use a smaller resistor than actually getting 6.3v, since you could go elsewhere, find a 110v outlet and starve the heater circuits! to make sure this does not happen, i'd suggest maybe a 3.3 ohm resistor, run the tubes at around 6.7vAC at your house (124vAC) and be ok when you go jam at 110vAC venues. (around 6.0vAC)

power is being dissipated on this 3.3 ohm resistor, 0.3A x 1.0v = 0.3w, so I'd be safe and get a 1w resistor. 3.3 ohm (or any low resistance resistors) are hard to find, and u might have to search a bit.

and about the heater resistance, ya. heaters are like light bulbs, small resistance on a thin piece of metal to generate heat. too much resistance won't give us any heat. the math, voltage across the two nodes (+ and - of the heaters, 6.3v) divided by the current through them (.3A) is about 23 ohms. power dissipated on the heaters is VI, 6.3v x .3A = 1.89w

[Structo]

@ ecisthebest
How do you assume that filament transformer is rated for 117vac on the primary?
More correctly, that transformer is rated for 6 amps and you are only drawing 330ma.
So not enough current to pull the voltage down.

I would monitor the wall voltage for a few different days at different times to see what it averages out to be, then install the correct resistors to pull down the voltage.
You could also use a zener diode.

If you don't gig out then you will not need to worry about voltages elsewhere.

[jelle]

What Structo said....IMHO, The nicest way would be to use a voltage regulator like the furman and set the heater series resistor for that.

Just my $.02

jelle

[ChrisM]

Transformer is 0.6A not 6A so that would be enough of a load.
Don't know this Zener trick your speaking of.

I'd just put some resistors or diodes in series with the heaters to drop them.

[SoundPerf]

Transformer is 0.6A not 6A so that would be enough of a load.
Don't know this Zener trick your speaking of.

I'd just put some resistors or diodes in series with the heaters to drop them.

First. Thanks all!

Yes, the trafo is .6A or 600mA. As far as monitoring the wall voltage. I have and it stays constant at 123 to 124VAC. Which is the case in most the area I live in. I thought I mentioned the primary rating. It's 115VAC. sorry!

I'm thinking the in line resitors is probably the best solution.

Structo, I have read about using regular high current diodes to essentially use the voltage drop accross the diodes. Is this the same as the zener?

So, you all agree that the 30mA overage is too much?

[ChrisM]

If you use high power silicon diodes your only dropping .3V per diode. Check out Valve Wizard for more on this.

Maybe the Zener trick is...
Creating an artificial CT with two resistors. And then instead of taking them to ground, take them to ground through a Zener diode (cathode to ground). 1V should do the trick.

[SoundPerf]

If you use high power silicon diodes your only dropping .3V per diode. Check out Valve Wizard for more on this.

Maybe the Zener trick is...
Creating an artificial CT with two resistors. And then instead of taking them to ground, take them to ground through a Zener diode (cathode to ground). 1V should do the trick.

Thanks Chris, Yeah the diode Vdrop isn't really enough. The CT zener idea is interesting.

I have been thinking about the fact that IMO the mains in US will only all end up more like mine than not. It's a PITA to some degree. I know my amp voltages are higher than spec due to this. But, I haven't really notices any ill effects though. But, I'm still concerned about starving the heaters when that's not the case.

I guess a $1000 true mains regulator would be the perfect solution, but that is sort of out of question. My wallet will not support it at this point it time.

[markusw]

http://www.tubecad.com/july2000/page6.html

Four Schottky diodes, two caps, one resistor and a low drop out voltage regulator would be cheaper and give you regulated DC

Peace,

Markus

[SoundPerf]

http://www.tubecad.com/july2000/page6.html

Four Schottky diodes, two caps, one resistor and a low drop out voltage regulator would be cheaper and give you regulated DC

Peace,

Markus

hmm...I didn't think I have enough voltage to rectify and use a regulator and still get 6.3V.

[Structo]

Sorry about that, totally missed the decimal point.

[markusw]

http://www.tubecad.com/july2000/page6.html

Four Schottky diodes, two caps, one resistor and a low drop out voltage regulator would be cheaper and give you regulated DC

Peace,

Markus

hmm...I didn't think I have enough voltage to rectify and use a regulator and still get 6.3V.

With a low drop out regulator and Schottky diodes it should work.
6.3VAC minus 2x 0.35VAC (for the diodes) would be 5.6VAC. Rectified *1,414 = 7,9VDC.
The LT1086 e.g. has a maximum voltage drop of about 1V at 0.3A. This would leave about 0,6V headroom for the LT1086. As far as I remember there are regulators with even slightly lower drop out voltages.

You could also wire the LT1086 as a current regulator set to 0.3A. This way you would also avoid the current inrush into the cold heater which might even prolong tube life.

Hope this helps.

Maybe check out Duncan's PSU designer. I think it's very useful.
http://www.duncanamps.com/psud2/index.html

Peace,

Markus

[SoundPerf]

Thanks Markus.

I have the Duncan PSU Designer. I will have to seriously consider this. One issue I will have is fitting any more circuit into an already cramped enclousure. I didn't use a 1U rack enclosure.

Thanks again.

[glasman]

When I designed the power supply for the matchbox and reverb a loops. I had Heyboer wind me a transformer with a 15V 1 amp winding. I use a bridge rectifier into a 7812 with a 1N4007 in the ground lead. I always have 12.5 to 12.6 for the heaters. Plus the noise benefit of DC on the filaments.

Plus I have a nice and stiff 12V DC supply for other uses.

Gary