Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fluzo18 on September 13, 2010, 06:38:48 am

Title: 12AX7 heaters in series at 13.4V AC
Post by: fluzo18 on September 13, 2010, 06:38:48 am

Hello,

For a 12AX7, is it safe to run the heaters in series at 13.4V AC instead of 12.6V?

I'm building the tube pedal project at the library of information and I find the DC option with the 7812 regulator to be very noisy.

I've eliminated the 7812 and now I run the heaters at (ideally) 12V AC directly from the AC socket. I elevate the heater reference about 65V, using two 100 ohm resistors to a voltage divider from the HT circuit. The thing is I get 13.6V instead of 12V, and I presume it would fluctuate even more as the wall voltage changes.

It now sounds great and has no noise at all. But, is it safe?

Thank you.

Title: Re: 12AX7 heaters in series at 13.4V AC
Post by: FYL on September 13, 2010, 07:51:31 am

The spec is 12.6V ±10%, 13.4V under load is on the high side but OK. You may consider adding a dropping R.

OTOH, a properly used 7812 isn't noisy.

Title: Re: 12AX7 heaters in series at 13.4V AC
Post by: eleventeen on September 13, 2010, 10:15:21 am

I agree w/FYL, a 7812 should be utterly quiet. I am skeptical of dropping the heater volts off the HV supply, but if it works, OK, I guess. I haven't looked at the circuit, hence I don't have an idea of the ampacity of the overall power supply, but heaters use a solid order of magnitude more current than (the cathode > plate of) preamp type circuits...even running a 12AX7 at 12.6 volts. Maybe you could use two diodes connected head to tail and tail to head to drop .7 volts in your current configuration.

Title: Re: 12AX7 heaters in series at 13.4V AC
Post by: fluzo18 on September 13, 2010, 10:48:46 am

Quote from: eleventeen on September 13, 2010, 10:15:21 am

I agree w/FYL, a 7812 should be utterly quiet. I am skeptical of dropping the heater volts off the HV supply, but if it works, OK, I guess. I haven't looked at the circuit, hence I don't have an idea of the ampacity of the overall power supply, but heaters use a solid order of magnitude more current than (the cathode > plate of) preamp type circuits...even running a 12AX7 at 12.6 volts. Maybe you could use two diodes connected head to tail and tail to head to drop .7 volts in your current configuration.

I'm not taking the heater volts off the HV supply, but elevating their reference to +60V to reduce hum. Normally, the artificial center tap formed by the two 100 ohm resistors go to ground. In my case, I connect them to a voltage divider coming from the HT supply (bottom image):

(http://www.freewebs.com/valvewizard/heater5.jpg)
http://www.freewebs.com/valvewizard/heater.html

How would you connect the two diodes to lower .7V? Do you have a schematic?

Thank you.

Title: Re: 12AX7 heaters in series at 13.4V AC
Post by: kagliostro on September 13, 2010, 10:56:25 am

Quote

How would you connect the two diodes to lower .7V? Do you have a schematic?

here the schematic

http://www.freewebs.com/valvewizard/heater.html  :laugh: (sometime that happen also to me ...)

Kagliostro

Title: Re: 12AX7 heaters in series at 13.4V AC
Post by: stingray_65 on September 13, 2010, 11:07:26 am

Funny This should come up about the7812 chip as a friend and I were discussing that series recently.

I have been using that family of regulators for 20+years now and have always found it to be EXTREMELY quiet.

I would look much closer to what is going on before modding things heavily.

Just my .02 take it or leave it.

Ray

Title: Re: 12AX7 heaters in series at 13.4V AC
Post by: stingray_65 on September 13, 2010, 11:11:42 am

do you have a photo of your project?

Ray