MA-2 mk1 bias & AC hum

OTL Asylum

OTL, Output Transformerless Amplifier User Group.

0

Hello again ,
as i have been with my MA-2's for some weeks now, thing's are
shaking down, and besides that they sound better the longer i
actually listen, [i know this is an open door] they prove very
reliable, but also sensitive to ac power, [ i have no power line filter ]
now i am still left with some hum, mechanically it is fully acceptable,
but thru the speaker its not ideal, so i would like to get this down.
the AC measured across the outputs is 7mv for l/h and 14mv for r/h amp.
after swapping tubes input / output [ i did every thinkable swap ]
the amps still hold on to the 7/14 mv. output tubes are not measured actually, but the amps are used as demo's
by the previous [shop] owner and have low hours.
i have put in nos telefunken's at the front, and this actually got the
AC down to the mentioned 7 / 14 mv.[ it started all much worse when the
amp's first arrived, with figure's up to 36mv and even 45mv one day !
also the bias is still more than 2a, measured with a good dmm,
i understood that it should be around 1.5 amp, cant get it down also.
could it be that this has a relation with the [high] ac at the outputs somehow ? i presume the ac should be zero ideally spoken.
the 7mv figure is reasonable quiet, but still..
DC offset is very easy to adjust and is also very stable.[just to mention]
anyone ??
Yes i am saving for the upgrade to MK-2, but in the meantime..
Thank's on beforehand, Rudolf

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Topic - MA-2 mk1 bias & AC hum - Rudolf 03:45:06 01/29/00 (10)

Re: MA-2 mk1 bias & AC hum - mark 09:46:37 01/29/00 (9)

Re: MA-2 mk1 bias & AC hum - rudolf 01:49:56 01/30/00 (8)

Re: MA-2 mk1 bias & AC hum - mark 07:49:39 01/30/00 (7)

Re: MA-2 mk1 bias & AC hum - rudolf 12:36:27 01/30/00 (6)

Re: MA-2 mk1 bias & AC hum - mark 10:19:55 01/31/00 (5)

Re: MA-2 mk1 bias & AC hum - Rudolf 13:42:55 01/31/00 (4)

Re: MA-2 mk1 bias & AC hum - mark 19:12:45 01/31/00 (3)