• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

heater voltage: why ac, and not dc?

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#21
in one 2A3 amp i made, i used a humdinger pot, i set it to null the hum,
but then the owner told me, "you know what? i liked the sound of the amp with a wee bit of hum, the hum that you do not hear at your listening spot...." so who am i to tell him otherwise?
 
#22
My first all-valve project was an RIAA-correction amplifier for moving-magnet cartridges. As I wanted to make a circuit without semiconductor devices and as using a battery or a mercury vapour rectifier with smoothing for the heaters seemed unattractive, I used an AC heater supply. I took several precautions to reduce hum, such as using EF86 valves (with their low heater-to-grid capacitance) with decoupled cathodes for the input stages, using shielded twisted audio cable for the heater connections and using a voltage divider with two 100 ohm resistors as a "centre tap".

Much to my surprise, the hum was below the (quite low) noise floor as long as the transformer was far enough away from the circuit. The heater circuit didn't cause any hum issues at all, only the transformer's stray field did.
 
#23
Back in 1930'Th rectification and filtering were too expensive for ordinary consumers. As well as regulation of screen grid voltages. Today it is cheap and easy, and there is no excuse not to do that making high end amplifiers.

TonyTecson said:
in one 2A3 amp i made, i used a humdinger pot, i set it to null the hum,
but then the owner told me, "you know what? i liked the sound of the amp with a wee bit of hum, the hum that you do not hear at your listening spot...." so who am i to tell him otherwise?
Click to expand...

I tell them, "Sorry, you are not my customer. 😀
 
#25
Wavebourn said:
Back in 1930'Th rectification and filtering were too expensive for ordinary consumers. As well as regulation of screen grid voltages. Today it is cheap and easy, and there is no excuse not to do that making high end amplifiers.
Click to expand...

For indirectly heated valves, it's also a complete waste of energy unless you make a switched-mode power supply, which much complicates matters.
 
#27
anatech said:
New designs tend to abuse everything, like they didn't have a tube manual.
-Chris
Click to expand...

Yikes! One must watch the dates on these posts. But this one is too good to let pass. Well said; often wondered myself!


Dc on heaters: Have become a 'why not' matter in the last decades, what with regulators and easy diodes freely available. Just a warning regarding those smacked-on rectifier + small capacitor jobbies: 'D.C.' poorly filtered with spikes like digs' hairs can cause more interference than properly laid-out a.c.. One does talk of a moderately well-filtered smooth d.c. if there is any ripple left.

But I come from the days when it was possible to have a hum-free RIAA circuit (possibly preceded by a stage of quiet amplification) with a.c. on heaters and often not clean mains a.c. at that.
 
#28
Well; anything can be done wrong easily. 🙂
So, if you know how to get great results even from AC, from DC you can get even better results.

The most common mistake is, designers keep forgetting that wires have resistance and inductance, and mix rectifier and input currents through the same wire. Even if it is a thick bus, it has a resistance.
 
#30
Why I love compactrons with 8V filaments, because it is easy to get clean as a teardrop DC from 6.3V using a Shottky diode and a capacitor. 🙂
Here as an example; a board with B+ rectifier and regulator, and 8.4V filament rectifier. Why would I refuse such a convenience?
 

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#35
Hi tony,
You might want to use one that has the metal shield installed. This will really help contain the RFI emitted by the switcher.

I just ordered two 24 volt power supplies, one for each project I'm working on. The supplies in the case cost more, but less switching interference is well worth the extra. There will be some 24 VDC to 12 VDC switchers and I might have to go linear, or find a box for them.

Best, Chris
 
#36
Hi Anatoliy,
Yes, I agree. You could use a 0~199 mV one to measure bias current. They are cheap enough! I have to admit that I haven't tried one yet. The price seemed to low to be true.

How do you like them?

-Chris
 
#37
Chris, i just received a totally enclosed 24vdc 20A power supply, cost was around U$20..the one i linked was used to power our kampana amps and even if you located it close to the amplifier board, there was no interference whatsover that can be heard thru the speakers....that is why i am convinced that smps is the way to go forward...
 
#39
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