DC heaters question

[catalin gramada]

Hello. Does the dc supply artificial CT helps to get rid of AC components in an dc heaters supply ? Or will not do any difference in respect with tied negative to ground please ? Take in consideration an independent dc heaters power supply with its output referenced to ground by 2x 100 ohm resistors vs directly tied negative to ground please... Merry Christmas anyone !
late: I still get 2mVpk AC ripple over my 6.3V dc supply at 0.9A. May I get a better ac100cps rejection referencing the supply to ground by two 100 ohm or is equal if just reference the negative side to ground, ? It will do any difference please ? Thanks.

[x-pro]

I don't get it.
There is no such thing as a DC transformer.
Physics doesn't allow them to be.

[catalin gramada]

On the pics there are presented both references versions of the supply to ground. My question is version A or version B ? There will do any differences in terms of noise please ? Thanks.
late: you're right is not quite a center tap but balancing resistors which act in same manner for ac components. And I wonder if will do more in terms of noise canceling for those tiny amount of residual ripple - more than I tie directly the suply to ground like in version A ?

[catalin gramada]

Or better ...maybe ...?

[x-pro]

Simply put.
In circuit A, heater leakage current appears on the Common conductor of the useful signal of a specific polarity and it will be an element of unwanted feedback.
Even if we put any resistor, and to minimize the background it should be of small nominal value, the leakage current will have polarity. To minimize background, the lamp heater is isolated from the signal circuits.
According to circuit B- the background component is isolated at the connection of two identical resistors (current difference is eliminated). For DC voltage this point is zero. For induced current, it is variable and it flows to the Common.

[Jon Snell]

If you want a common, it is common practice and in my view better practice, to common the negative to ground.
There is no requirement for a balanced centre tapping as it s not AC and balancing does not exist with a DC path.
If you use good quality valves, DC heaters are not required.

[catalin gramada]

No. I use crappy noise valves. Extremely sensitive to ac heaters supply the only way I can get rid of noise is to elevate heaters, only with negative voltage (positive elevation not work). Still quiet with DC supply. The balanced arrangement is supposed to address to the remanent 2mVpk AC residual ripple. No good ? Thx.

[Jon Snell]

2mv peak ripple equates to an ablolute maximum 2mv ripple on the anode, assuming the heaters are short circuit to the cathode, which is unlikely.
All valve and solid state amplifiers have an inherrant noise level, usually -80db or better which renders the noise, acceptable and unnoticeable.

[catalin gramada]

Well a 100cps ripple will make more noticeable perceived noise than 50cps do.

[g1]

The balanced arrangement is supposed to address to the remanent 2mVpk AC residual ripple. No good ? Thx.

What kind of filter capacitance are you using? Some manufacturers that run DC heaters are using tens of thousands of uF, or using multi-stage filtering like this:
(mF values are milli not micro)

[catalin gramada]

I used pair of 15mF/25V, two filtering stages With a 7.5 ohm resistor it drop right on 6.3V loaded for 0.95A. I splited the resistor in couple of pieces. It get pretty hot. There are just two tubes to supply : one 6sl7 and one 6sn7 type.

[Helmholtz]

I don't think a few mVs of 100Hz ripple matter.
And I don't think the DC supply needs to be "balanced".
But, c'mon It's easy to try and find out.

[catalin gramada]

I don't have such of sensitive rig to precise measure the coupling into the stages to see a diference. Then, how I may do a difference between plate supply ripple and induced riple from heaters ? There are both 100 hz. Please to suggest a method how can I do a measurement to see if changing the reference of DC heaters to ground is matter in terms of coupling to cathode please ? Thanks.

[Helmholtz]

Measure output ripple with either positive or negative side of heater supply grounded and see if it makes a difference.

[catalin gramada]

I measured the output of first stage. Well...with my 1x probe on 2mV scale I cannot see any ripple on plate...it's a straight line, the instrument have not enough sensitivity to show some.