The AM Forum

Is there a rule of thumb when increasing power supply capacitance?  It has been suggested to me to increase the capacitance in the input (up to 100uf from 40uf) and output capacitors (up to 200uf from 40uf) for my Heathkit DX-60B.  I am no engineer by any means and have been cautioned that at some point you could damage your transformer.  I do use a thermistor for inrush protection.  The rig works well but I have been told by some that there is a little hum on my transmitted signal.

Thank you,
Bill KO4NR

 

I'd check the caps first, out of circuit.  They were once enough to keep it quiet.

Also, RF getting into your mic input can sound like 120Hz hum.

73DG

All the capacitors are new.  I did move the mic jack and input circuit to the top of the deck.
Bill

Online schematics show silicon diodes rather than tube rectification.  Added capacitance can exceed tube rectifier peak current specs, but this is rarely an issue with solid state. The transformer is far more likely to be overheated by leaky caps increasing overall power than it is by larger peak loads charging capacitors. You’ve already taken steps to limit inrush..

If you want to play around with this in simulation to see what the differences are, google Duncan amplifiers psu design tool, a simple but good simulator.

Ed

I was told by Bill Moyer of Moyer Electronics in Pottsville and Sunbury, PA, that his father insisted the values for electrolytics varied quite a bit. I thought he said that -20%~+80% was not uncommon for the old 'twist lok' multi-section electrolytics.

Mic input and (any) resistor shielded?   Gud Luk..

Stated by other poster you exceed tube current ratings, but the DX-60 does not have a rectifier tube. Use the higher rated capacitors, the increased charging current is very small. Rectifier tubes are rated for maximum capacitance with capacitor input filter, follow the recommendation.

I like to try and define the problem first before pondering solutions...
I would try to find an online SDR receiver that you can hit with a good signal. Is there any audible hum?
 If so, can you determine if it 60hz or 120hz? Does the hum go away in CW mode, key down? If the hum peaks as QSB drops the carrier level, you might have some FM.
Try a crystal instead of a VFO. Any difference?

Jim
wd5JKO

  I agree with Jim.  Figure out if the hum is 120Hz or 60Hz before you start messing with capacitors.  All DX60's in AM mode have some 60Hz induced in their audio because of the 12AX7's  filament wiring being unbalanced and using the chassis for the filament return.  Twisted pair wiring for the filaments improves this with the ground placed back where the filament winding actually grounds to the chassis.  Making up a DC supply will improve it more.  It never goes away easily.

    Remember these are not Hi-Fi tube audio amps where single point grounding techniques were employed to minimize 120Hz and 60Hz. Filaments don't float with a hum balance pot.  When it was designed, no one cared if the audio dropped precipitately below 200Hz anyway. No one cared if it had some hum.

Steve
KB1VWC