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RE: PING to Audio Note Kit One owners

Posted by Acoustic-Dimension (S) on May 5, 2015 at 23:22:33

In Reply to: PING to Audio Note Kit One owners posted by beto1 on May 5, 2015 at 15:29:53:

Hello Beto,

Quite a strange problem, because some of the things you mention are contradiction each other.

When then high tension on the 300B's are higher then normal, that would indicate that no (or only little) current is flowing through power supply/output tubes, but then RED plates in the 300b would not be possible.

However something must be wrong, because else R1 should not get hot.
R1 and C4 are used to "prevent" some humm/noise issues, but in most amplifiers these parts are not used at all and R1 is replace with a wire/direct connection. It is quite possible that R1 is now acting as a fuse and is indicating a problem.

Finding a problem, is more finding what can't be the problem and then the one thing that remains will be the problem.

There are a few things that you can try/test.

First: replace R1 with a wire (short R1), make sure that you have the correct fuse in the amplifier (in the USA probably a 0,5A slow). Probably you will need a few in the process.

One of the things I would first inspect is if there is a connection between the regulator chip on the filament board and the chassis. (there should be NO connection, the back of the regulator is not isolated and a washer is used to isolate it)
Use your multimeter in the "resistance/ohm" position and test the resistance between the metal bar/chassis//GND of the amplifier and the metal part of the regulator(s). It could read something like 750R , but it should not read a value lower then this.
When it is lower, replace the washer. Do this for both tubes.

Remove all tubes from the amplifier and switch on the amplifier. When the fuse does not blow, I would say, the power transformer is Ok.
Test the (AC volt) filament tension of the driver tubes. (for example tube pin 7 and 8 of the 6SN7). Tension will be a above 6,3Vac (because there are no tubes in the amplifier, the power transformer is not loaded and all tension will be higher then normal.

You can also test the filament voltages for the 300B tubes. They should read close to 5V DC (but from your post, they measure correct)

Next : let's test the 300B tubes. Many problems come from capacitors and there are 2 capacitors around the 300B that can change the idle current when these parts are defective.
The coupling capacitor between the driver tube and 300B and the cathode capacitor of the 300B. To remove their influence, do the following.
*Connect a wire between the grid of the 300B (direct from pin 3 !) and GND. Even when the coupling capacitor would be defective (and as a result a positive tension would appear on the grid of the 300) this tension will be shorted to GND and the 300B will not notice it.
* remove the cathode capacitor/de-solder the + lead of this capacitor.
This capacitor is needed when you want to have music, but for setting the idle current through the 300B, this capacitors is not needed.

Insert 3000B's, do not insert the small tubes, insert the rectifier tube.

Switch on the amplifier and watch the fuse. When the fuse stay Ok, then test the anode tension and the cathode tension of the 300B's.

Let us know the results.

Best regards, Peter

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