| Romy the Cat wrote:|
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…
Yes, the ventilation is sort of "wait and see" at this stage, but it is based on convection. In the photos above you can see some stainless steel mesh that I have glued over the exterior ventilation holes...there is some on the bottom shelf and some on the front and back panels. The industrial footers that I have chosen for the PS sit it quite high, about 30mm+, above the floor which means that the bottom ventilation has plenty of room to draw in air from the floor. The ventilation channels are virtually full length of the case and are aligned like a chimney with the same slots cut in the sides of the middle shelf to let the warm air rise more or less without impedance. When it reaches the roof of the PS chassis it will push out of the ventilation slots on the front and back panels. The system seems to work. I let the smoke out of a resistor during power-on testing and it dribbled out the convection slots just as I had imagined it would, so hopefully I will not have to install any fans (but there is allowance for fans at a later date).
The iron in the power supplies has also been designed and wound for low temperature rise (on big cores)...I think the biggest temperature rise is expected to be 14C-16C above ambient. When the transformer winder long-term tested his mathematics outside the confinement of a case he managed a 9C temperature rise on the biggest piece of iron, so my fingers are crossed that I can keep temperatures manageable without a fan.
| Romy the Cat wrote:|
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 Cat
Thank-you for this information Romy. Hopefully I will get a chance to test this in the coming week. I should state that I had no problem getting to 0VDC at input, rather it would not stay at 0VDC...for example after a few minutes it might have wandered up to 60mVDC or down to -20mVDC.