dc heaters question on preamps or tube mics

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SUPERMAGOO

Well-known member
Joined
Mar 12, 2005
Messages
348
Location
argentina
I need to know if its true than using lower voltage (heaters)on preamplifiers and tubes mics i will have less noise.
If anyone knows about it lets me know it .
if a apply low voltage. How much ? :roll:
 
Using a lower voltage will improve things if you're powering heaters with AC voltage, but if you're using well-filtered DC, it shouldn't make a difference.

Tube microphones and preamplifiers should use DC for the heaters, for sure.

E.
 
Lower voltage will lower noise with both AC and DC powered heaters. But don't go too low as that will shorten the life of the tube.

Best regards,

Mikkel C. Simonsen
 
It's not clear what noise you're talking about. If it is induced noise (hum) from the heater then as stated it will go down proportional to voltage. Usually hum can be nulled out though, and there is always the d.c. heater strategy. Also as mentioned in Langford-Smith you can bias the filament positive a bit with respect to the cathode to counter the tendency for that subsystem to act like a little vacuum diode.

But the random noise in the tube is a complex function of cathode temperature, and although some contributions are going to go down as the temp. is reduced others will get a lot worse. Most likely the designers knew what they were doing.
 
go to the steve bench site there is an article about underheating.

underheating has been written about in some of the microphone threads.

Underheating a tube is tricky

Alot depends on the alloy of the cathode and the oxides used

Some tubes can be underheated some can not.

there are no easy rules to underheating

some smaller 6.3V tubes get noisy at 5.9V some can go to about 5V (DC)

The C800G microphone seems to be an extreme case of underheating and tube oxides.

With a few of my microphone with underheated fils you will hear what sound like whisles and other strange noises (at a very low level) for about 1/2 hour to an hour before the oxides "quiet down".

The noise level drop is not alot from underheating: however with a tube microphone any noise you can reduce tends to be a good thing.
 
A lot of (old) audio equipment underheats the tubes to lower noise - including Fisher receivers and Tandberg tape decks. Both use standard tubes like ECC81/83 etc. The Fishers use elevated heaters also, as far as I remember. So if standard equipment like mentioned use the technique there must be some benefits without any problems at the same time.

Best regards,

Mikkel C. Simonsen
 
Between powerline drift and sloppy massed produced power transformers, underheating and overheating used to happen in voulentearily. Take that, Keef!

Gus, you have any links from Ollie and Klaus on this? I think, like you said, certain tubes like it and certain ones do not.

Nowdays, everything is a v reg/pass trans heated so we lose out on all that good cycling. But Lance Amrmstong is good at cycling. Maybe I will ask him.

What?
 


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