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In Reply to: RE: using cathode bias voltage for preamp tube heaters posted by DAK on July 20, 2014 at 23:10:45
John says the same thin I did. After a quick search, here is your article.
Free DC Voltage for Heaters
Thinking about the 6082’s 26.5V heater led to my thinking about the trick I mentioned long ago: using the input and driver tubes’ heaters as a cathode resistor for the output stage in a power amplifier. On the mono Aikido PCBs, the two tube heaters can be placed in parallel or in series, so this trick would be easy to accomplish. For example, a 12AX7 as input tube and 12AU7 as buffer tube have 12.6V heaters, which placed in series yield 25.2V. (Fortunately, these two tubes share the same heater current draw: 150mA.) Given that 25.2V divided by 150mA equals 168-ohms of resistance, we can use the heater string as part of a cathode resistor’s total resistance, as long as the current flowing through the cathode resistor is equal to or greater than 150mA. Now 150mA is far too much current for a single 300B or 6550/KT88, but not for a 6AS7 or 6C33 nor for parallel 300Bs and EL34s and 6550/KT88s...
http://www.tubecad.com/july2000/
Follow Ups:
I read this one but it isn't the one I found awhile back. He actually followed up and built an amp using these ideas. It was a recent post and it included a schematic of the amp he built. Its too bad that there is no search engine in his website. cheers, Dak
You could ask him.
Still, you should be able to back into his implementation if you use some basic current analysis. You cannot get around the physics that the heater will consume 600mA and therefore must be supplied with +600mA. There is no way the plate current from 2 6550 tubes can do this. He would have to put a CCS below the heaters, but that would mitigate any "free lunch" supply.
I think this article should clarify the "free DC" heater supply concept, the heaters are connected in series, not in parallel which consumes too much current as you pointed out.
You can restrict Google to his site by appending site:tubecad.com to the end of your search terms.
Hi!
In his article Broskie also describes a theoretical OTL amp with 13 6AS7 tubes and their heaters wired in series to make up their own cathode resistor. How the hell should such a construct light up? The current will only flow when the tubes are warmed up.
Obvoiusly such a concept only works if you supply some fronted heaters with the current from the output tubes. But if they share a common B+ supply, the input tubes will be hit with B+ before way before the heaters are warmed up.
Best regards
Thomas
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