12V DC heater supply

[Leevi]

I have a battery driven amp where I have a common 12V DC heater supply for one 12AX7 and 6V6.
Those heaters are serial connected. When I measure the voltages across the heaters the 6V6 has voltage drop ~4V
and 12AX7 ~8V. Is that an issue I should take into consideration?


/Leevi

[PRR]

Those tubes will NOT be happy.

You want a 12V6. Wire 12AX7 for 12V. Give each tube its full 12V.
https://tubedepot.com/products/12v6gt

[Leevi]

Thanks PRR, yes that is probably the solution without changing the filament which
is too much power consuming way in a battery driven amp. I made a fast check that it's
difficult find those in Europe so a U.S. order is ahead;)


/Leevi

[sluckey]

Tube filaments operating in series must have the same current rating, since the same current flows through each tube. 12AX7, 6V6, and 12V6 all have different current requirements. 12V6 will only solve the problem if you rewire the filaments to be parallel. If rewiring is an option, just use the common 6V6 and rewire so the filaments are parallel. But use a power resistor in series with the 6V6 to drop the voltage to 6.3v.

[PRR]

EL84 will replace 6V6 with a change of bias resistor. There must be a 12V version of EL84 but I don't have the letters memorized. EDIT-- LL84 is a 10.5V EL84. (This seems to be very rare.)

Yes, series resistor... but that is somewhat "dumb" for a battery power amplifier, you waste half your precious charge dropping 12V to 6V. Using the 10.5V version you only waste 1/6th the total power.

There are DC-DC converters to drop 12V to lower voltages at high efficiency. Two types: over-priced and cheap auction-market junk.

Back in the very old days if we changed a 6V car to 12V to run modern radio and get better cold-start, we'd drive a screw mid-way in the battery to get a 6V tap for the stuff we didn't want to change-over. Of course with modern over-stuffed super-energy batteries you'd probably blow your face off. And any heavy load in the half-tap unbalanced the battery charge.

[Leevi]

But use a power resistor in series with the 6V6 to drop the voltage to 6.3v.

This is not an option because the amp is battery driven and I have to minimize the current consumption.


Yes I'll connect 12V6 and 12AX7 heaters parallel.


I found a 12V6 Nos from eBay (Europe) for very reasonable price.


/Leevi

[kagliostro]

Ciao Risto

Just curious

I've seen a spech sheet of the 12V6, the 12.6v heater consumption is 225mA (good)

Only one thing, B+ for the 12V6 seems in the order of 250V, which is the B+ level on your battery amp ?

http://tdsl.duncanamps.com/show.php?des=12V6



EDIT:

I found a datasheet of the GT version and seems that max B+ is 315V

https://frank.pocnet.net/sheets/127/1/12V6GT.pdf

Franco

[Leevi]

Ciao Franco,


225mA is a very good news;)


B+ is now 248v and it can be controlled on the DC-DC inverter board I'm using. The amp uses 10×4000mAh rechargeable batteries that allow currently ~4h playing. I think the operation time will be longer with 12V6 because heater current is lower than with 6V6.


/Leevi

[kagliostro]

248v, surely within spec

Thanks

Franco

[PRR]

12V6 "IS" a 6V6 with a longer thinner heater. Twice the volts, half the amps. Same difference as car lamps for 6V or 12V cars. You just re-scale the filament wire, you don't change the rest of the lamp or tube.

One thing about 12AV6: they are ALL "golden age production" (even though maybe late gold-age), not modern recreations "works like" the good old bottles. (At least I don't think there are any new-made 12V6.) (Unless the Russians had a similar tube now being re-badged as 12V6?)

"Sensible" designers worked 6V6(12V6) at 250V maybe 285V because you really do not get more power at higher voltages without more costly OT winding and greater stress. However I've never seen guitar-amp 6V6 cry about 350V, and some were used in TVs at 300V as V-sweep (very high kick-voltage) reliably.

"Car radio" may have been in mind when 6V6 was introduced. Some old data has the note about up to 7V on heater, which would be the max-RPM charge of a speeding Buick in 1937; with the understanding that most of the time the heater voltage would be nearer 6.3V. Car radios routinely used 250V vibrator supplies; only a Cadillac got 300V.

[kagliostro]

Thanks PRR

Franco

[Leevi]

One thing about 12AV6: they are ALL "golden age production" (even though maybe late gold-age), not modern recreations "works like" the good old bottles. (At least I don't think there are any new-made 12V6.) (Unless the Russians had a similar tube now being re-badged as 12V6?)

The 12V6 I ordered is manufactured by K-R.


https://i.ebayimg.com/images/g/LnQAAOSwvcFcOL6o/s-l300.jpg


/Leevi