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using cathode bias voltage for preamp tube heaters

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Posted on July 20, 2014 at 09:36:49
DAK
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 I lost my bookmark for a john broskie article on doing this. This old trick was used in a PSE 6550 with a 6sn7 front end? If anyone has the link I thank you for posting it. I checked his website and did not find it. thanx again. Dak

 

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RE: using cathode bias voltage for preamp tube heaters, posted on July 20, 2014 at 16:59:28
Chip647
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 Just curious, but how could a 6550 (biased at, say 100mA) or even 2 in parallel passing 200mA at the extreme, ever pass the 600mA required to light a 6SN7? (who's heater is effectively a 10 ohm resistor). You must supply the tube heater with 600mA or it will not function right, there is no free lunch of magic current being created.

I have seen this using a 12AX7 with the heater in series passing 150 mA which will light a 12AX7 with about 12.6 volts. It is effectively a 84 Ohm resistor. Are you sure about the tubes he used?

 

RE: using cathode bias voltage for preamp tube heaters, posted on July 20, 2014 at 23:10:45
DAK
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 Well, I am pretty sure that is the tube line up, but that is why I am looking for the article. My computer died a few months back and I lost the bookmark. It was an interesting implementation and the author felt that DC on the heaters was a sonic improvement. I am searching for a good PSE 6550 circuit and this one looks promising. cheers, Dak

 

Here you go., posted on July 21, 2014 at 05:12:58
Chip647
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 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/

 

RE: Here you go., posted on July 21, 2014 at 07:24:33
Thomas Mayer
Manufacturer

Posts: 602
Joined: May 8, 2001
 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

 

RE: Not the one., posted on July 21, 2014 at 08:14:06
DAK
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 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

 

RE: using cathode bias voltage for preamp tube heaters, posted on July 21, 2014 at 08:54:33
FenderLover
 Just outta curiosity, what advantages are there to draw heater current from the cathode bias voltage? Doesn't this put heater noise (albeit DC) into the tube? I would think that one should isolate the cathode from all sources of current draw, outside of the tube's needs.

 

RE: using cathode bias voltage for preamp tube heaters, posted on July 21, 2014 at 09:02:39
Thomas Mayer
Manufacturer

Posts: 602
Joined: May 8, 2001
 Hi!

As I see it just a cost advantage. This is a way to make use of the current which would otherwise be wasted in the cathode resistor.

Thomas

 

RE: Not the one., posted on July 21, 2014 at 12:11:25
drsx
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 You can restrict Google to his site by appending site:tubecad.com to the end of your search terms.

 

RE: Not the one., posted on July 21, 2014 at 12:20:14
Chip647
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Posts: 2651
Location: The South
Joined: December 24, 2012
 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.

 

And a great new failure mode of having a shorted input tube heater take out a pair of 6550s (nr), posted on July 21, 2014 at 13:59:08
Chip647
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Joined: December 24, 2012

 

RE: using cathode bias voltage for preamp tube heaters, posted on July 21, 2014 at 22:10:26
shane.
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 The heater current goes through the bias resistor.. which means you can use a substantially lower value for Rk and avoid using Ck altogether.

 

Not really., posted on July 22, 2014 at 10:53:28
DAK
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 This method was used to good effect in many vintage units with high reliability. Eico, Fisher, and others would take the "wasted" current from power tube bias and instead putting it to work through heating the filaments of the front end tubes. Since it is pure DCV there is no hum induced by AC which is a good thing. regards, Dak

 

RE: Not really., posted on July 22, 2014 at 12:04:38
Triode_Kingdom
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Posts: 10047
Location: Central Texas
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 Harman Kardon also used this method to good end. I've heard a few disparaging remarks about the technique here in the forum, but I don't know why. I've never personally seen it cause a problem. When they're powered by the output bias, the phono tubes don't exhibit that annoying flash at turn-on, and they operate virtually hum free.

 

I had a Fisher melt all of it output tube because the bias was not held right by the 12AX7 heaters, posted on July 23, 2014 at 06:37:17
Chip647
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 The concept of "waste" is an economic concept.
Done well the heater on a 12AX7 can be very quiet with AC, after all, that is one of the benefits of the indirect heater.

 

RE: I had a Fisher melt all of it output tube because the bias was not held right by the 12AX7 heaters, posted on July 23, 2014 at 08:26:59
Triode_Kingdom
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Posts: 10047
Location: Central Texas
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 "Done well the heater on a 12AX7 can be very quiet with AC"

AC on a 12AX7 is absolutely NOT quiet enough for a high-end phono preamp. Perhaps the Fisher wasn't properly designed. HK used a circuit that combined resistors with the filament drive circuit, and they also used output tubes operating well under their ratings. These two techniques pretty well ensure that the choice of preamp tubes won't be critical. I can't say it's impossible to damage the outputs, but judging by the schematics, it seems extremely unlikely.

 

RE: using cathode bias voltage for preamp tube heaters, posted on July 24, 2014 at 12:41:43
Thermionic27609
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 I've mainly seen this done in less expensive 1960s tube integrated amps like some small Bogens where they used cathode current to supply DC to one or two 12AX7s in the phono stage. Based on the type of amps in which this was done, I'd say it was mainly a cost-cutting measure.

 

RE: Not the one., posted on July 25, 2014 at 22:10:59
jazbo8
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Posts: 70
Location: Europe
Joined: January 16, 2012
 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.

 

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