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papparazzi 10th March 2006 11:10 PM

Hum 300b Again

This has been discussed here over and over again,but still again.
On DHT amp fully AC heated 300b.

If i dont have mains hum what ican get rid with humpot ,and i dont have groundloop ;so how can i still hear about
100hz hum ?
Hum level vary depending what type of input tube Im using. Amp is simple Reichart 6sn7/300b -2,5k /370 vdc - 1kohm/100uf amp.
Ihave tried Dc heating ,no hum but soundstage and naturallness
vanished? No good!

In one article (what i can not find anymore) was suggested on Simple
autobias, humpot ; Ac heaters grounded directly from both sides of heater pins with small 0,1uf? capto ground. I haven seen this kind of
of any schematics. Does this Help I doint know.
My speakers are 92w/db so I should live with AC heating on this level of sensitivity?.


lndm 11th March 2006 02:06 AM

Get onto John Broskie's site . He dislikes noise an amps. He has great ideas such as adding hum in antiphase to cancel existing hum.

kmtang 11th March 2006 05:21 AM

Try run it with DC filament supply to see how much improvement it can get. Also, check the ripple voltage at B+. The ripple at B+ also cause some hum noise at output.


papparazzi 12th March 2006 12:07 AM


Try run it with DC filament supply to see how much improvement it can get.
Dc filaments no problem with hum. My experience ,adding DC to filaments moves towards Transistor analytical sound. Same happens
with using bypass caps in cathodes where they are not actually necessarily needed.


lndm 12th March 2006 12:48 AM

I know what you're saying. Bypassing the cathode resistor completely, removes the local feedback and often doesn't sound as good. This is the same as using DC to provide fixed bias. But this is not the same as using DC for the filament.

BTW Fully bypassing the cathode resistor is a good way to ensure low noise in a DHT so maybe you should do it.

sgerus 12th March 2006 04:15 AM

Check ot this thread, post#10.

What is the best input/driver for 2A3 SE

I wents through the same problems with my 6sl7/300b

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