So I'm experimenting with an new build at the moment, a 3 channel all tube guitar preamp, and I'm running the heaters on regulated 12.6VDC. I've got the heater PSU all hooked up and decide to give it a quick test to see if it will take the load of 7 12AX7's without going up in smoke. I've decided to go cautiously while I hook it up so I've got a DMM on the output of the regulator, a DMM on the secondary of the transformer, and a clamp-on ammeter on one of the secondaries feeding the input to the PSU, just so I can see what's going on as I run the PSU up.
I turn on the PSU and all is fine - 12.6VDC on the output, 15VAC on the input, no appreciable current shown on the clamp-on. Plug in a 12AX7 (an old Philips Miniwatt), all goes smoothly, 12.6VDC, 15VAC and curent has increased by 150mA (as you would expect for a 12AX7 run with it's heaters in series). Continue plugging in 12AX7's noting that the voltages stay largely the same and the current goes up by 150mA each time.
So I decide to plug in the last four 12AX7's in one hit. They're Sovtek 12AX7WA's, nothing too special. I look over at the meters and I've suddenly got 10VDC on the reg, 12VAC from the transformer and I'm pulling close to 1.8A from the tranny! Something's amiss. So I pull all the tubes out and start plugging in the Sovtek's one by one. As I plug in each one the heater current rises by 300mA instead of the usual 150mA. Thinking maybe I got a dud batch I try another four Sovtek's I bought in a different shopping spree - all the same.
So there you have it. I've got no idea why Sovtek have done it, but they've decided to stick in different heaters in their 12AX7WA's. Every datasheet I've ever seen for a 12AX7 has always listed the heaters as 150mA in series/300mA in parallel. For some reason the Sovteks are twice this.
A couple of years back I made a preamp using four Sovtek 12AX7WA's run on regulated 12VDC. I always thought the regulator ran hot for a load of only 600mA, even though the preamp worked fine. I guess I need to revisit it and double-check I'm not pulling 1.2A instead!
For those of you who use the Sovtek's in your amps/preamps I'd recommend checking how much heater current they're drawing in case you're pushing the heater supply too far.
I turn on the PSU and all is fine - 12.6VDC on the output, 15VAC on the input, no appreciable current shown on the clamp-on. Plug in a 12AX7 (an old Philips Miniwatt), all goes smoothly, 12.6VDC, 15VAC and curent has increased by 150mA (as you would expect for a 12AX7 run with it's heaters in series). Continue plugging in 12AX7's noting that the voltages stay largely the same and the current goes up by 150mA each time.
So I decide to plug in the last four 12AX7's in one hit. They're Sovtek 12AX7WA's, nothing too special. I look over at the meters and I've suddenly got 10VDC on the reg, 12VAC from the transformer and I'm pulling close to 1.8A from the tranny! Something's amiss. So I pull all the tubes out and start plugging in the Sovtek's one by one. As I plug in each one the heater current rises by 300mA instead of the usual 150mA. Thinking maybe I got a dud batch I try another four Sovtek's I bought in a different shopping spree - all the same.
So there you have it. I've got no idea why Sovtek have done it, but they've decided to stick in different heaters in their 12AX7WA's. Every datasheet I've ever seen for a 12AX7 has always listed the heaters as 150mA in series/300mA in parallel. For some reason the Sovteks are twice this.
A couple of years back I made a preamp using four Sovtek 12AX7WA's run on regulated 12VDC. I always thought the regulator ran hot for a load of only 600mA, even though the preamp worked fine. I guess I need to revisit it and double-check I'm not pulling 1.2A instead!
For those of you who use the Sovtek's in your amps/preamps I'd recommend checking how much heater current they're drawing in case you're pushing the heater supply too far.
