grounding heater/filament center

[metebalci]

Hi,

Which is better if heater/filament supply is DC ?

1) Grounding the center of the heater/filament
2) Grounding one end of the heater/filament

and why ?

Thanks in advance...

MB

[dhaen]

With DHT's, the recommendation of valve (tube) manufacturers is to ground the negative side. For indirectly heated valves it doesn't matter.

The practice of grounding the center of an AC supply is to attempt to cancel hum. You can also use this method if have any residual ripple on the DC. It is possible to reduce hum by using this method.

What you must take into account with DC, is that the grid and anode voltages will be offset by the mean DC heater value.

[Bryan]

Hey,

What you must take into account with DC, is that the grid and anode voltages will be offset by the mean DC heater value.

Could you please explain this a bit more?

Thanks,

Bryan

[dhaen]

Hi Bryan,

I should have made clear that the offset is for Directly Heated valves only, and my original wording was unclear.

In the case of anode (plate) voltage: Suppose a 5v heater valve has a maximum rating of 250v. If the positive end of the cathode were grounded, the anode to cathode voltage would be 255v. If the centre is grounded then the max is 252.5v negative end is grounded, 250v. Of course this is small stuff, and hardly affects the operating point.
However on the grid, 5v may make a big difference to the operating point, and must be considered.

[metebalci]

Hi dhaen,

> With DHT's, the recommendation of valve (tube)
> manufacturers is to ground the negative side

Thanks for this information...

Since one end of the cathode is at higher potential than other end, does that mean the cathode is not an equivalent potential surface ? If it is not, isnt it a problem ?

MB

[EC8010]

Originally posted by dhaen
With DHT's, the recommendation of valve (tube) manufacturers is to ground the negative side.

It's odd you should mention this, because I was testing a DHT with cathode bias earlier today...

metebalci, you are exactly right. A DH valve is not a unipotential cathode, and this was one of the advantages of the indirectly heated cathode.

The valve I tested earlier had a 4V AC heater and required 7V cathode bias. The cathode drops 4V across it, so the average potential is 4V/2 adrift from the end potentials. If I had connected to the negative end of the DC heater supply, I would have required 5V across my cathode resistor to set an average of 7V Vgk. Alternatively, if I had connected to the positive end, the average potential would have been 2V lower, so I would have needed 9V across the cathode bias resistor.

Setting 9V rather than 5V across a resistor for the same current would give better stabilisation of operating conditions, but is less efficient, and efficiency was very important when you powered valves from batteries.

[dhaen]

I was testing a DHT with cathode bias earlier today...

Oops, another thing I didn't make clear
This is the scheme for a stage using grid bias.
For cathode bias we refer to the point at the top of the cathode resistor.

Sorry for the confusion my ill thougt out answers may have caused.