DC Heaters..

Cathode Ray · Post by **Cathode Ray** » Sun Jan 06, 2019 7:22 pm

.. who invented it ?

Is this an old idea that has found a use in hi-gain pre-amp circuits ?

Tony Bones · Post by **Tony Bones** » Sun Jan 06, 2019 7:35 pm

It's a very, very old idea. 6.3V, 12.6V, ... these are the voltages of lead / acid batteries.

Cathode Ray · Post by **Cathode Ray** » Sun Jan 06, 2019 7:40 pm

Tony Bones wrote: ↑Sun Jan 06, 2019 7:35 pm It's a very, very old idea. 6.3V, 12.6V, ... these are the voltages of lead / acid batteries.

Makes perfect sense.

Thanks again, Tony

R.G. · Post by **R.G.** » Sun Jan 06, 2019 11:34 pm

Just to put a bit more on that skeleton, DC heaters predate AC power. Originally all tube gear was run on batteries. The term "B+" comes from the fact that early, early tube gear ran on an A battery, a B battery, and a C battery. B battery? Yep, B+. A batteries? A cells.

The tube heater voltages were designed for the batteries that drove them. When AC mains power became possible, then widespread, they came up with ways to make this into the DC needed for the tubes that already existed.

It persisted.

Cathode Ray · Post by **Cathode Ray** » Sun Jan 06, 2019 11:40 pm

R.G. wrote: ↑Sun Jan 06, 2019 11:34 pm Just to put a bit more on that skeleton, DC heaters predate AC power. Originally all tube gear was run on batteries. The term "B+" comes from the fact that early, early tube gear ran on an A battery, a B battery, and a C battery. B battery? Yep, B+. A batteries? A cells.

The tube heater voltages were designed for the batteries that drove them. When AC mains power became possible, then widespread, they came up with ways to make this into the DC needed for the tubes that already existed.

It persisted.

Damn, this place makes MetroAmp look like Radio Shack.

You guys rock

martin manning · Post by **martin manning** » Mon Jan 07, 2019 1:31 am

A for filament, B for plate, and C for bias. See: https://en.wikipedia.org/wiki/Battery_(vacuum_tube)

Cathode Ray · Post by **Cathode Ray** » Mon Jan 07, 2019 2:12 am

martin manning wrote: ↑Mon Jan 07, 2019 1:31 am A for filament, B for plate, and C for bias. See: https://en.wikipedia.org/wiki/Battery_(vacuum_tube)

So, they were able to get up to 300V DC by connecting several batteries together for the B+

R.G. · Post by **R.G.** » Mon Jan 07, 2019 3:09 am

Cathode Ray wrote: ↑Mon Jan 07, 2019 2:12 am So, they were able to get up to 300V DC by connecting several batteries together for the B+

Still works. A friend told me his brother got a job at a place that recalibrates medical instrumentation. Some of it used coin cells, the 3V lithium things. It was part of procedure to replace the batteries with a fresh one, no matter the condition, and without even a test. They were simply replaced.

Being a frugal kind of guy, friend's brother started stashing the removals, which were nearly universally in almost-new condition. Pretty soon, he had quite a pile, and instead of storing them some other way, he found a cardboard tube of about the right diameter for the batteries, tamping them in place with wads of crumpled paper. Being a neat, tidy kind of guy, he put them all in in one orientation, positive to negative every single one. After a few months he was putting the newest arrival in, touched the ends, and was thrown back into the floor.

Turns out he had nearly a hundred of them in this tube by then and 100 times 3V = ...

Reeltarded · Post by **Reeltarded** » Mon Jan 07, 2019 4:30 am

Cathode Ray wrote: ↑Sun Jan 06, 2019 11:40 pm
R.G. wrote: ↑Sun Jan 06, 2019 11:34 pm Just to put a bit more on that skeleton, DC heaters predate AC power. Originally all tube gear was run on batteries. The term "B+" comes from the fact that early, early tube gear ran on an A battery, a B battery, and a C battery. B battery? Yep, B+. A batteries? A cells.

The tube heater voltages were designed for the batteries that drove them. When AC mains power became possible, then widespread, they came up with ways to make this into the DC needed for the tubes that already existed.

It persisted.
Damn, this place makes MetroAmp look like Radio Shack.

You guys rock

roflmao you get it

The Ballzz · Post by **The Ballzz** » Mon Jan 07, 2019 5:10 am

Soooo,
This is a very interesting subject! Now, please allow the dunce to ask the burning dumb question: What is the most convenient and practical way to convert (at least for the preamp tubes) the AC heater circuit to DC. Even as a dunce, it seems fairly clear that heating with DC could alleviate some of the noise/hum issues associated with using AC for the purpose.
Please Inform The Uninformed?
Gene

Cathode Ray · Post by **Cathode Ray** » Mon Jan 07, 2019 5:15 am

The Ballzz wrote: ↑Mon Jan 07, 2019 5:10 am Soooo,
This is a very interesting subject! Now, please allow the dunce to ask the burning dumb question: What is the most convenient and practical way to convert (at least for the preamp tubes) the AC heater circuit to DC. Even as a dunce, it seems fairly clear that heating with DC could alleviate some of the noise/hum issues associated with using AC for the purpose.
Please Inform The Uninformed?
Gene

I'm definitely learning a lot here too. These guys are awesome.

The reason I asked to begin with is; I'm building a 50W hi-gain amp kit that has DC heaters. There's a 14V tap off the power xformer that goes to 7812 voltage regulator.

This is what supplies the DC voltage to the pre-amp tubes.

martin manning · Post by **martin manning** » Mon Jan 07, 2019 12:04 pm

The Ballzz wrote: ↑Mon Jan 07, 2019 5:10 amWhat is the most convenient and practical way to convert (at least for the preamp tubes) the AC heater circuit to DC.

There are any number of solutions, most commonly one or two 12A_7 tubes are converted to DC filaments. The difficulty is that the current required is relatively high, and there is a start-up surge. Here is one of the simplest ways to convert the input tube to DC, from the Marshall Studio 15:

Screen Shot 2019-01-07 at 6.19.20 AM.png

It was crude, and had a lot of ripple remaining, but it was deemed good enough at the time. 10x more capacitance will reduce the ripple 10x, and suitable 10,000u capacitors are easily found today. The diodes will have to stand the high initial current, though, so better to go with a 3A device.

Cathode Ray · Post by **Cathode Ray** » Mon Jan 07, 2019 1:57 pm

Here's that section of the layout of the amp I'm workin on...

The RC206 (little round black circle at the top) is the bridge rectifier.

F3 and F4 supply the heaters for all the pre-amp tubes.

martin manning · Post by **martin manning** » Mon Jan 07, 2019 2:26 pm

Works if you have that 14VAC available. Going to 12V means you can power three 12AX7's within the 1A capability of the LM7812.

Cathode Ray · Post by **Cathode Ray** » Mon Jan 07, 2019 2:42 pm

martin manning wrote: ↑Mon Jan 07, 2019 2:26 pm Works if you have that 14VAC available. Going to 12V means you can power three 12AX7's within the 1A capability of the LM7812.

Right, so I'm catching on.

You need to have a custom wound transformer and plan ahead to get that 14V tap.

..or I guess if you're modding an existing amp, you could add in a small, dedicated transformer for that purpose.

So.. the 6 components, you're looking at about $10 ~ $15 in parts to make this a mod in an existing amp

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