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

[Kenneth G. Gordon]

On 20 Jan 2015 at 12:59, Leslie Smith wrote:

> Filaments - 24V AC. B+ 60V, zener regulated, supplied by doubler
> from 24V AC.

Hmmm...I tried operating a limiter using 24 VDC from the filament supply, 
very well filtered, and had tons of hum. When I used a dropping resistor from 
the B+ supply, I had no hum. A clue, perhaps?

Besides, your voltage doubler would output ~68 VDC: to attempt to regulate 
that with Zeners to 60 VDC would not provide you with enough "overhead". 
Try disconnecting the Zeners completely to see if the hum goes away.

If you are only using 470 MFD, you may not have enough filtering, either.
 
> 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.

It may be. As I remember it, the output transformers of that model have two 
output taps: one at something like 4K and the other at 600 ohms, but it would 
depend on how it is connected as to whether or not your particular receiver is 
actually set to 600 ohms. Normally, as I recall it, the BC-453B was always 
set up for 600 ohms....except when it wasn't.

> (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.

Hmmm...that sounds like an odd result to me...I don't know how to explain 
that...

> (5) Problem. Heaps of hum. 50 or 100Hz hum (I can't tell the
>     difference).
> 
> The bottom plate from the set is missing.

Doesn't matter.

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

No. Not one whit. Absolutely not.
 
> Hi-Z signal path between the (diode) detector & the 12A6 grid.
> How much hum is picked up here.

Next to none.

> 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!)

At ~68 VDC, 470 mfd is not nearly enough filtering, IMHO.

I have seen a chart which listed the amount of filtering needed at what 
voltage for so much ripple, but I don't know where it is at the moment.

I also just ran a program which calculates the minimum capacitance needed 
for 68 mV ripple at 68 VDC at 50 mA: that value is ~3700 uFd.

As I said, in my opinion, 470 uFd is not NEARLY enough to keep the ripple 
to a reasonable value.

At 470 uFd at 68 VDC at 50 mA, your ripple would be 3.5 volts, according to 
my calculations. I am very curious how my calculations stack up against a 
'scope view of the output. In any case, 3.5 V PP ripple is too much. That is 
about 5.8%, which in a receiver is too high.

> Where should I start looking for the hum?

In the output of your ~68 VDC B+ supply...

I would try the following: 

1) increase your filter capacitor to at least 3700 uFd. and 

2) for now, get rid of the Zeners completely.

I think that if you want to use Zeners to regulate the B+ (which, in my 
opinion, is a very valuable and worthwhile effort), you may have to go to a 
voltage tripler to get enough overhead....but the necessary overhead can be 
calculated too.

> 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 ....

Or use a dynamotor... 

Ken W7EKB