Heater Voltages

[gearhead]

Was idly doing a search and came across a Wikipedia (the big one) reference to 12AX7s. It cites heater voltage as both 6.3 (at 300mA) and 12.6 (at 150mA), with the later as being what "most" use.

I understand a number of things, but my knowledge drops over the ledge for transformers. Do the TW 12AX7 heaters run at 6.3V each or doubled to 12.6V between the two taps?

[UR12]

They are running at 6.3. if you think of the heater as a coil of wire between pin 4 and 5 and then add a centertap to it and run the centertap to pin 9, you can put 12.6 volts accross pin 4 and 5 and not use pin 9 or you can tie pins 4 and 5 to gether and put 6.3 volts accross 4&5 to 9.

Kind of like a tranny secondary like 300-0-300you can get 600 volts out of it if you leave off the CT and use a bridge recto. Or you can ground theCT and use 1/2 of the total or each side and get 300v

[unklmickey]

... It cites heater voltage as both 6.3 (at 300mA) and 12.6 (at 150mA), with the later as being what "most" use. ...

read it again. most of this family of tubes uses the same configuration and specs on the heater.

the 12.6 volt (series) configuration is almost never used, except in automobile radios (think cave-man days).

the 'wrecks and almost all other guitar amps use the 6.3 volt PARALLEL connection of the heater elements.

the TRANSFORMERS are 6.3 volts (RMS) end-to-end.


hope that helped,

unk


EDIT:

oops. sorry Dana, i type slow. i guess i was still typing this while you answered.

[David Root]

The reason for using 6.3V with pins 4,5 together and pin 9 for the other side is this is a 60Hz humbucking arrangement and is why every manufacturer since Leo changed to 12A*7s from octals and Gibson/Magnatone changed from 6EU7s to 12A*7s in the '50s/early '60s.

[gearhead]

... It cites heater voltage as both 6.3 (at 300mA) and 12.6 (at 150mA), with the later as being what "most" use. ...

read it again.

Hmm(?), here it is, Wikipedia:

"The 12AX7 was the most common member of what eventually became a huge family of twin-triode vacuum tubes, manufactured all over the world, all sharing the same pinout (EIA 9A) Most used a 150 mA heater which could also be connected to run on 6.3V at 300 mA. The variations include the 12AT7, 12AU7, 12AV7, and the low-voltage 12U7, plus many 5XXX series dual triodes too numerous to mention. The variations span a wide range of voltage gain, ruggedness, and transconductance.
http://en.wikipedia.org/wiki/12AX7"

Unkie, I thought their reference to "150 mA heater" was for a 12.6V config (cited earlier in the listing), which struck me as odd, as I thought 6.3V is the standard, as you say!

Dana, the TW does have the center tap grounded. Where does the extra 95V for B1 come from?

[unklmickey]

yeah, it one of those lame sentences that can only be correctly deciphered if you break it up correctly:

"The 12AX7 was the most common member of what eventually became ......... a huge family of twin-triode vacuum tubes, manufactured all over the world, all sharing the same pinout (EIA 9A)......... Most used....... a 150 mA heater which could also be connected to run on 6.3V at 300 mA. "

so, you see they aren't actually saying anything about how the heaters are usually connected. only that most of this family shares the same heater construction.


although i usually hate sentences that are misleading, i HAVE been know to ask the waitress, who brings my coffee:

"Can I have some, sugar?"

[drz400]

IF you you DC heaters it makes more sense to use 12.6
In fact almost everyone using regultaed DC heaters uses 12.6 because of less current draw

[David Root]

I've been using UNregulated but highly smoothed 6.3 VDC heaters, with only 1% ripple (-40dB). Costs about $25 for the bridge rectifier, 2 A choke and pair of 10,000 uF 6.3 VDC caps.

I'd be interested in opinions both technical and cost-wise vs. 12.6 VDC regulated supply at the same ripple performance. And, most importantly, are there any tone implications?

[drz400]

I've been using UNregulated but highly smoothed 6.3 VDC heaters, with only 1% ripple (-40dB). Costs about $25 for the bridge rectifier, 2 A choke and pair of 10,000 uF 6.3 VDC caps.

I'd be interested in opinions both technical and cost-wise vs. 12.6 VDC regulated supply at the same ripple performance. And, most importantly, are there any tone implications?

12.6VDC cost you a regulator and a cheap transformer. You can use a 12V and float the ground with a diode too to get 12.5vdc
$20 and you have no hum in your supply? A no brainer for me, Also it isnt picky about the quality of preamp tubes. Maybe an extra $10 if you want to go hog wild with 6 preamp tubes load to splurge for a 350ksteel regulator, or you can just use DC on the first couple of stages where it matters the most ....Maybe $30 tops for pure DC why screw around?

[David Root]

Any particular regulator(s) you would recommend now? I seem to recall something about this in one of my TUT books but those regulators could be obsolete by now.

[mhuss]

*Any* 3-terminal regulator out of Mouser or DigiKey that provides enough current is fine. They still make the good 'ol 7812, which is good for up to 1A.

Note that you'll likely need to heat sink whatever regulator you use.

--mark h

[David Root]

I found a modern version of the 7812 type, made by Linear Technology, LT317A, which doesn't need the protective diode. $3.75 at Mouser, also available in the big can as 117A. Downloaded the spec/data sheet. I think I'll try it.

BTW, Apex Jr. has surplus 7812s for $0.40 each, I noticed.

[drz400]

Well probably most Audio guys do run at 12.6 150ma DC
It is really the best choice, many amp guys do this as well
VHT, Custom Audio and I believe Bogner, Diezel, maybe Engl and anyone into high gain