hum problems with my amp, still

[mig-ru]

i seem to develop very audible hum running my amp using 6A3's with AC fils. i know this isnt due to the 6SL7 because it runs on DC fils. i did read somewhere that the point of low voltage heaters was to reduce hum, so i switched the 6A3's to run on the 2.5V winding and see what happens. the output level dropped (duh), and the hum seemed to go away. so should i get some 2A3's (expensive) or try and DC the 6A3 filament supply? could it also be that i have bad hum pots (alpha wire wound 100ohm)? although after the adjustment of the pots to eliminate as much hum as possible there is a somewhat different hum present (what im trying to get rid of) i though it was power supply problems at first because the hum is somewhat different from the type i get when i turn the hum pot to one extreme. any suggestions?

[Positron]

Running the 6A3s on 2.5 volt filament will dramatically shorten the tube life, as well as change the transfer function. Thorium must be "boiled" to the surface and requires a "hot" temp.

Hope this helps.

[mig-ru]

yes i know that, i just wanted to see what would happen; this was done for a short time only (30 sec max).

[Colt45]

i dont think 2A3 has thorium filament.. thoriated tungsten is more common in higher power tubes..

you can usually tell a thoriated tungsten tube by the fact it is very bright, it needs to be to get sufficient electron dissipation.. (not as bright as lightbulb usually, but much brighter the the dull orange most tubes have..)

i think 2A3 uses just oxide coating (oxide of what? i dont remember.. not thorium though)

[Sch3mat1c]

Uhm, geez... calcium or barium oxide (applied as a carbonate)... I forget.

Check out Eimac's (unfortunately incomplete) Care And Feeding Of Tubes.

Tim

[316a]

Originally posted by mig-ru
i seem to develop very audible hum running my amp using 6A3's with AC fils. i know this isnt due to the 6SL7 because it runs on DC fils. i did read somewhere that the point of low voltage heaters was to reduce hum, so i switched the 6A3's to run on the 2.5V winding and see what happens. the output level dropped (duh), and the hum seemed to go away. so should i get some 2A3's (expensive) or try and DC the 6A3 filament supply? could it also be that i have bad hum pots (alpha wire wound 100ohm)? although after the adjustment of the pots to eliminate as much hum as possible there is a somewhat different hum present (what im trying to get rid of) i though it was power supply problems at first because the hum is somewhat different from the type i get when i turn the hum pot to one extreme. any suggestions?

could it be that there is a hum loop ? Did you float the filaments of the 6SL7 ? Only other suggestion is psu wiring could be at fault (dry joints , 0v take-off point etc)

316a

[mig-ru]

6SL7 filaments are floated. the 6.3V winding is shared between all tubes.

[dhaen]

Originally posted by mig-ru
snip...i switched the 6A3's to run on the 2.5V winding and see what happens. the output level dropped (duh), and the hum seemed to go away..snip..

Well the anode current dropped too; Thus the PSU current dropped.
Have you thought it might be as simple as more smoothing required?

Cheers,

[mig-ru]

yeah im thinking of altering the PSU, changing the choke to a larger value and increasing the values of the caps after the choke. although i dont want to do this if it's still filament related.

[SHiFTY]

Don't suffer with AC filaments... I got bored of the hum on my 6B4 SET.

Get a 50 cent diode bridge into a C-R-C filter (I use 1000uF - 2 ohm (5W) - 6800uF) 10V or 25V caps are cheap.

Silent, smooth and 6.25V. If you don't like it then you have spent about $5! no great loss...

If you want to be flash use shottkey or UF4007 diodes for clean switching.

Hum is not hi-fi.