Capacitor polarity

pre65 · Post by **pre65** » Mon Sep 17, 2007 2:23 pm

Hi-i have a 100uf polarised cap that goes in the PT15 cathode circuit,only thing thats bothering me is the + terminal is wired to the negative side of the cathode (DC heating) heater circuit.
Is that OK ?

If not how can i do it ?

Lee S · Post by **Lee S** » Mon Sep 17, 2007 2:39 pm

You are putting the capacitor in circuit with respect to ground, not the heater wiring. On a DHT (or DH pentode such as the PT15), the heater wiring should be floating. No reference to ground at all. As long as the cathode bypass (I assume it is this) cap is the correct polarity with respect to ground you will be OK. The positive of the cap would go to the "top" of the cathode resistor and the negative to ground. It wouldn't matter which way round you then added the heater wiring but apparently the accepted standard is to use the cathode leg as negative IIRC.

Post by **Nick** » Mon Sep 17, 2007 4:34 pm

Just to add to Lee's perfectly correct answer. Any voltages only make sense with respect to some other point. Its conventional to measure with respect to ground, but its worth getting used to thinking about voltages as a difference not a absolute value. It is after all a potential difference.

The classic example of that is cathode bias. the cathode is at a positive potential with respect to ground. So the grid, which is at the same potential as ground is at a negative potential with respect to the cathode.

pre65 · Post by **pre65** » Mon Sep 17, 2007 4:48 pm

Hi-so if i power my onboard LCD voltmeter from the 6.3V non regulated supply (for the C3g) do i need to take the negative wire back to that circuit ?

And if i want to monitor the two heater supplies and the HT supply (via a multi way switch) can i reference that to ground,or use both halfs of the switch to keep the original reference (meaning two wires between measured voltage and switch).

Post by **Nick** » Mon Sep 17, 2007 5:04 pm

The 6.3 supply for the c3g will either be floating or referenced to ground, or some voltage above ground. I would guess if you intend to measure any voltages in the circuit, it will need referencing to ground. But how are you going to creat DC to power the meter from the heater supply?

Its going to be hard to measur ethe voltage across the fillament supply with it referenced to ground, as that will be floating at the cathode voltage.

I think you need to think carefully about how that meter is going to behave in the circuit.

Not only that, I would have doubts about powering a digital thingy of the heater supply to the pentode.

pre65 · Post by **pre65** » Mon Sep 17, 2007 5:26 pm

Hi Nick-the C3G heater supply is a completely separate DC supply.

6.3V from transformer,schottky bridge,2 caps and a 1R5 WW resistor.

I can leave the meter off for now.

Post by **Nick** » Mon Sep 17, 2007 6:17 pm

Hi Nick-the C3G heater supply is a completely separate DC supply.

Yes, but it will need referencing to the signal ground in some way, otherwise it will float and introduce noise in to the pentode. If you reference it to 0v, then you can use the meter for most things (assuming its not going to introduce any hash back into the pentode), but you will still be unable to measure the pt15 fill voltage unless you switch the 0v to the meter in that case.

pre65 · Post by **pre65** » Mon Sep 17, 2007 6:31 pm

Hi-OK,i wont worry about the meter for the moment.

How do i reference the C3g DC supply to ground then-noise is the last thing i want.

Lee S · Post by **Lee S** » Mon Sep 17, 2007 6:47 pm

Wouldn't you just take a link cable from the DC heater's negative supply cable to the amp's ground?

Post by **Nick** » Mon Sep 17, 2007 7:18 pm

Wouldn't you just take a link cable from the DC heater's negative supply cable to the amp's ground?

Yep, thats the first thing I would try.

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Capacitor polarity

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