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In the Forum: Melquiades Amplifier
In the Thread: Bias help
Post Subject: Ok, you got me here.Posted by Romy the Cat on: 4/14/2018
Gintaras, this is
funny as in my last post I meant to write that I might not remember correctly
and you need to look in my old posts. I did not write it for whatever reasons
but what you did was right. Whatever I wrote in my old post was correct. My apologies
that I mislead you now. Indeed, the current means not a lot in your
configuration, when 5E6P drives
6C33C. The 6C33C is indirect heated tube and can’t work with plate current. So,
you do not need current from 5E6P to push 6C33C into class A2. The critical current
from 5E6P 35-40mA and anything under is fine. The 15-17mA would be
a good zone of comfort.
If you do want go with higher current then you drop a little
bit a plate resistor R2. I do not know if you need to. The change of plate resistor
does impact sound but not because of current but because of other things. At
the time I was making the Milq we had a lot of debate what plate resistor
should be. Some people I know use very low value, 5K with lower voltage PS.
They had the same current on the tube and slightly different current. I feel
that high plate resonance is good for this tube. Again, I can’t recollect now any
specific sonic changes I remember with using high plate resistor vs low plate resistor.
I assure you that using a resistor for 1-2K lower you will not hear any sonic difference.
Another, purer, way to get more plate current is supply different
negative voltage to the 5E6P grid. You
can do it by reducing R6 resistor. This will not impact the 5E6P operation point.
Now about the 5E6P current and
my 25-30mA misleading. I did not look at my old posts and was referenced to
what I was using in my current multi-channel version of Milq.
http://www.goodsoundclub.com/PDF/6-Chennal_Melquiades_DSET_Amplifier.pdf
If you look at the schematic than you can see that channels E, F and D have in the negative (green)
power supplies 100K trimers. I use the trimers to change plate currents and I usually
stay with 25-30mA. The channels E, F and D are not the same as what you have as
they are single channel where 5E6P drive speakers directly. You need current
there but not so much in the channels where 5E6P drives 6C33C.
If you are uncomfortable with your plate current at 5E6P then you might do the same and put a trimmer
after R10. After you find the best for sound current (do not go over 30mA as
you might burn the tube) then measure the final resistance and put in there a permanent
resistor. This supply to directly to the tube grid and the less cap you have in
there the best it will be.
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