[ARC5] Converters for Q5-ers (PS hum)

[AKLDGUY .]

Hi Leslie

> DC filament supply v AC filament supply. Does AC to the filaments
> matter?

It will not matter one iota since all the receiver tubes are cathode type,
and what's more, no bias voltages or AGC are derived from the heater
line (unlike in transmitters such as the BC-230).

> Hi-Z signal path between the (diode) detector & the 12A6 grid.
> How much hum is picked up here.

My opinion is: not enough to matter, since there is no high gain preamp
before the 12A6 like there is in HiFi amplifiers. The audio level at the
12A6
grid should be well above hum level at that point.

> B+ supply, from voltage doubler. 1/2 wave doubler, followed by x5 12V
> zeners in parallel with 470uF filter cap. (always thought 470uF would
> take out a lot of hum!)
>
> Where should I start looking for the hum?

Describe how you have connected the zener string. Is the string connected
directly to the 470uF decoupling cap or is it fed the proper way via a
dropping
resistor? If you haven't designed this properly, I suspect the problem is
not
hum but low frequency motorboating like this:

The 470uF charges to the point where the zeners abruptly conduct (as they
are meant to), which drags down the line voltage to the point where the
zener
with the lowest drop out voltage drops out again. The voltage then rises and
the same thing happens again. Long term, it's an oscillating circuit.

Series connected zeners are not good practice and need special design
considerations. Zeners, as with tube regulators of the VR-105 and VR-150
type, require a line voltage considerably more than the regulated B+ voltage
(such as 105 or 150 V) that you intend to end up with.

A calculation shows that your 24V when doubled (2.828 x RMS = 67.2 V)
has not sufficient head room over and above the 'striking' voltages of all
the
zeners added together.

It would be very interesting to remove the zener string and run the power
supply unregulated to see what happens. It's easy to do, so try it.

73 de Neil ZL1ANM



On Tue, Jan 20, 2015 at 2:59 PM, Leslie Smith <vk2bcu at operamail.com> wrote:

>
> #2 Power supply hum in my set.
>
> Hello Bill,
>
> I'm diverging from the points you raised about the M.F. converter. The
> converter is still the topic of interest. It's working again - I found a
> transformer to replace the burnt out 24V AC job; the intermittent fault
> is still there, but not evident at the moment. The current interruption
> involves sound quality.
>
>
> (1) I think I can describe the set-up adequately with out a schematic.
>
> (2) The set is running according to my usual practice.
>
> Filaments - 24V AC. B+ 60V, zener regulated, supplied by doubler
> from 24V AC.
>
> I take the AF from the rear connector - J3, pin #2. This is (I
> believe) 500 ohms.
>
> The set is BC-453-B, not ARA. That's why I think - 600 ohms.
>
> (3) Until recently I fed the AF into a power transformer, 110V :
>     11V. Small.
>
> With this the AF lacks bass, and in the "middle" of the tuning is
> muffled.
>
> Remember, I'm using an HF converter to listen to the BC stations on the
> 'range' rx.
>
> If I tune the '453 across the station I hear 'tinny' high pitched sound
>
> (like you might expect from a cheap transistor radio)
>
> After that the signal becomes 'wooly' lacking highs.
>
> Tuning further gives a second region of 'tinny' sound; the set is
> clearly
>
> mis-tuned, and on the edge of the pass-band.
>
> (4) The result is quite different when I use (modern) headphones.
>
> The sound quality is quite good (except for the point below)
> considering the
>
> narrow I.F. (The I.F. 'slugs' are pushed 'in'.)
>
> (5) Problem. Heaps of hum. 50 or 100Hz hum (I can't tell the
>     difference).
>
> The bottom plate from the set is missing.
>
> My desk/work-bench has a steel frame that is 'infected' with hum.
>
> The set has previously been 'hacked' and I have no idea (at this moment)
>
> where the under-chassis A.F. cables 'go'. (Actually they 'go' from
>
> the detector to the 12A6, but that's not what I mean.)
>
> My question: Can anyone on this list comment on the following.
>
>
> DC filament supply v AC filament supply. Does AC to the filaments
> matter?
>
> Hi-Z signal path between the (diode) detector & the 12A6 grid.
> How much hum
>
> is picked up here.
>
> B+ supply, from voltage doubler. 1/2 wave doubler, followed by x5 12V
>
> zeners in parallel with 470uF filter cap. (always thought 470uF would
>
> take out a lot of hum!)
>
> Where should I start looking for the hum?
>
>
> First thought: Get a 24V battery and build a 'clever' voltage
> doubling circuit
>
> to supply 50V B+. That way I can cart the whole she-bang into the middle
> of
>
> a paddock and see if the hum is still there ....
>
> All comments received with thanks! Les
>
>
> --
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