Page 55 voltage check wrong need help again.

Kurthaudio · 7233

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Offline Paul Birkeland

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Reply #30 on: December 05, 2021, 10:20:54 AM
With one probe clipped to terminal 4, what voltages do you get at:

14, 15, 21, 26.

Paul "PB" Birkeland

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Offline Kurthaudio

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Reply #31 on: December 06, 2021, 06:32:56 AM
I get 0 Vdc at 14, 15, 21, 26



Offline Paul Birkeland

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Reply #32 on: December 06, 2021, 06:45:26 AM
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.


Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Kurthaudio

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Reply #33 on: December 06, 2021, 06:48:45 AM
Paul, maybe I’m missing something here but what 130v are you referring to?
« Last Edit: December 06, 2021, 06:50:37 AM by Kurthaudio »



Offline Paul Birkeland

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Reply #34 on: December 06, 2021, 07:07:24 AM
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?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Kurthaudio

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Reply #35 on: December 06, 2021, 07:18:27 AM
21: o
22: 4.7
24: -1.4
25:1.3



Offline Kurthaudio

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Reply #36 on: December 06, 2021, 07:38:25 AM
Paul, do you see any possibility that the tubes themselves could be causing this?



Offline Paul Birkeland

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Reply #37 on: December 06, 2021, 07:45:30 AM
If a tube was the problem, the problem would move to the other side of the amp when swapping tubes around.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Paul Birkeland

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Reply #38 on: December 06, 2021, 08:02:41 AM
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?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Kurthaudio

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Reply #39 on: December 06, 2021, 08:11:02 AM
With tube in socket B and nothing in socket A I get tab 1: 216.8vdc   Tab 6: 0vdc



Offline Paul Birkeland

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Reply #40 on: December 06, 2021, 08:14:31 AM
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?


Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Kurthaudio

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Reply #41 on: December 06, 2021, 08:23:30 AM
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.



Offline Kurthaudio

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Reply #42 on: December 06, 2021, 08:37:35 AM
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



Offline Paul Birkeland

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Reply #43 on: December 06, 2021, 08:40:08 AM
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. 


Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Kurthaudio

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Reply #44 on: December 06, 2021, 08:43:16 AM
Yes sir.