Hum cancelling scheme?

[slor]

I'm working on my first amplifier build (a Supro clone, parallel single-ended 6v6) and I'm seeking clarification on hum cancelling. I've been told that one scheme is to reference the heater/filament circuit to ground via a 100 ohm pot; better yet to reference it via the "hot" end of the cathode resistor. Does this crude diagram I've made do the trick? Seems simple enough but I'm still new to this stuff....

Many thanks,
Seth

[billr]

I'm not too sure about the hot end thing,

but this is the normal way of doing things,

1. if the heater transformer has a centre tap, earth that.

2. failing that, put a 100 ohm pot across the heater circuit, put the earth on the wiper, listen to the amp, and twiddle the pot until hum is minimised.

does that help.

also, read up on aikido and other noise canceling things at www.tubecad.com, john broskie talks a lot of sense on noise in preamps. you may find that useful.

kind regards

bill

[ray_moth]

If you make the heater about 30v more positive than the cathode, you can prevent thermionic emission from heater to cathode. Instead of taking the CT to ground, connect a 10uF cap from the CT to ground and arrange two resistors as a voltage divider between B+ and ground, to give you ~30v at the heater. Resistors should be high in value, around 220k - 1Meg. If necessary, to minimise hum still more, you can use a pot instead of the CT.

[Jeb-D.]

If you make the heater about 30v more positive than the cathode, you can prevent thermionic emission from heater to cathode. Instead of taking the CT to ground, connect a 10uF cap from the CT to ground and arrange two resistors as a voltage divider between B+ and ground, to give you ~30v at the heater. Resistors should be high in value, around 220k - 1Meg. If necessary, to minimise hum still more, you can use a pot instead of the CT.

Is there any benifit to this if the heater is DC supplied?

[boergars]

Yes

[ray_moth]

I've never bothered with DC heaters, so I don't know first hand, but I imagine it might help if the rectified heater supply is noisy.

[refference]

Jeb-D wrote :

Is there any benifit to this if the heater is DC supplied?

YES . No doubt about that !!!

Carlos

[cbutterworth]

Using regulated DC on the heaters worked wonders on my Aikido. Hum is barely audible, whereas before it was quite obvious. While the circuit is relatively simple, I ended-up buying the PSHREG from Welborne labs. It works, although customer support could be better, and some of the parts were substitutes requiring some alteration of PCB mounting. Furthermore, I referenced the center-tap of the 6.3VAC trafo to 1/4 B+ using a cap and voltage divider. It is possible that some of the remaining hum could be due to an slightly off-center center-tap, although I have never measured it.

Going DC on the heaters was the single biggest improvement to my Aikido.

Charlie

[slor]

Gentlemen:

Thanks much for the replies. A few follow-ups:

1. I considered doing a regulated DC filament supply, but seeing as this is a smallish guitar amp (and a fairly primitive one at that) it seems like overkill. I'll know more once the thing is built (still waiting on the cabinet maker).

2. Ray Moth (or anyone), excuse my inexperience but could you clarify a little more for me? Specifically:

"Instead of taking the CT to ground, connect a 10uF cap from the CT to ground"

-You're still talking about the "fake" CT on the filament circuit, right?

"...and arrange two resistors as a voltage divider between B+ and ground, to give you ~30v at the heater."

-At what point in the B+ supply should I install these? And frankly, I don't know yet how to construct a voltage divider!

3. I understand more or less HOW to do this, but can someone explain WHY? Specifically, from what I can understand, I'm raising the filament voltage relative to ground. Why does this make a difference?

Many thanks,
Seth

[poobah]

Ok...

Remember... votage is a relative difference... THIS point compared to THAT point.

Now, there is a limit, and sometimes a polarity on the voltage you can impose on the heater relative to the cathode of the tube.

Look at tube data sheet and you will see this spec. Let's say the rating is 50 Volts.

If the cathodes of your tubes rested at say 70 Volts... relative to chassis ground, and the heater supply CT was grounded (O volts), then you would be exceeding the 50 Volt difference allowed between heater and cathode.

Tying the heater CT to a 30 Volt source cures this problem... 70 minus 30 equals 40 Volts difference between the anode and the cathode.... problem solved... that's the "why". This is a crude analysis... we have overlooked the fact that cathode voltage has a min and a max. the heater is actually two voltages on two wires. But, you get the idea.

EDIT: this is the most common reason to use a voltage divider for the heater reference. Ray's suggestion is about hotrodding. How it works is the same in either case.

The divider does not require a low DC impedance ( with high divider current, power loss, and heat)... It does require a low AC impedance... that's where the 10uF cap comes in. It provides a low impedance path for AC garbage to ground.