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Old 28th May 2003, 06:46 PM   #1
mig-ru is offline mig-ru  Russian Federation
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Default DC filament supply AC ripple affect on overall hum

I am wondering how much does the AC ripple in a DC filament supply affect the overall hum? Would a simple cap filter suffice? Guessing that if filament can be run at AC a 500mV ripple wouldnt do much, but I want to make sure.
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Old 28th May 2003, 09:48 PM   #2
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Default HUMMM...

Hi,

Quote:
I am wondering how much does the AC ripple in a DC filament supply affect the overall hum?
For preamp service I'd regulate.
For amp service I'd regulate the input valves filament supply and use A.C. on the output.

Unregulated D.C. would be O.K too but I doubt there'd be a benefit IMHO.

Cheers,
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Old 28th May 2003, 09:54 PM   #3
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Default Pre-amplifiers: Regulate and rule

I would be less happy about 500mV of ripple than 6.3V AC! The reason for this is that the act of rectification produces high frequencies that crawl easily into the audio, whereas the 50 or 60Hz has more trouble getting in. There's no question about it, regulate, and enjoy the slight noise improvement, plus the much greater life and stability improvement. A 317 regulator plus the (small) handful of components required to set 6.30V at the heater pins is dirt cheap.

Power amplifiers occassionally need regulated heater supplies, but it's very rare...
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Old 28th May 2003, 10:05 PM   #4
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Hi,

It strikes me that you seem to insist on exact voltage for the heaters.

Any particular reason?

Most databooks agree on a +/- 10 % deviation and fron experience I can only sya that a - couple of % gives no problems whatsoever and saves on component count as well...

On a transcondustance tester I never noticed a difference wheter I heated at 6 or 6.3V.

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Old 28th May 2003, 10:27 PM   #5
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Quote:
Originally posted by fdegrove
It strikes me that you seem to insist on exact voltage for the heaters.
Damn right. We're talking about thermionic valves here. DVMs and regulators are cheap. Why get it wrong for the same price as getting it right?

Let's avoid the learning experience of having to find any problems caused by changing/incorrect heater voltage. The valve designers aimed for optimum operation at their stated heater voltage. Emission is critically dependent on cathode temperature, and although the heater has an element (ouch!) of self-regulation, it doesn't make sense to deliberately stretch a design.
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Old 28th May 2003, 10:34 PM   #6
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Hi,

+/- 10 for heater voltage.

With all due respect but...

I doubt that running a valve at 6V or 12V iso 6.3 or 12.6 heater voltage makes a difference.

At least, I haven't noticed any...the valves last longer, it's easier (avoids extra components when usings regs) and it makes them that little bit quieter...

O.K., I know I'm being me usual smart ***...
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Old 28th May 2003, 10:53 PM   #7
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Ah, but you've split into two different issues here.

You've said that you're happy to run at 6V or 12V (rather than 6.3V or 12.6V) because it's easier on regulators. I don't have a problem with that, because you are bending operation within the manufacturers' limits for a known reason.

The second issue is that by regulating you have eliminated mains voltage variation from cathode temperature. This is very important.

The manufacturers' curves were produced using the nominal heater voltage, and if we want our designs to work as predicted from their curves, then we use exactly the right heater voltage - especially if it's trivial to do so.

Of course, if we are able to plot our own curves, then we can operate the valve completely differently. But I'm prepared to bet that you wouldn't like to operate a design optimised for 6V regulated DC with 6V AC complete with mains voltage variation?
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Old 28th May 2003, 10:59 PM   #8
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Hi,

Quote:
I'm prepared to bet that you wouldn't like to operate a design optimised for 6V regulated DC with 6V AC complete with mains voltage variation?
Touche.
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Old 28th May 2003, 11:15 PM   #9
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Quote:
Originally posted by fdegrove
Touche.
You're welcome. Traditional wisdom was that reducing heater voltage reduced noise, but do you think this is actually true with modern (post-1945) triodes?
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Old 28th May 2003, 11:51 PM   #10
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Default Heaters...

Hi,

Quote:
Traditional wisdom was that reducing heater voltage reduced noise, but do you think this is actually true with modern (post-1945) triodes?
That's not the complete picture though...

While I agree with the quote, with modern post 1960 valves the same trick can be applied with no penalty.
Neither measurably nor audibly..with the added benefit of relaxed regulator requirements and extended life.

Plots we did back in the mid-eighties showed the valve actually performed better when underheated...(within reason, say +/- 5%)

Current however was paramount to perfomance...admittedly most tests were done on what we call " small signal valves"..ECCs etc.

So, I'd say keep current constant or at least ample, relax the voltage a bit and all is bliss...

Surely, I'll be the last person to generalise on this.

Cheers,

EDIT: It often makes valves less prone to microphony too.
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