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previous: gazza HELP convert el34 amp to 6v6s ? -- 2/2/2004 10:45 AM

busy tweaking - where to bias ?

2/2/2004 9:35 PM
gazza	busy tweaking - where to bias ? Hello all ! thanks for the replys. I'm not sure I quite follow everything that has been suggested yet. i;ve got the amp open at the moment and I've put some zeners in the ground of the solid state bridge rectifier so my B+ at the EL34 plates now reads 375V from 420V. I hope there isn't so much sag that 2 6v6gt tubes will allow the B+ to go up too much. my next problem is changing the fixed bias from the -36 volts to something a bit more sensible. I'm a bit lost as to where to try as a first step biasing. can people check the following ? ok, so assuming a plate voltage of 370V and a max dissipation of 14 watts, my max plate current is 37 mAmps, so 50% is 19 mA and 70% is 26mA. am i right in thinking i want to get an idle current between 19 and 26 mA ? I will probably try to put a pot in the amp to get the bias point roughly in the right place. any suggestions as to what voltage this might be ?? i'll try for about -20 V aa a first attempt before putting the tubes in. shout if i'm going wrong here ! thanks people Gareth. PS. I might go cathode biased - but I'd quite like to have this amp feeling fairly tight thus I'll think about keeping a SS recto and the fixed bias. tell me if this is silly.

2/2/2004 9:35 PM

gazza

busy tweaking - where to bias ?

Hello all !

thanks for the replys.

I'm not sure I quite follow everything that has been suggested yet.

i;ve got the amp open at the moment and I've put some zeners in the ground of the solid state bridge rectifier so my B+ at the EL34 plates now reads 375V from 420V.

I hope there isn't so much sag that 2 6v6gt tubes will allow the B+ to go up too much.

my next problem is changing the fixed bias from the -36 volts to something a bit more sensible.

I'm a bit lost as to where to try as a first step biasing. can people check the following ?

ok, so assuming a plate voltage of 370V and a max dissipation of 14 watts, my max plate current is 37 mAmps, so 50% is 19 mA and 70% is 26mA.

am i right in thinking i want to get an idle current between 19 and 26 mA ?

I will probably try to put a pot in the amp to get the bias point roughly in the right place.

any suggestions as to what voltage this might be ??

i'll try for about -20 V aa a first attempt before putting the tubes in.

shout if i'm going wrong here !

thanks people

Gareth.

PS. I might go cathode biased - but I'd quite like to have this amp feeling fairly tight thus I'll think about keeping a SS recto and the fixed bias. tell me if this is silly.

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