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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: It look like the project progress wellPosted by Romy the Cat on: 2/3/2017
It look like the project progress well. I have a little concern
about heat the will be building up in your PS enclosure. I do not know what
kind ventilation hole you have in there but be advised that it will be quite
hot. In worst case you might put a little 4-6” fan in there…
Regarding your bias
problem. This is normal and it is very easy to deal with. Short the amplifier
input jack. You should have about minus 3.4V at the driver tube grid or about
200V on plate. That would be depends the quality of your tube and if you have
185-215V on plate then it is still OK. Your bias is set by negative (green)
supply chain and resistor R6. The positive
supply chain (red) is just compensates the same voltage that negative cleat on
the left side of the resistor R5. This is bias resistor and keep it exactly as
it is (unless you build filters at input). The values that are given in the schematic
are very accurate but they presume that you use gas tubes that give 150V. In reality
they never do. Some of them stabilize voltage at 148, or 149, or 151 volts.
They are not bad tube, this is how they works normally. So, you might have a
positive gas tube give 151.6 and negative gives 148V and then you do not have enough
voltage on positive side to set zero at input. Pay attention in negative side
we have R10 = 10K and on positive side we have R9 which is hale of it and then trimmer
run +/- 10K. So, of you a few KOhm not enough then just change the value of R9.
Alternately you can do what I usually do: switch the tubes, positive and
negative between each other, or get another gas tube. Between 2-3 tubes I always
find a configuration where I can get zero at my input. Sine you find a right
set of tube and adjust to zero input it should reminds like this for years. If
you use a direct coupled preamp that accidently runs DC at output then you can
even to correct if at your power amp input.
Another aspect that you need to recognize is that 8mA at amp input is VERY little and it should not
affect sound at all. All that it will do “bad” will be creating auditable click
when you connect and disconnect cables. As you understand when your cables are connected
then your positive supple (red) is grounded to your preamp output impedance. My
preamp has 8R output impedance, so when my cables are connected I might even to
pull the positive tube out or turn off positive supply – it will have very little
impact to anything. Well, if I disconnect the cable after this then I have voltage
burst that might burn my driver tube and might send the speakers diaphragm
flying. This is what we have positive supply and balance the input to zero, because
we want the amp to be balanced independent from the environment and to be
stable.
Rgs, Romy the CatRerurn to Romy the Cat's Site