Page 55 voltage check wrong need help again.

[Paul Birkeland]

With one probe clipped to terminal 4, what voltages do you get at:

14, 15, 21, 26.

[Kurthaudio]

I get 0 Vdc at 14, 15, 21, 26

[Paul Birkeland]

There is something that's missing in all this, but I have no idea what it is.

0.9V across ~400 ohms is 2mA. 

The 130V you are measuring across the 3K plate load resistor is more like 43mA.

There should be no difference between these two.

These two measurements are contradictory, and when I see stuff like this, I tend to expect that a flaky solder joint is at work.  I know you are not going to be happy with that suggestion, but you have more or less ruled everything else out.  The only other explanations I could think of are ruled out by taking the bypass cap out and confirming appropriate resistance to ground from terminals 1 and 4 on the 4 pin socket. 

It may also help to get a photo of the 21-25 terminal strip with no cap installed, just to double check everything.

[Kurthaudio]

Paul, maybe I’m missing something here but what 130v are you referring to?

[Paul Birkeland]

130V is the voltage across the 3K plate loading resistor.  130V/3K=43mA.

When looking at the 300B datasheet, 75V on the plate and 43mA of current does indeed happen with approximately 0V of bias.

The question with your preamp is why the bias voltage is so low (43mA drawn through that 330 ohm cathode resistor is 14V of bias!).  Something that would be super typical that we have run into in a situation like this is that the cathode bypass cap gets installed backwards, fails, but the builder will notice and reinstall it.  Instead of the DC operating current flowing through the 330 ohm resistor, it flows through the shorted cap.  In your case the problem exists without the cap there which further extends the mystery.

For your kit, can  you post the DC voltages between terminal 4 and:
21, 22, 24, and 25?

[Kurthaudio]

21: o
22: 4.7
24: -1.4
25:1.3

[Kurthaudio]

Paul, do you see any possibility that the tubes themselves could be causing this?

[Paul Birkeland]

If a tube was the problem, the problem would move to the other side of the amp when swapping tubes around.

[Paul Birkeland]

OK, here's what I can see from your measurements...

The 6.3V filament supply sits on terminals 22 and 24.  Those terminals are both connected to pin 25 by 130 ohm resistors.  6.3V divided by the 260 ohms (the two resistors in series) is 24mA, which would be the expected current with a 300B sitting in the socket but no current being drawn from the HV supply. 

Between 22 and 25, we see 3.4V, 3.4V/130R=24mA.

Between 24 and 25, we see 2.7V, 2.7V/130R=21mA.

These measurements tell me that nearly no current is being drawn across these resistors beyond just having the 6.3V filament supply active.

1.3V at terminal 25 indicates 4mA of cathode current, but this is a contradiction to what's going on with the HV side.

Just for curiosity's sake, if you run no 300B in the A socket with just a 300B in the B socket, what are the voltages at terminals 1 and 6?

[Kurthaudio]

With tube in socket B and nothing in socket A I get tab 1: 216.8vdc   Tab 6: 0vdc

[Paul Birkeland]

With tube in socket B and nothing in socket A I get tab 1: 216.8vdc   Tab 6: 0vdc

0V at terminal 6 is nearly an impossible result.  Are both ends of the 3K resistor well connected? Perhaps the solder joint at 6U isn't doing its job?

[Kurthaudio]

I just double checked and yes I’m getting 0vdc measuring from tab 4 (ground) to tab 6……….

The resistor measures 2995 ohms


P.S.
Just in case this is the first time in my life I’ve ever been wrong (lol) I reflowed all my solder joints. Related to socket A. Unfortunately Same results.

[Kurthaudio]

I cut out that resistor and soldered in a new one. Now I get about 215vdc on 6. And 216 on 1

Installed the tube and voltage drops to about 50 vdc @6

[Paul Birkeland]

Can I see the other side of terminal 6?

With no tube in the socket, there's nothing to draw current through that resistor, so seeing 0V at terminal 6 indicates an issue with the amp that isn't tube related. 

[Kurthaudio]

Yes sir.