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Preamp tube DC heater circuit

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ronnyelectron

New Member
Hi guys,

I have some questions about running 12ax7 preamp tube heaters on DC.

The filaments are pins 4,9ct, 5.

Typically they are run at 6.3 VAC in parallel ..pins 4&5 connected to one leg of Ac and pin 9(center tap of the filament) to the other leg of ac. In parallel the current draw is 300ma.

I'd like to run multiple tubes on regulated 12.6 DC. The filaments will be run in series, pin 9 not used. In series the current draw is 150ma.

How do you wire this? Put 12.6 DC on pin 5s and wire pin 4s to ground? Is it that easy? Or what am I missing?

Thanks!
 

MikeMl

Well-Known Member
Most Helpful Member
It is just that easy. Leave pin 9 open. When running multiple tubes, they will be wired in parallel with the first tube.
 

gabeNC

Member
Or the Marshalls of the 1970's! :D

'Cmon Audioguru... don't you like the sound of smooth tube amps over solid state?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Or the Marshalls of the 1970's! :D

'Cmon Audioguru... don't you like the sound of smooth tube amps over solid state?

I suspect he likes quality - low distortion and good frequency response.

'Smooth' isn't an audio term, but means low quality, poor frequency response and high distortion.
 

audioguru

Well-Known Member
Most Helpful Member
In 1950, solid state amplifiers had crossover distortion and no high frequencies. Then tube amplifiers sounded distorted but better.

How times have changed today.
Now solid state amplifiers have distortion so low that it is difficult to measure and their frequency response is a straight line from DC to green light.
 

Mr RB

Well-Known Member
...
Now solid state amplifiers have distortion so low that it is difficult to measure and their frequency response is a straight line from DC to green light.

And they are great for playing CDs but they sound really crap playing an instrument through.

Use A when A is best, use B when B is best, and be smart enough to know which is best and when. ;)
 

Bob Scott

New Member
In 1950, solid state amplifiers had crossover distortion and no high frequencies.

And that amplifier was hard to make in 1950, because the transistor was not developed until 1954!

I think some people confuse the "smooth" characteristic of tubes with the pleasing sound of even order harmonic distortion, the kind you get with a class A amplifier where the top half of the waveform is compressed. I adds a nice "chorus" effect to the sound.
 
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gabeNC

Member
I like what RB said... "Use A when A is best"

As a guitar player for almost 20 years, I've had several of both and it really just depends on what genre you want to sound like. Both have their pros & cons. But hey, if it sounds good...
 

Nigel Goodwin

Super Moderator
Most Helpful Member
And they are great for playing CDs but they sound really crap playing an instrument through.

They make the instrument sound as it's supposed to, a valve amplifier colours the sound to make it different (limited frequency response and high distortion).

If you want such a coloured sound a valve amp is the best way to go.
 

Mr RB

Well-Known Member
Sure they affect the sound, but with modern instruments (say electric guitar) the amp is very much a PART of the instrument.

As an example you could say the spruce top of a classic violin gives it a "smooth" sound by attenuating the high frequencies. You could make the violin top out of high tensile steel and it would have "much better high frequency response" but then it would sound tinny, harsh etc. Having an instrument where all the frequencies are reproduced equally is NOT a guarantee it will sound better.
 

Roff

Well-Known Member
Sure they affect the sound, but with modern instruments (say electric guitar) the amp is very much a PART of the instrument.

As an example you could say the spruce top of a classic violin gives it a "smooth" sound by attenuating the high frequencies. You could make the violin top out of high tensile steel and it would have "much better high frequency response" but then it would sound tinny, harsh etc. Having an instrument where all the frequencies are reproduced equally is NOT a guarantee it will sound better.
Tube amps generate frequencies (due to harmonic distortion) that are not present in the input. This is different from modifying the frequency response. I'm not saying this is necessarily bad, but it is different from your example.
 

audioguru

Well-Known Member
Most Helpful Member
You don't need to use a vacuum tube amplifier to rolloff the high frequencies. A tone control circuit will do the same but will not add distortion like vacuum tubes unless the tone control circuit also has vacuum tubes.
 

MikeMl

Well-Known Member
Most Helpful Member
Question: How many engineers does it take to answer a question about series-parallel connections?

Answer: ONE to answer the question; and FIVE to argue about deep-seated personal preferences!
 
To the OP,

make sure you follow convention with the wiring. Keep the two wires twisted and as close to the valve sockets as possible. Solid hookup wire is prefered so that you can position the twisted pairs and keep them there.

Cheers
 

MikeMl

Well-Known Member
Most Helpful Member
To the OP,

make sure you follow convention with the wiring. Keep the two wires twisted and as close to the valve sockets as possible. Solid hookup wire is prefered so that you can position the twisted pairs and keep them there.

Cheers

Now you got me going. Why bother twisting filament wires if the OP is powering them on DC? Twisting the filament wires is supposed to be a hum-bucking technique, but it can only apply if the filaments are heated with AC.
 
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