HT power supply high?

[Nuuk]

In the following circuit I would have expected to get around 332v instead of the 354v that I measure. Is this due to the traffo windings or is there another reason?

I want to build another valve pre using the same power supply but that requires 320 v.

 

[palmas]

the 5687 can take 300V anode voltage, so no problem there.
You measured it with the load?
about 8% is not a lot, but if you want there are many ways to reduce the voltage.

 

[Nuuk]

Thanks palmas. Yes measured under load.

Presumably as the other circuit I want to build (using ECC82) calls for 320v, I can change the values of those 390r resistors?

BTW - can you tell me what the gain of that 5687 circuit is please?

 

[palmas]

Thanks palmas. Yes measured under load.

Presumably as the other circuit I want to build (using ECC82) calls for 320v, I can change the values of those 390r resistors?

BTW - can you tell me what the gain of that 5687 circuit is please?

Gain should be about 22,6dB (13,5x).

I found a few odd things about that circuit:

if you have only 2,4V bias, the plate current should be about 30mA, and that has two problems:
the voltage drop on the 8k2 resistor will limit the output swing (lower than 100V plate voltage), power dissipation on that resistor is above 7W and the battery will discharge fast.

You should: reduce the 8k2 resistor, or increase the bias voltage or place the bias battery in the grid (virtually no current consumption), or both. Place it in series with the grid stopper resistor, negative side to the grid, and increase it to 6,0V at least.

 

[DF96]

Won't the cathode current keep the cell charged? The risk at 30mA is continuous trickle charging, which some NiMH cells don't like.

 

[palmas]

Won't the cathode current keep the cell charged? The risk at 30mA is continuous trickle charging, which some NiMH cells don't like.

You're right off course, the problem is too much current, overheating and overcharging

 

[Nuuk]

Thanks again Palmas. The gain is a bit high to be ideal in my system. I'm looking for about half that, hence looking at the other circuit (or can I modify that one?).

As regards the battery, you guys know infinitely more than I do about these things. All I can say is that I've used the pre amp years without any problems.

 

[tomchr]

So your mains voltage is 2.6 % high, resulting in a B+ that's 3.9 % high. There are so many variables factoring into where the B+ settles, so if you want a high-precision B+, I suggest going with a regulated supply. You can see my website for an example of a B+ regulator.

But seriously... 3.9 % variation from target is actually pretty good for an unregulated supply. I wouldn't worry about it. If you really want to reduce the B+, I suggest increasing the 100 ohm resistor you have in series with the diode bridge.

~Tom

 

[DF96]

Most classic valve circuits, with cathode resistor bias, can cope with 10-20% voltage variation as the bias varies with the supply. By fixing the bias you have made a rod for your own back. However 5% ought to be OK.

 

[palmas]

Thanks again Palmas. The gain is a bit high to be ideal in my system. I'm looking for about half that, hence looking at the other circuit (or can I modify that one?).

As regards the battery, you guys know infinitely more than I do about these things. All I can say is that I've used the pre amp years without any problems.

What gain would you like to have?

 

[Nuuk]

What gain would you like to have?

Around 10 db max.

 

[palmas]

something like this?

 

[Nuuk]

Palmas, I'm nowhere near competent enough to assess your design. I was thinking more of being able to adapt the existing circuit to have lower gain, rather than building a complete new one.

 

[DF96]

In that circuit the gain is more or less set by the valve. To get different gain you need a different valve or a different circuit. A different valve might mean circuit changes anyway. It is only circuits with significant negative feedback which can be simply altered to get different gain. Even then, less gain means higher loop gain which can mean instability.

 

[Nuuk]

Thanks DF96. I'll leave that one well alone then, and think about building a new one using the same power supply (which is in a separate box).