Page 55 voltage check wrong need help again.

[Kurthaudio]

That’s what I’m doing, I’m at the point where I should be getting 10vdc and 100vdc and I’m only getting 0.9vdc and 45vdc. I cant get past this thats why I am asking for help right now.

[Kurthaudio]

That’s exactly what I’m doing.

I should be getting 10vdc and 100vdc on 25 and 6 respectively

I’m getting 0.9vdc and 45vdc……that is what this thread is all about. I’m asking for help to figure this out.

[Thermioniclife]

I'm sure either PB or PJ or if you are lucky PB&J will chime in and get you sorted out. I did about all I can do without being familiar of that product. Don't get to stressed out, you'll be fine in the end.

[Kurthaudio]

Thanks brother. I needed to hear that. I am getting a bit stressed about this. Lol. Thank you for your help. I did learn a little.

[Paul Birkeland]

A seems to be my problem. I’m getting 14- 25:.9vdc and 14- 6: 45vdc.

This needs to be cleared up before you continue.  There could be a loose connection at pin 3 on that socket which isn't providing the grid with a solid reference to ground.

It's also possible to plug the 300B into the socket with the fat pins not in the bigger holes, though it takes some force to do that.  If this is done, you can get weird voltages. 

It's going to be tough to provide much more assistance beyond that, as all the parts in this preamp are not the parts we provided, so there isn't much of a way to visually compare.

[Kurthaudio]

The parts used in this preamp are either the same quality or higher. The values have been checked double checked then rechecked time and time again.  I used my fluke 902 to measure most values I used my LCR meter to measure the rest.  to be 100% certain they are the correct values. I also had these exact voltages with the stock parts

I can re-install all the original parts if you would like to show I’m getting the exact same problem.

The tubes have not been installed backwards

The tube sockets are installed with larger pins to the back as stated in the instructions.

Let’s just assume  all values are installed correctly and all pins are in good contact. All solder joints are properly soldered. Would a set of bad tubes (possibly damaged from my first attempt) possibly cause this? If not……. What should be my next step to get this resolved?

[Kurthaudio]

I just want to throw this out as well, everything measures good without the tubes I stalled

[Paul Birkeland]

If you swap the tubes from side to side, do the problematic voltages stay in the same place or migrate to the other side of the preamp?

[Kurthaudio]

Paul, That’s what makes this even more confusing. Swapping the tubes makes no difference.

[Paul Birkeland]

OK, then you have an issue with the wiring on that side of the amp.

As I mentioned before, if A3 isn't well referenced to ground, that will throw things way off.  This is extra possible if there are cold solders or unsoldered joints.

If you replaced the cathode resistor with a different value (like 33 ohms instead of 330), then that would create this exact problem.

[Kurthaudio]

Agin. No cold solder joints ( I know how to solder very well) no un- soldered joints as shown in my last picture. 330 ohm is used.

I have built many kits. Several from Tubes for hi fi including 6 m-125 monoblock amps. Two Chinese phono stages, a bottlehead phono stage. Countless speakers over the last 20 years. I have made miles of soldered connections. I know what a cold joint is and I know what it looks like.

I understand you get a lot of solder issues on this forum. This is not a solder issue. Something else is going on here and to keep referring to a cold solder joint is not helping at all.

[Paul Birkeland]

0.9V of bias and 45V on the plate means the 300B is drawing too much current.

You can temporarily solder in a wire between pin 3 on the 300B socket at terminal 4 to give the grid a solid ground reference to see if that changes anything.

Otherwise there is something amiss with the three resistors and the cap on 21-25.  The 0.9V would indicate a near short in that area. If you take the capacitor off that terminal strip, what is the DC resistance between pin 1 and ground? ...and pin 4 and ground?

[Kurthaudio]

With the 220 cap removed

Pin 1 - tab 3: 447.3 ohm
Pin 4 - tab 3: 449.2 ohm


Jumping pin 3 to tab 4 doesn’t change anything still .9vdc at 25 and 45vdc at 6

[Paul Birkeland]

How about the voltages with no 220uF cap?

[Kurthaudio]

Voltage does not change with the 220 cap removed.

I tried it with the 220 cap removed. No change.

Jumped pin 3 to ground (tab4) with 220 cap removed. No change